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RingTheory\Ideal\Basis.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen
-/
import Mathlib.Algebra.Algebra.Bilinear
import Mathlib.LinearAlgebra.Basis
import Mathlib.RingTheory.Ideal.Basic
/-!
# The basis of ideals
Some results involving `Ideal` and `Basis`.
-/
namespace Ideal
variable {ι R S : Type*} [CommSemiring R] [CommRing S] [IsDomain S] [Algebra R S]
/-- A basis on `S` gives a basis on `Ideal.span {x}`, by multiplying everything by `x`. -/
noncomputable def basisSpanSingleton (b : Basis ι R S) {x : S} (hx : x ≠ 0) :
Basis ι R (span ({x} : Set S)) :=
b.map <|
LinearEquiv.ofInjective (Algebra.lmul R S x) (LinearMap.mul_injective hx) ≪≫ₗ
LinearEquiv.ofEq _ _
(by
ext
simp [mem_span_singleton', mul_comm]) ≪≫ₗ
(Submodule.restrictScalarsEquiv R S S (Ideal.span ({x} : Set S))).restrictScalars R
@[simp]
theorem basisSpanSingleton_apply (b : Basis ι R S) {x : S} (hx : x ≠ 0) (i : ι) :
(basisSpanSingleton b hx i : S) = x * b i := by
simp only [basisSpanSingleton, Basis.map_apply, LinearEquiv.trans_apply,
Submodule.restrictScalarsEquiv_apply, LinearEquiv.ofInjective_apply, LinearEquiv.coe_ofEq_apply,
LinearEquiv.restrictScalars_apply, Algebra.coe_lmul_eq_mul, LinearMap.mul_apply']
@[simp]
theorem constr_basisSpanSingleton {N : Type*} [Semiring N] [Module N S] [SMulCommClass R N S]
(b : Basis ι R S) {x : S} (hx : x ≠ 0) :
(b.constr N).toFun (((↑) : _ → S) ∘ (basisSpanSingleton b hx)) = Algebra.lmul R S x :=
b.ext fun i => by
erw [Basis.constr_basis, Function.comp_apply, basisSpanSingleton_apply, LinearMap.mul_apply']
end Ideal
-- Porting note: added explicit coercion `(b i : S)`
/-- If `I : Ideal S` has a basis over `R`,
`x ∈ I` iff it is a linear combination of basis vectors. -/
theorem Basis.mem_ideal_iff {ι R S : Type*} [CommRing R] [CommRing S] [Algebra R S] {I : Ideal S}
(b : Basis ι R I) {x : S} : x ∈ I ↔ ∃ c : ι →₀ R, x = Finsupp.sum c fun i x => x • (b i : S) :=
(b.map ((I.restrictScalarsEquiv R _ _).restrictScalars R).symm).mem_submodule_iff
/-- If `I : Ideal S` has a finite basis over `R`,
`x ∈ I` iff it is a linear combination of basis vectors. -/
theorem Basis.mem_ideal_iff' {ι R S : Type*} [Fintype ι] [CommRing R] [CommRing S] [Algebra R S]
{I : Ideal S} (b : Basis ι R I) {x : S} : x ∈ I ↔ ∃ c : ι → R, x = ∑ i, c i • (b i : S) :=
(b.map ((I.restrictScalarsEquiv R _ _).restrictScalars R).symm).mem_submodule_iff'
|
RingTheory\Ideal\Colon.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.LinearAlgebra.Quotient
import Mathlib.RingTheory.Ideal.Operations
/-!
# The colon ideal
This file defines `Submodule.colon N P` as the ideal of all elements `r : R` such that `r • P ⊆ N`.
The normal notation for this would be `N : P` which has already been taken by type theory.
-/
namespace Submodule
open Pointwise
variable {R M M' F G : Type*} [CommRing R] [AddCommGroup M] [Module R M]
variable {N N₁ N₂ P P₁ P₂ : Submodule R M}
/-- `N.colon P` is the ideal of all elements `r : R` such that `r • P ⊆ N`. -/
def colon (N P : Submodule R M) : Ideal R :=
annihilator (P.map N.mkQ)
theorem mem_colon {r} : r ∈ N.colon P ↔ ∀ p ∈ P, r • p ∈ N :=
mem_annihilator.trans
⟨fun H p hp => (Quotient.mk_eq_zero N).1 (H (Quotient.mk p) (mem_map_of_mem hp)),
fun H _ ⟨p, hp, hpm⟩ => hpm ▸ ((Quotient.mk_eq_zero N).2 <| H p hp)⟩
theorem mem_colon' {r} : r ∈ N.colon P ↔ P ≤ comap (r • (LinearMap.id : M →ₗ[R] M)) N :=
mem_colon
@[simp]
theorem colon_top {I : Ideal R} : I.colon ⊤ = I := by
simp_rw [SetLike.ext_iff, mem_colon, smul_eq_mul]
exact fun x ↦ ⟨fun h ↦ mul_one x ▸ h 1 trivial, fun h _ _ ↦ I.mul_mem_right _ h⟩
@[simp]
theorem colon_bot : colon ⊥ N = N.annihilator := by
simp_rw [SetLike.ext_iff, mem_colon, mem_annihilator, mem_bot, forall_const]
theorem colon_mono (hn : N₁ ≤ N₂) (hp : P₁ ≤ P₂) : N₁.colon P₂ ≤ N₂.colon P₁ := fun _ hrnp =>
mem_colon.2 fun p₁ hp₁ => hn <| mem_colon.1 hrnp p₁ <| hp hp₁
theorem iInf_colon_iSup (ι₁ : Sort*) (f : ι₁ → Submodule R M) (ι₂ : Sort*)
(g : ι₂ → Submodule R M) : (⨅ i, f i).colon (⨆ j, g j) = ⨅ (i) (j), (f i).colon (g j) :=
le_antisymm (le_iInf fun _ => le_iInf fun _ => colon_mono (iInf_le _ _) (le_iSup _ _)) fun _ H =>
mem_colon'.2 <|
iSup_le fun j =>
map_le_iff_le_comap.1 <|
le_iInf fun i =>
map_le_iff_le_comap.2 <|
mem_colon'.1 <|
have := (mem_iInf _).1 H i
have := (mem_iInf _).1 this j
this
@[simp]
theorem mem_colon_singleton {N : Submodule R M} {x : M} {r : R} :
r ∈ N.colon (Submodule.span R {x}) ↔ r • x ∈ N :=
calc
r ∈ N.colon (Submodule.span R {x}) ↔ ∀ a : R, r • a • x ∈ N := by
simp [Submodule.mem_colon, Submodule.mem_span_singleton]
_ ↔ r • x ∈ N := by simp_rw [fun (a : R) ↦ smul_comm r a x]; exact SetLike.forall_smul_mem_iff
@[simp]
theorem _root_.Ideal.mem_colon_singleton {I : Ideal R} {x r : R} :
r ∈ I.colon (Ideal.span {x}) ↔ r * x ∈ I := by
simp only [← Ideal.submodule_span_eq, Submodule.mem_colon_singleton, smul_eq_mul]
theorem annihilator_quotient {N : Submodule R M} :
Module.annihilator R (M ⧸ N) = N.colon ⊤ := by
simp_rw [SetLike.ext_iff, Module.mem_annihilator, colon, mem_annihilator, map_top,
LinearMap.range_eq_top.mpr (mkQ_surjective N), mem_top, forall_true_left, forall_const]
theorem _root_.Ideal.annihilator_quotient {I : Ideal R} : Module.annihilator R (R ⧸ I) = I := by
rw [Submodule.annihilator_quotient, colon_top]
end Submodule
|
RingTheory\Ideal\Cotangent.lean | /-
Copyright (c) 2022 Andrew Yang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Andrew Yang
-/
import Mathlib.RingTheory.Ideal.Operations
import Mathlib.Algebra.Module.Torsion
import Mathlib.Algebra.Ring.Idempotents
import Mathlib.LinearAlgebra.Dimension.FreeAndStrongRankCondition
import Mathlib.LinearAlgebra.FiniteDimensional.Defs
import Mathlib.RingTheory.LocalRing.ResidueField.Basic
import Mathlib.RingTheory.Filtration
import Mathlib.RingTheory.Nakayama
/-!
# The module `I ⧸ I ^ 2`
In this file, we provide special API support for the module `I ⧸ I ^ 2`. The official
definition is a quotient module of `I`, but the alternative definition as an ideal of `R ⧸ I ^ 2` is
also given, and the two are `R`-equivalent as in `Ideal.cotangentEquivIdeal`.
Additional support is also given to the cotangent space `m ⧸ m ^ 2` of a local ring.
-/
namespace Ideal
-- Porting note: universes need to be explicit to avoid bad universe levels in `quotCotangent`
universe u v w
variable {R : Type u} {S : Type v} {S' : Type w} [CommRing R] [CommSemiring S] [Algebra S R]
variable [CommSemiring S'] [Algebra S' R] [Algebra S S'] [IsScalarTower S S' R] (I : Ideal R)
-- Porting note: instances that were derived automatically need to be proved by hand (see below)
/-- `I ⧸ I ^ 2` as a quotient of `I`. -/
def Cotangent : Type _ := I ⧸ (I • ⊤ : Submodule R I)
instance : AddCommGroup I.Cotangent := by delta Cotangent; infer_instance
instance cotangentModule : Module (R ⧸ I) I.Cotangent := by delta Cotangent; infer_instance
instance : Inhabited I.Cotangent := ⟨0⟩
instance Cotangent.moduleOfTower : Module S I.Cotangent :=
Submodule.Quotient.module' _
instance Cotangent.isScalarTower : IsScalarTower S S' I.Cotangent :=
Submodule.Quotient.isScalarTower _ _
instance [IsNoetherian R I] : IsNoetherian R I.Cotangent :=
inferInstanceAs (IsNoetherian R (I ⧸ (I • ⊤ : Submodule R I)))
/-- The quotient map from `I` to `I ⧸ I ^ 2`. -/
@[simps! (config := .lemmasOnly) apply]
def toCotangent : I →ₗ[R] I.Cotangent := Submodule.mkQ _
theorem map_toCotangent_ker : I.toCotangent.ker.map I.subtype = I ^ 2 := by
rw [Ideal.toCotangent, Submodule.ker_mkQ, pow_two, Submodule.map_smul'' I ⊤ (Submodule.subtype I),
Algebra.id.smul_eq_mul, Submodule.map_subtype_top]
theorem mem_toCotangent_ker {x : I} : x ∈ LinearMap.ker I.toCotangent ↔ (x : R) ∈ I ^ 2 := by
rw [← I.map_toCotangent_ker]
simp
theorem toCotangent_eq {x y : I} : I.toCotangent x = I.toCotangent y ↔ (x - y : R) ∈ I ^ 2 := by
rw [← sub_eq_zero]
exact I.mem_toCotangent_ker
theorem toCotangent_eq_zero (x : I) : I.toCotangent x = 0 ↔ (x : R) ∈ I ^ 2 := I.mem_toCotangent_ker
theorem toCotangent_surjective : Function.Surjective I.toCotangent := Submodule.mkQ_surjective _
theorem toCotangent_range : LinearMap.range I.toCotangent = ⊤ := Submodule.range_mkQ _
theorem cotangent_subsingleton_iff : Subsingleton I.Cotangent ↔ IsIdempotentElem I := by
constructor
· intro H
refine (pow_two I).symm.trans (le_antisymm (Ideal.pow_le_self two_ne_zero) ?_)
exact fun x hx => (I.toCotangent_eq_zero ⟨x, hx⟩).mp (Subsingleton.elim _ _)
· exact fun e =>
⟨fun x y =>
Quotient.inductionOn₂' x y fun x y =>
I.toCotangent_eq.mpr <| ((pow_two I).trans e).symm ▸ I.sub_mem x.prop y.prop⟩
/-- The inclusion map `I ⧸ I ^ 2` to `R ⧸ I ^ 2`. -/
def cotangentToQuotientSquare : I.Cotangent →ₗ[R] R ⧸ I ^ 2 :=
Submodule.mapQ (I • ⊤) (I ^ 2) I.subtype
(by
rw [← Submodule.map_le_iff_le_comap, Submodule.map_smul'', Submodule.map_top,
Submodule.range_subtype, smul_eq_mul, pow_two] )
theorem to_quotient_square_comp_toCotangent :
I.cotangentToQuotientSquare.comp I.toCotangent = (I ^ 2).mkQ.comp (Submodule.subtype I) :=
LinearMap.ext fun _ => rfl
@[simp]
theorem toCotangent_to_quotient_square (x : I) :
I.cotangentToQuotientSquare (I.toCotangent x) = (I ^ 2).mkQ x := rfl
lemma Cotangent.smul_eq_zero_of_mem {I : Ideal R}
{x} (hx : x ∈ I) (m : I.Cotangent) : x • m = 0 := by
obtain ⟨m, rfl⟩ := Ideal.toCotangent_surjective _ m
rw [← map_smul, Ideal.toCotangent_eq_zero, pow_two]
exact Ideal.mul_mem_mul hx m.2
lemma isTorsionBySet_cotangent :
Module.IsTorsionBySet R I.Cotangent I :=
fun m x ↦ m.smul_eq_zero_of_mem x.2
/-- `I ⧸ I ^ 2` as an ideal of `R ⧸ I ^ 2`. -/
def cotangentIdeal (I : Ideal R) : Ideal (R ⧸ I ^ 2) :=
Submodule.map (Quotient.mk (I ^ 2)|>.toSemilinearMap) I
theorem cotangentIdeal_square (I : Ideal R) : I.cotangentIdeal ^ 2 = ⊥ := by
rw [eq_bot_iff, pow_two I.cotangentIdeal, ← smul_eq_mul]
intro x hx
refine Submodule.smul_induction_on hx ?_ ?_
· rintro _ ⟨x, hx, rfl⟩ _ ⟨y, hy, rfl⟩; apply (Submodule.Quotient.eq _).mpr _
rw [sub_zero, pow_two]; exact Ideal.mul_mem_mul hx hy
· intro x y hx hy; exact add_mem hx hy
theorem to_quotient_square_range :
LinearMap.range I.cotangentToQuotientSquare = I.cotangentIdeal.restrictScalars R := by
trans LinearMap.range (I.cotangentToQuotientSquare.comp I.toCotangent)
· rw [LinearMap.range_comp, I.toCotangent_range, Submodule.map_top]
· rw [to_quotient_square_comp_toCotangent, LinearMap.range_comp, I.range_subtype]; ext; rfl
/-- The equivalence of the two definitions of `I / I ^ 2`, either as the quotient of `I` or the
ideal of `R / I ^ 2`. -/
noncomputable def cotangentEquivIdeal : I.Cotangent ≃ₗ[R] I.cotangentIdeal := by
refine
{ LinearMap.codRestrict (I.cotangentIdeal.restrictScalars R) I.cotangentToQuotientSquare
fun x => by { rw [← to_quotient_square_range]; exact LinearMap.mem_range_self _ _ },
Equiv.ofBijective _ ⟨?_, ?_⟩ with }
· rintro x y e
replace e := congr_arg Subtype.val e
obtain ⟨x, rfl⟩ := I.toCotangent_surjective x
obtain ⟨y, rfl⟩ := I.toCotangent_surjective y
rw [I.toCotangent_eq]
dsimp only [toCotangent_to_quotient_square, Submodule.mkQ_apply] at e
rwa [Submodule.Quotient.eq] at e
· rintro ⟨_, x, hx, rfl⟩
exact ⟨I.toCotangent ⟨x, hx⟩, Subtype.ext rfl⟩
@[simp]
theorem cotangentEquivIdeal_apply (x : I.Cotangent) :
↑(I.cotangentEquivIdeal x) = I.cotangentToQuotientSquare x := rfl
theorem cotangentEquivIdeal_symm_apply (x : R) (hx : x ∈ I) :
-- Note: #8386 had to specify `(R₂ := R)` because `I.toCotangent` suggested `R ⧸ I^2` instead
I.cotangentEquivIdeal.symm ⟨(I ^ 2).mkQ x, Submodule.mem_map_of_mem (R₂ := R) hx⟩ =
I.toCotangent ⟨x, hx⟩ := by
apply I.cotangentEquivIdeal.injective
rw [I.cotangentEquivIdeal.apply_symm_apply]
ext
rfl
variable {A B : Type*} [CommRing A] [CommRing B] [Algebra R A] [Algebra R B]
/-- The lift of `f : A →ₐ[R] B` to `A ⧸ J ^ 2 →ₐ[R] B` with `J` being the kernel of `f`. -/
def _root_.AlgHom.kerSquareLift (f : A →ₐ[R] B) : A ⧸ RingHom.ker f.toRingHom ^ 2 →ₐ[R] B := by
refine { Ideal.Quotient.lift (RingHom.ker f.toRingHom ^ 2) f.toRingHom ?_ with commutes' := ?_ }
· intro a ha; exact Ideal.pow_le_self two_ne_zero ha
· intro r
rw [IsScalarTower.algebraMap_apply R A, RingHom.toFun_eq_coe, Ideal.Quotient.algebraMap_eq,
Ideal.Quotient.lift_mk]
exact f.map_algebraMap r
theorem _root_.AlgHom.ker_kerSquareLift (f : A →ₐ[R] B) :
RingHom.ker f.kerSquareLift.toRingHom = f.toRingHom.ker.cotangentIdeal := by
apply le_antisymm
· intro x hx; obtain ⟨x, rfl⟩ := Ideal.Quotient.mk_surjective x; exact ⟨x, hx, rfl⟩
· rintro _ ⟨x, hx, rfl⟩; exact hx
instance Algebra.kerSquareLift : Algebra (R ⧸ (RingHom.ker (algebraMap R A) ^ 2)) A :=
(Algebra.ofId R A).kerSquareLift.toAlgebra
instance [Algebra A B] [IsScalarTower R A B] :
IsScalarTower R (A ⧸ (RingHom.ker (algebraMap A B) ^ 2)) B :=
IsScalarTower.of_algebraMap_eq'
(IsScalarTower.toAlgHom R A B).kerSquareLift.comp_algebraMap.symm
/-- The quotient ring of `I ⧸ I ^ 2` is `R ⧸ I`. -/
def quotCotangent : (R ⧸ I ^ 2) ⧸ I.cotangentIdeal ≃+* R ⧸ I := by
refine (Ideal.quotEquivOfEq (Ideal.map_eq_submodule_map _ _).symm).trans ?_
refine (DoubleQuot.quotQuotEquivQuotSup _ _).trans ?_
exact Ideal.quotEquivOfEq (sup_eq_right.mpr <| Ideal.pow_le_self two_ne_zero)
/-- The map `I/I² → J/J²` if `I ≤ f⁻¹(J)`. -/
def mapCotangent (I₁ : Ideal A) (I₂ : Ideal B) (f : A →ₐ[R] B) (h : I₁ ≤ I₂.comap f) :
I₁.Cotangent →ₗ[R] I₂.Cotangent := by
refine Submodule.mapQ ((I₁ • ⊤ : Submodule A I₁).restrictScalars R)
((I₂ • ⊤ : Submodule B I₂).restrictScalars R) ?_ ?_
· exact f.toLinearMap.restrict (p := I₁.restrictScalars R) (q := I₂.restrictScalars R) h
· intro x hx
refine Submodule.smul_induction_on hx ?_ (fun _ _ ↦ add_mem)
rintro a ha ⟨b, hb⟩ -
simp only [SetLike.mk_smul_mk, smul_eq_mul, Submodule.mem_comap, Submodule.restrictScalars_mem]
convert (Submodule.smul_mem_smul (M := I₂) (r := f a)
(n := ⟨f b, h hb⟩) (h ha) (Submodule.mem_top)) using 1
ext
exact _root_.map_mul f a b
@[simp]
lemma mapCotangent_toCotangent
(I₁ : Ideal A) (I₂ : Ideal B) (f : A →ₐ[R] B) (h : I₁ ≤ I₂.comap f) (x : I₁) :
Ideal.mapCotangent I₁ I₂ f h (Ideal.toCotangent I₁ x) = Ideal.toCotangent I₂ ⟨f x, h x.2⟩ := rfl
end Ideal
namespace LocalRing
variable (R : Type*) [CommRing R] [LocalRing R]
/-- The `A ⧸ I`-vector space `I ⧸ I ^ 2`. -/
abbrev CotangentSpace : Type _ := (maximalIdeal R).Cotangent
instance : Module (ResidueField R) (CotangentSpace R) := Ideal.cotangentModule _
instance : IsScalarTower R (ResidueField R) (CotangentSpace R) :=
Module.IsTorsionBySet.isScalarTower _
instance [IsNoetherianRing R] : FiniteDimensional (ResidueField R) (CotangentSpace R) :=
Module.Finite.of_restrictScalars_finite R _ _
variable {R}
lemma subsingleton_cotangentSpace_iff [IsNoetherianRing R] :
Subsingleton (CotangentSpace R) ↔ IsField R := by
refine (maximalIdeal R).cotangent_subsingleton_iff.trans ?_
rw [LocalRing.isField_iff_maximalIdeal_eq, Ideal.isIdempotentElem_iff_eq_bot_or_top_of_localRing]
simp [(maximalIdeal.isMaximal R).ne_top]
lemma CotangentSpace.map_eq_top_iff [IsNoetherianRing R] {M : Submodule R (maximalIdeal R)} :
M.map (maximalIdeal R).toCotangent = ⊤ ↔ M = ⊤ := by
refine ⟨fun H ↦ eq_top_iff.mpr ?_, by rintro rfl; simp [Ideal.toCotangent_range]⟩
refine (Submodule.map_le_map_iff_of_injective (Submodule.injective_subtype _) _ _).mp ?_
rw [Submodule.map_top, Submodule.range_subtype]
apply Submodule.le_of_le_smul_of_le_jacobson_bot (IsNoetherian.noetherian _)
(LocalRing.jacobson_eq_maximalIdeal _ bot_ne_top).ge
rw [smul_eq_mul, ← pow_two, ← Ideal.map_toCotangent_ker, ← Submodule.map_sup,
← Submodule.comap_map_eq, H, Submodule.comap_top, Submodule.map_top, Submodule.range_subtype]
lemma CotangentSpace.span_image_eq_top_iff [IsNoetherianRing R] {s : Set (maximalIdeal R)} :
Submodule.span (ResidueField R) ((maximalIdeal R).toCotangent '' s) = ⊤ ↔
Submodule.span R s = ⊤ := by
rw [← map_eq_top_iff, ← (Submodule.restrictScalars_injective R ..).eq_iff,
Submodule.restrictScalars_span]
· simp only [Ideal.toCotangent_apply, Submodule.restrictScalars_top, Submodule.map_span]
· exact Ideal.Quotient.mk_surjective
open FiniteDimensional
lemma finrank_cotangentSpace_eq_zero_iff [IsNoetherianRing R] :
finrank (ResidueField R) (CotangentSpace R) = 0 ↔ IsField R := by
rw [finrank_zero_iff, subsingleton_cotangentSpace_iff]
lemma finrank_cotangentSpace_eq_zero (R) [Field R] :
finrank (ResidueField R) (CotangentSpace R) = 0 :=
finrank_cotangentSpace_eq_zero_iff.mpr (Field.toIsField R)
open Submodule in
theorem finrank_cotangentSpace_le_one_iff [IsNoetherianRing R] :
finrank (ResidueField R) (CotangentSpace R) ≤ 1 ↔ (maximalIdeal R).IsPrincipal := by
rw [Module.finrank_le_one_iff_top_isPrincipal, isPrincipal_iff,
(maximalIdeal R).toCotangent_surjective.exists, isPrincipal_iff]
simp_rw [← Set.image_singleton, eq_comm (a := ⊤), CotangentSpace.span_image_eq_top_iff,
← (map_injective_of_injective (injective_subtype _)).eq_iff, map_span, Set.image_singleton,
Submodule.map_top, range_subtype, eq_comm (a := maximalIdeal R)]
exact ⟨fun ⟨x, h⟩ ↦ ⟨_, h⟩, fun ⟨x, h⟩ ↦ ⟨⟨x, h ▸ subset_span (Set.mem_singleton x)⟩, h⟩⟩
end LocalRing
|
RingTheory\Ideal\IdempotentFG.lean | /-
Copyright (c) 2018 Mario Carneiro, Kevin Buzzard. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro, Kevin Buzzard
-/
import Mathlib.Algebra.Ring.Idempotents
import Mathlib.RingTheory.Finiteness
import Mathlib.Order.Basic
/-!
## Lemmas on idempotent finitely generated ideals
-/
namespace Ideal
/-- A finitely generated idempotent ideal is generated by an idempotent element -/
theorem isIdempotentElem_iff_of_fg {R : Type*} [CommRing R] (I : Ideal R) (h : I.FG) :
IsIdempotentElem I ↔ ∃ e : R, IsIdempotentElem e ∧ I = R ∙ e := by
constructor
· intro e
obtain ⟨r, hr, hr'⟩ :=
Submodule.exists_mem_and_smul_eq_self_of_fg_of_le_smul I I h
(by
rw [smul_eq_mul]
exact e.ge)
simp_rw [smul_eq_mul] at hr'
refine ⟨r, hr' r hr, antisymm ?_ ((Submodule.span_singleton_le_iff_mem _ _).mpr hr)⟩
intro x hx
rw [← hr' x hx]
exact Ideal.mem_span_singleton'.mpr ⟨_, mul_comm _ _⟩
· rintro ⟨e, he, rfl⟩
simp [IsIdempotentElem, Ideal.span_singleton_mul_span_singleton, he.eq]
theorem isIdempotentElem_iff_eq_bot_or_top {R : Type*} [CommRing R] [IsDomain R] (I : Ideal R)
(h : I.FG) : IsIdempotentElem I ↔ I = ⊥ ∨ I = ⊤ := by
constructor
· intro H
obtain ⟨e, he, rfl⟩ := (I.isIdempotentElem_iff_of_fg h).mp H
simp only [Ideal.submodule_span_eq, Ideal.span_singleton_eq_bot]
apply Or.imp id _ (IsIdempotentElem.iff_eq_zero_or_one.mp he)
rintro rfl
simp
· rintro (rfl | rfl) <;> simp [IsIdempotentElem]
end Ideal
|
RingTheory\Ideal\IsPrimary.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.RingTheory.Ideal.Operations
/-!
# Primary ideals
A proper ideal `I` is primary iff `xy ∈ I` implies `x ∈ I` or `y ∈ radical I`.
## Main definitions
- `Ideal.IsPrimary`
-/
namespace Ideal
variable {R : Type*} [CommSemiring R]
/-- A proper ideal `I` is primary iff `xy ∈ I` implies `x ∈ I` or `y ∈ radical I`. -/
def IsPrimary (I : Ideal R) : Prop :=
I ≠ ⊤ ∧ ∀ {x y : R}, x * y ∈ I → x ∈ I ∨ y ∈ radical I
theorem IsPrime.isPrimary {I : Ideal R} (hi : IsPrime I) : IsPrimary I :=
⟨hi.1, fun {_ _} hxy => (hi.mem_or_mem hxy).imp id fun hyi => le_radical hyi⟩
theorem isPrime_radical {I : Ideal R} (hi : IsPrimary I) : IsPrime (radical I) :=
⟨mt radical_eq_top.1 hi.1,
fun {x y} ⟨m, hxy⟩ => by
rw [mul_pow] at hxy; cases' hi.2 hxy with h h
· exact Or.inl ⟨m, h⟩
· exact Or.inr (mem_radical_of_pow_mem h)⟩
theorem isPrimary_inf {I J : Ideal R} (hi : IsPrimary I) (hj : IsPrimary J)
(hij : radical I = radical J) : IsPrimary (I ⊓ J) :=
⟨ne_of_lt <| lt_of_le_of_lt inf_le_left (lt_top_iff_ne_top.2 hi.1),
fun {x y} ⟨hxyi, hxyj⟩ => by
rw [radical_inf, hij, inf_idem]
cases' hi.2 hxyi with hxi hyi
· cases' hj.2 hxyj with hxj hyj
· exact Or.inl ⟨hxi, hxj⟩
· exact Or.inr hyj
· rw [hij] at hyi
exact Or.inr hyi⟩
end Ideal
|
RingTheory\Ideal\IsPrincipal.lean | /-
Copyright (c) 2024 Xavier Roblot. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Xavier Roblot
-/
import Mathlib.RingTheory.PrincipalIdealDomain
/-!
# Principal Ideals
This file deals with the set of principal ideals of a `CommRing R`.
## Main definitions and results
* `Ideal.isPrincipalSubmonoid`: the submonoid of `Ideal R` formed by the principal ideals of `R`.
* `Ideal.isPrincipalNonZeroDivisorSubmonoid`: the submonoid of `(Ideal R)⁰` formed by the
non-zero-divisors principal ideals of `R`.
* `Ideal.associatesMulEquivIsPrincipal`: the `MulEquiv` between the monoid of `Associates R` and
the submonoid of principal ideals of `R`.
* `Ideal.associatesNonZeroDivisorsMulEquivIsPrincipal`: the `MulEquiv` between the monoid of
`Associates R⁰` and the submonoid of non-zero-divisors principal ideals of `R`.
-/
variable {R : Type*} [CommRing R]
namespace Ideal
open Submodule Associates
open scoped nonZeroDivisors
variable (R) in
/-- The principal ideals of `R` form a submonoid of `Ideal R`. -/
def isPrincipalSubmonoid : Submonoid (Ideal R) where
carrier := {I | IsPrincipal I}
mul_mem' := by
rintro _ _ ⟨x, rfl⟩ ⟨y, rfl⟩
exact ⟨x * y, span_singleton_mul_span_singleton x y⟩
one_mem' := ⟨1, one_eq_span⟩
theorem mem_isPrincipalSubmonoid_iff {I : Ideal R} :
I ∈ isPrincipalSubmonoid R ↔ IsPrincipal I := Iff.rfl
theorem span_singleton_mem_isPrincipalSubmonoid (a : R) :
span {a} ∈ isPrincipalSubmonoid R := mem_isPrincipalSubmonoid_iff.mpr ⟨a, rfl⟩
variable (R) in
/-- The non-zero-divisors principal ideals of `R` form a submonoid of `(Ideal R)⁰`. -/
def isPrincipalNonZeroDivisorsSubmonoid : Submonoid (Ideal R)⁰ where
carrier := {I | IsPrincipal I.val}
mul_mem' := by
rintro ⟨_, _⟩ ⟨_, _⟩ ⟨x, rfl⟩ ⟨y, rfl⟩
exact ⟨x * y, by
simp_rw [Submonoid.mk_mul_mk, submodule_span_eq, span_singleton_mul_span_singleton]⟩
one_mem' := ⟨1, by simp⟩
variable [IsDomain R]
variable (R) in
/-- The equivalence between `Associates R` and the principal ideals of `R` defined by sending the
class of `x` to the principal ideal generated by `x`. -/
noncomputable def associatesEquivIsPrincipal :
Associates R ≃ {I : Ideal R // IsPrincipal I} where
toFun := Quotient.lift (fun x ↦ ⟨span {x}, x, rfl⟩)
(fun _ _ _ ↦ by simpa [span_singleton_eq_span_singleton])
invFun I := .mk I.2.generator
left_inv := Quotient.ind fun _ ↦ by simpa using
Ideal.span_singleton_eq_span_singleton.mp (@Ideal.span_singleton_generator _ _ _ ⟨_, rfl⟩)
right_inv I := by simp only [Quotient.lift_mk, span_singleton_generator, Subtype.coe_eta]
@[simp]
theorem associatesEquivIsPrincipal_apply (x : R) :
associatesEquivIsPrincipal R (.mk x) = span {x} := rfl
@[simp]
theorem associatesEquivIsPrincipal_symm_apply {I : Ideal R} (hI : IsPrincipal I) :
(associatesEquivIsPrincipal R).symm ⟨I, hI⟩ = .mk hI.generator := rfl
theorem associatesEquivIsPrincipal_mul (x y : Associates R) :
(associatesEquivIsPrincipal R (x * y) : Ideal R) =
(associatesEquivIsPrincipal R x) * (associatesEquivIsPrincipal R y) := by
rw [← quot_out x, ← quot_out y]
simp_rw [mk_mul_mk, associatesEquivIsPrincipal_apply, span_singleton_mul_span_singleton]
@[simp]
theorem associatesEquivIsPrincipal_map_zero :
(associatesEquivIsPrincipal R 0 : Ideal R) = 0 := by
rw [← mk_zero, associatesEquivIsPrincipal_apply, Submodule.zero_eq_bot, span_singleton_eq_bot]
@[simp]
theorem associatesEquivIsPrincipal_map_one :
(associatesEquivIsPrincipal R 1 : Ideal R) = 1 := by
rw [one_eq_mk_one, associatesEquivIsPrincipal_apply, span_singleton_one, one_eq_top]
variable (R) in
/-- The `MulEquiv` version of `Ideal.associatesEquivIsPrincipal`. -/
noncomputable def associatesMulEquivIsPrincipal :
Associates R ≃* (isPrincipalSubmonoid R) where
__ := associatesEquivIsPrincipal R
map_mul' _ _ := by
erw [Subtype.ext_iff, associatesEquivIsPrincipal_mul]
rfl
variable (R) in
/-- A version of `Ideal.associatesEquivIsPrincipal` for non-zero-divisors generators. -/
noncomputable def associatesNonZeroDivisorsEquivIsPrincipal :
Associates R⁰ ≃ {I : (Ideal R)⁰ // IsPrincipal (I : Ideal R)} :=
calc Associates R⁰ ≃ (Associates R)⁰ := associatesNonZeroDivisorsEquiv.toEquiv.symm
_ ≃ {I : {I : Ideal R // IsPrincipal I} // I.1 ∈ (Ideal R)⁰} :=
Equiv.subtypeEquiv (associatesEquivIsPrincipal R)
(fun x ↦ by rw [← quot_out x, mk_mem_nonZeroDivisors_associates,
associatesEquivIsPrincipal_apply, span_singleton_nonZeroDivisors])
_ ≃ {I : Ideal R // IsPrincipal I ∧ I ∈ (Ideal R)⁰} :=
Equiv.subtypeSubtypeEquivSubtypeInter (fun I ↦ IsPrincipal I) (fun I ↦ I ∈ (Ideal R)⁰)
_ ≃ {I : Ideal R // I ∈ (Ideal R)⁰ ∧ IsPrincipal I} := Equiv.setCongr (by simp_rw [and_comm])
_ ≃ {I : (Ideal R)⁰ // IsPrincipal I.1} := (Equiv.subtypeSubtypeEquivSubtypeInter _ _).symm
@[simp]
theorem associatesNonZeroDivisorsEquivIsPrincipal_apply (x : R⁰) :
associatesNonZeroDivisorsEquivIsPrincipal R (.mk x) = Ideal.span {(x : R)} := rfl
theorem associatesNonZeroDivisorsEquivIsPrincipal_coe (x : Associates R⁰) :
(associatesNonZeroDivisorsEquivIsPrincipal R x : Ideal R) =
(associatesEquivIsPrincipal R (associatesNonZeroDivisorsEquiv.symm x)) := rfl
theorem associatesNonZeroDivisorsEquivIsPrincipal_mul (x y : Associates R⁰) :
(associatesNonZeroDivisorsEquivIsPrincipal R (x * y) : Ideal R) =
(associatesNonZeroDivisorsEquivIsPrincipal R x) *
(associatesNonZeroDivisorsEquivIsPrincipal R y) := by
simp_rw [associatesNonZeroDivisorsEquivIsPrincipal_coe, _root_.map_mul, Submonoid.coe_mul,
associatesEquivIsPrincipal_mul]
@[simp]
theorem associatesNonZeroDivisorsEquivIsPrincipal_map_one :
(associatesNonZeroDivisorsEquivIsPrincipal R 1 : Ideal R) = 1 := by
rw [associatesNonZeroDivisorsEquivIsPrincipal_coe, map_one, OneMemClass.coe_one,
associatesEquivIsPrincipal_map_one]
variable (R) in
/-- The `MulEquiv` version of `Ideal.associatesNonZeroDivisorsEquivIsPrincipal`. -/
noncomputable def associatesNonZeroDivisorsMulEquivIsPrincipal :
Associates R⁰ ≃* (isPrincipalNonZeroDivisorsSubmonoid R) where
__ := associatesNonZeroDivisorsEquivIsPrincipal R
map_mul' _ _ := by
erw [Subtype.ext_iff, Subtype.ext_iff, associatesNonZeroDivisorsEquivIsPrincipal_mul]
rfl
end Ideal
|
RingTheory\Ideal\Maps.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.RingTheory.Ideal.Operations
/-!
# Maps on modules and ideals
-/
assert_not_exists Basis -- See `RingTheory.Ideal.Basis`
assert_not_exists Submodule.hasQuotient -- See `RingTheory.Ideal.QuotientOperations`
universe u v w x
open Pointwise
namespace Ideal
section MapAndComap
variable {R : Type u} {S : Type v}
section Semiring
variable {F : Type*} [Semiring R] [Semiring S]
variable [FunLike F R S] [RingHomClass F R S]
variable (f : F)
variable {I J : Ideal R} {K L : Ideal S}
/-- `I.map f` is the span of the image of the ideal `I` under `f`, which may be bigger than
the image itself. -/
def map (I : Ideal R) : Ideal S :=
span (f '' I)
/-- `I.comap f` is the preimage of `I` under `f`. -/
def comap (I : Ideal S) : Ideal R where
carrier := f ⁻¹' I
add_mem' {x y} hx hy := by
simp only [Set.mem_preimage, SetLike.mem_coe, map_add f] at hx hy ⊢
exact add_mem hx hy
zero_mem' := by simp only [Set.mem_preimage, map_zero, SetLike.mem_coe, Submodule.zero_mem]
smul_mem' c x hx := by
simp only [smul_eq_mul, Set.mem_preimage, map_mul, SetLike.mem_coe] at *
exact mul_mem_left I _ hx
@[simp]
theorem coe_comap (I : Ideal S) : (comap f I : Set R) = f ⁻¹' I := rfl
variable {f}
theorem map_mono (h : I ≤ J) : map f I ≤ map f J :=
span_mono <| Set.image_subset _ h
theorem mem_map_of_mem (f : F) {I : Ideal R} {x : R} (h : x ∈ I) : f x ∈ map f I :=
subset_span ⟨x, h, rfl⟩
theorem apply_coe_mem_map (f : F) (I : Ideal R) (x : I) : f x ∈ I.map f :=
mem_map_of_mem f x.2
theorem map_le_iff_le_comap : map f I ≤ K ↔ I ≤ comap f K :=
span_le.trans Set.image_subset_iff
@[simp]
theorem mem_comap {x} : x ∈ comap f K ↔ f x ∈ K :=
Iff.rfl
theorem comap_mono (h : K ≤ L) : comap f K ≤ comap f L :=
Set.preimage_mono fun _ hx => h hx
variable (f)
theorem comap_ne_top (hK : K ≠ ⊤) : comap f K ≠ ⊤ :=
(ne_top_iff_one _).2 <| by rw [mem_comap, map_one]; exact (ne_top_iff_one _).1 hK
variable {G : Type*} [FunLike G S R]
theorem map_le_comap_of_inv_on [RingHomClass G S R] (g : G) (I : Ideal R)
(hf : Set.LeftInvOn g f I) :
I.map f ≤ I.comap g := by
refine Ideal.span_le.2 ?_
rintro x ⟨x, hx, rfl⟩
rw [SetLike.mem_coe, mem_comap, hf hx]
exact hx
theorem comap_le_map_of_inv_on (g : G) (I : Ideal S)
(hf : Set.LeftInvOn g f (f ⁻¹' I)) :
I.comap f ≤ I.map g :=
fun x (hx : f x ∈ I) => hf hx ▸ Ideal.mem_map_of_mem g hx
/-- The `Ideal` version of `Set.image_subset_preimage_of_inverse`. -/
theorem map_le_comap_of_inverse [RingHomClass G S R] (g : G) (I : Ideal R)
(h : Function.LeftInverse g f) :
I.map f ≤ I.comap g :=
map_le_comap_of_inv_on _ _ _ <| h.leftInvOn _
/-- The `Ideal` version of `Set.preimage_subset_image_of_inverse`. -/
theorem comap_le_map_of_inverse (g : G) (I : Ideal S) (h : Function.LeftInverse g f) :
I.comap f ≤ I.map g :=
comap_le_map_of_inv_on _ _ _ <| h.leftInvOn _
instance IsPrime.comap [hK : K.IsPrime] : (comap f K).IsPrime :=
⟨comap_ne_top _ hK.1, fun {x y} => by simp only [mem_comap, map_mul]; apply hK.2⟩
variable (I J K L)
theorem map_top : map f ⊤ = ⊤ :=
(eq_top_iff_one _).2 <| subset_span ⟨1, trivial, map_one f⟩
theorem gc_map_comap : GaloisConnection (Ideal.map f) (Ideal.comap f) := fun _ _ =>
Ideal.map_le_iff_le_comap
@[simp]
theorem comap_id : I.comap (RingHom.id R) = I :=
Ideal.ext fun _ => Iff.rfl
@[simp]
theorem map_id : I.map (RingHom.id R) = I :=
(gc_map_comap (RingHom.id R)).l_unique GaloisConnection.id comap_id
theorem comap_comap {T : Type*} [Semiring T] {I : Ideal T} (f : R →+* S) (g : S →+* T) :
(I.comap g).comap f = I.comap (g.comp f) :=
rfl
theorem map_map {T : Type*} [Semiring T] {I : Ideal R} (f : R →+* S) (g : S →+* T) :
(I.map f).map g = I.map (g.comp f) :=
((gc_map_comap f).compose (gc_map_comap g)).l_unique (gc_map_comap (g.comp f)) fun _ =>
comap_comap _ _
theorem map_span (f : F) (s : Set R) : map f (span s) = span (f '' s) := by
refine (Submodule.span_eq_of_le _ ?_ ?_).symm
· rintro _ ⟨x, hx, rfl⟩; exact mem_map_of_mem f (subset_span hx)
· rw [map_le_iff_le_comap, span_le, coe_comap, ← Set.image_subset_iff]
exact subset_span
variable {f I J K L}
theorem map_le_of_le_comap : I ≤ K.comap f → I.map f ≤ K :=
(gc_map_comap f).l_le
theorem le_comap_of_map_le : I.map f ≤ K → I ≤ K.comap f :=
(gc_map_comap f).le_u
theorem le_comap_map : I ≤ (I.map f).comap f :=
(gc_map_comap f).le_u_l _
theorem map_comap_le : (K.comap f).map f ≤ K :=
(gc_map_comap f).l_u_le _
@[simp]
theorem comap_top : (⊤ : Ideal S).comap f = ⊤ :=
(gc_map_comap f).u_top
@[simp]
theorem comap_eq_top_iff {I : Ideal S} : I.comap f = ⊤ ↔ I = ⊤ :=
⟨fun h => I.eq_top_iff_one.mpr (map_one f ▸ mem_comap.mp ((I.comap f).eq_top_iff_one.mp h)),
fun h => by rw [h, comap_top]⟩
@[simp]
theorem map_bot : (⊥ : Ideal R).map f = ⊥ :=
(gc_map_comap f).l_bot
variable (f I J K L)
@[simp]
theorem map_comap_map : ((I.map f).comap f).map f = I.map f :=
(gc_map_comap f).l_u_l_eq_l I
@[simp]
theorem comap_map_comap : ((K.comap f).map f).comap f = K.comap f :=
(gc_map_comap f).u_l_u_eq_u K
theorem map_sup : (I ⊔ J).map f = I.map f ⊔ J.map f :=
(gc_map_comap f : GaloisConnection (map f) (comap f)).l_sup
theorem comap_inf : comap f (K ⊓ L) = comap f K ⊓ comap f L :=
rfl
variable {ι : Sort*}
theorem map_iSup (K : ι → Ideal R) : (iSup K).map f = ⨆ i, (K i).map f :=
(gc_map_comap f : GaloisConnection (map f) (comap f)).l_iSup
theorem comap_iInf (K : ι → Ideal S) : (iInf K).comap f = ⨅ i, (K i).comap f :=
(gc_map_comap f : GaloisConnection (map f) (comap f)).u_iInf
theorem map_sSup (s : Set (Ideal R)) : (sSup s).map f = ⨆ I ∈ s, (I : Ideal R).map f :=
(gc_map_comap f : GaloisConnection (map f) (comap f)).l_sSup
theorem comap_sInf (s : Set (Ideal S)) : (sInf s).comap f = ⨅ I ∈ s, (I : Ideal S).comap f :=
(gc_map_comap f : GaloisConnection (map f) (comap f)).u_sInf
theorem comap_sInf' (s : Set (Ideal S)) : (sInf s).comap f = ⨅ I ∈ comap f '' s, I :=
_root_.trans (comap_sInf f s) (by rw [iInf_image])
theorem comap_isPrime [H : IsPrime K] : IsPrime (comap f K) :=
⟨comap_ne_top f H.ne_top, fun {x y} h => H.mem_or_mem <| by rwa [mem_comap, map_mul] at h⟩
variable {I J K L}
theorem map_inf_le : map f (I ⊓ J) ≤ map f I ⊓ map f J :=
(gc_map_comap f : GaloisConnection (map f) (comap f)).monotone_l.map_inf_le _ _
theorem le_comap_sup : comap f K ⊔ comap f L ≤ comap f (K ⊔ L) :=
(gc_map_comap f : GaloisConnection (map f) (comap f)).monotone_u.le_map_sup _ _
-- TODO: Should these be simp lemmas?
theorem _root_.element_smul_restrictScalars {R S M}
[CommSemiring R] [CommSemiring S] [Algebra R S] [AddCommMonoid M]
[Module R M] [Module S M] [IsScalarTower R S M] (r : R) (N : Submodule S M) :
(algebraMap R S r • N).restrictScalars R = r • N.restrictScalars R :=
SetLike.coe_injective (congrArg (· '' _) (funext (algebraMap_smul S r)))
theorem smul_restrictScalars {R S M} [CommSemiring R] [CommSemiring S]
[Algebra R S] [AddCommMonoid M] [Module R M] [Module S M]
[IsScalarTower R S M] (I : Ideal R) (N : Submodule S M) :
(I.map (algebraMap R S) • N).restrictScalars R = I • N.restrictScalars R := by
simp_rw [map, Submodule.span_smul_eq, ← Submodule.coe_set_smul,
Submodule.set_smul_eq_iSup, ← element_smul_restrictScalars, iSup_image]
exact (_root_.map_iSup₂ (Submodule.restrictScalarsLatticeHom R S M) _)
@[simp]
theorem smul_top_eq_map {R S : Type*} [CommSemiring R] [CommSemiring S] [Algebra R S]
(I : Ideal R) : I • (⊤ : Submodule R S) = (I.map (algebraMap R S)).restrictScalars R :=
Eq.trans (smul_restrictScalars I (⊤ : Ideal S)).symm <|
congrArg _ <| Eq.trans (Ideal.smul_eq_mul _ _) (Ideal.mul_top _)
@[simp]
theorem coe_restrictScalars {R S : Type*} [CommSemiring R] [Semiring S] [Algebra R S]
(I : Ideal S) : (I.restrictScalars R : Set S) = ↑I :=
rfl
/-- The smallest `S`-submodule that contains all `x ∈ I * y ∈ J`
is also the smallest `R`-submodule that does so. -/
@[simp]
theorem restrictScalars_mul {R S : Type*} [CommSemiring R] [CommSemiring S] [Algebra R S]
(I J : Ideal S) : (I * J).restrictScalars R = I.restrictScalars R * J.restrictScalars R :=
le_antisymm
(fun _ hx =>
Submodule.mul_induction_on hx (fun _ hx _ hy => Submodule.mul_mem_mul hx hy) fun _ _ =>
Submodule.add_mem _)
(Submodule.mul_le.mpr fun _ hx _ hy => Ideal.mul_mem_mul hx hy)
section Surjective
variable (hf : Function.Surjective f)
open Function
theorem map_comap_of_surjective (I : Ideal S) : map f (comap f I) = I :=
le_antisymm (map_le_iff_le_comap.2 le_rfl) fun s hsi =>
let ⟨r, hfrs⟩ := hf s
hfrs ▸ (mem_map_of_mem f <| show f r ∈ I from hfrs.symm ▸ hsi)
/-- `map` and `comap` are adjoint, and the composition `map f ∘ comap f` is the
identity -/
def giMapComap : GaloisInsertion (map f) (comap f) :=
GaloisInsertion.monotoneIntro (gc_map_comap f).monotone_u (gc_map_comap f).monotone_l
(fun _ => le_comap_map) (map_comap_of_surjective _ hf)
theorem map_surjective_of_surjective : Surjective (map f) :=
(giMapComap f hf).l_surjective
theorem comap_injective_of_surjective : Injective (comap f) :=
(giMapComap f hf).u_injective
theorem map_sup_comap_of_surjective (I J : Ideal S) : (I.comap f ⊔ J.comap f).map f = I ⊔ J :=
(giMapComap f hf).l_sup_u _ _
theorem map_iSup_comap_of_surjective (K : ι → Ideal S) : (⨆ i, (K i).comap f).map f = iSup K :=
(giMapComap f hf).l_iSup_u _
theorem map_inf_comap_of_surjective (I J : Ideal S) : (I.comap f ⊓ J.comap f).map f = I ⊓ J :=
(giMapComap f hf).l_inf_u _ _
theorem map_iInf_comap_of_surjective (K : ι → Ideal S) : (⨅ i, (K i).comap f).map f = iInf K :=
(giMapComap f hf).l_iInf_u _
theorem mem_image_of_mem_map_of_surjective {I : Ideal R} {y} (H : y ∈ map f I) : y ∈ f '' I :=
Submodule.span_induction H (fun _ => id) ⟨0, I.zero_mem, map_zero f⟩
(fun _ _ ⟨x1, hx1i, hxy1⟩ ⟨x2, hx2i, hxy2⟩ =>
⟨x1 + x2, I.add_mem hx1i hx2i, hxy1 ▸ hxy2 ▸ map_add f _ _⟩)
fun c _ ⟨x, hxi, hxy⟩ =>
let ⟨d, hdc⟩ := hf c
⟨d * x, I.mul_mem_left _ hxi, hdc ▸ hxy ▸ map_mul f _ _⟩
theorem mem_map_iff_of_surjective {I : Ideal R} {y} : y ∈ map f I ↔ ∃ x, x ∈ I ∧ f x = y :=
⟨fun h => (Set.mem_image _ _ _).2 (mem_image_of_mem_map_of_surjective f hf h), fun ⟨_, hx⟩ =>
hx.right ▸ mem_map_of_mem f hx.left⟩
theorem le_map_of_comap_le_of_surjective : comap f K ≤ I → K ≤ map f I := fun h =>
map_comap_of_surjective f hf K ▸ map_mono h
theorem map_eq_submodule_map (f : R →+* S) [h : RingHomSurjective f] (I : Ideal R) :
I.map f = Submodule.map f.toSemilinearMap I :=
Submodule.ext fun _ => mem_map_iff_of_surjective f h.1
end Surjective
section Injective
theorem comap_bot_le_of_injective (hf : Function.Injective f) : comap f ⊥ ≤ I := by
refine le_trans (fun x hx => ?_) bot_le
rw [mem_comap, Submodule.mem_bot, ← map_zero f] at hx
exact Eq.symm (hf hx) ▸ Submodule.zero_mem ⊥
theorem comap_bot_of_injective (hf : Function.Injective f) : Ideal.comap f ⊥ = ⊥ :=
le_bot_iff.mp (Ideal.comap_bot_le_of_injective f hf)
end Injective
/-- If `f : R ≃+* S` is a ring isomorphism and `I : Ideal R`, then `map f.symm (map f I) = I`. -/
@[simp]
theorem map_of_equiv (I : Ideal R) (f : R ≃+* S) :
(I.map (f : R →+* S)).map (f.symm : S →+* R) = I := by
rw [← RingEquiv.toRingHom_eq_coe, ← RingEquiv.toRingHom_eq_coe, map_map,
RingEquiv.toRingHom_eq_coe, RingEquiv.toRingHom_eq_coe, RingEquiv.symm_comp, map_id]
/-- If `f : R ≃+* S` is a ring isomorphism and `I : Ideal R`,
then `comap f (comap f.symm I) = I`. -/
@[simp]
theorem comap_of_equiv (I : Ideal R) (f : R ≃+* S) :
(I.comap (f.symm : S →+* R)).comap (f : R →+* S) = I := by
rw [← RingEquiv.toRingHom_eq_coe, ← RingEquiv.toRingHom_eq_coe, comap_comap,
RingEquiv.toRingHom_eq_coe, RingEquiv.toRingHom_eq_coe, RingEquiv.symm_comp, comap_id]
/-- If `f : R ≃+* S` is a ring isomorphism and `I : Ideal R`, then `map f I = comap f.symm I`. -/
theorem map_comap_of_equiv (I : Ideal R) (f : R ≃+* S) : I.map (f : R →+* S) = I.comap f.symm :=
le_antisymm (Ideal.map_le_comap_of_inverse _ _ _ (Equiv.left_inv' _))
(Ideal.comap_le_map_of_inverse _ _ _ (Equiv.right_inv' _))
/-- If `f : R ≃+* S` is a ring isomorphism and `I : Ideal R`, then `comap f.symm I = map f I`. -/
@[simp]
theorem comap_symm (I : Ideal R) (f : R ≃+* S) : I.comap f.symm = I.map f :=
(map_comap_of_equiv I f).symm
/-- If `f : R ≃+* S` is a ring isomorphism and `I : Ideal R`, then `map f.symm I = comap f I`. -/
@[simp]
theorem map_symm (I : Ideal S) (f : R ≃+* S) : I.map f.symm = I.comap f :=
map_comap_of_equiv I (RingEquiv.symm f)
end Semiring
section Ring
variable {F : Type*} [Ring R] [Ring S]
variable [FunLike F R S] [RingHomClass F R S] (f : F) {I : Ideal R}
section Surjective
theorem comap_map_of_surjective (hf : Function.Surjective f) (I : Ideal R) :
comap f (map f I) = I ⊔ comap f ⊥ :=
le_antisymm
(fun r h =>
let ⟨s, hsi, hfsr⟩ := mem_image_of_mem_map_of_surjective f hf h
Submodule.mem_sup.2
⟨s, hsi, r - s, (Submodule.mem_bot S).2 <| by rw [map_sub, hfsr, sub_self],
add_sub_cancel s r⟩)
(sup_le (map_le_iff_le_comap.1 le_rfl) (comap_mono bot_le))
/-- Correspondence theorem -/
def relIsoOfSurjective (hf : Function.Surjective f) :
Ideal S ≃o { p : Ideal R // comap f ⊥ ≤ p } where
toFun J := ⟨comap f J, comap_mono bot_le⟩
invFun I := map f I.1
left_inv J := map_comap_of_surjective f hf J
right_inv I :=
Subtype.eq <|
show comap f (map f I.1) = I.1 from
(comap_map_of_surjective f hf I).symm ▸ le_antisymm (sup_le le_rfl I.2) le_sup_left
map_rel_iff' {I1 I2} :=
⟨fun H => map_comap_of_surjective f hf I1 ▸ map_comap_of_surjective f hf I2 ▸ map_mono H,
comap_mono⟩
/-- The map on ideals induced by a surjective map preserves inclusion. -/
def orderEmbeddingOfSurjective (hf : Function.Surjective f) : Ideal S ↪o Ideal R :=
(relIsoOfSurjective f hf).toRelEmbedding.trans (Subtype.relEmbedding (fun x y => x ≤ y) _)
theorem map_eq_top_or_isMaximal_of_surjective (hf : Function.Surjective f) {I : Ideal R}
(H : IsMaximal I) : map f I = ⊤ ∨ IsMaximal (map f I) := by
refine or_iff_not_imp_left.2 fun ne_top => ⟨⟨fun h => ne_top h, fun J hJ => ?_⟩⟩
· refine
(relIsoOfSurjective f hf).injective
(Subtype.ext_iff.2 (Eq.trans (H.1.2 (comap f J) (lt_of_le_of_ne ?_ ?_)) comap_top.symm))
· exact map_le_iff_le_comap.1 (le_of_lt hJ)
· exact fun h => hJ.right (le_map_of_comap_le_of_surjective f hf (le_of_eq h.symm))
theorem comap_isMaximal_of_surjective (hf : Function.Surjective f) {K : Ideal S} [H : IsMaximal K]:
IsMaximal (comap f K) := by
refine ⟨⟨comap_ne_top _ H.1.1, fun J hJ => ?_⟩⟩
suffices map f J = ⊤ by
have := congr_arg (comap f) this
rw [comap_top, comap_map_of_surjective _ hf, eq_top_iff] at this
rw [eq_top_iff]
exact le_trans this (sup_le (le_of_eq rfl) (le_trans (comap_mono bot_le) (le_of_lt hJ)))
refine
H.1.2 (map f J)
(lt_of_le_of_ne (le_map_of_comap_le_of_surjective _ hf (le_of_lt hJ)) fun h =>
ne_of_lt hJ (_root_.trans (congr_arg (comap f) h) ?_))
rw [comap_map_of_surjective _ hf, sup_eq_left]
exact le_trans (comap_mono bot_le) (le_of_lt hJ)
theorem comap_le_comap_iff_of_surjective (hf : Function.Surjective f) (I J : Ideal S) :
comap f I ≤ comap f J ↔ I ≤ J :=
⟨fun h => (map_comap_of_surjective f hf I).symm.le.trans (map_le_of_le_comap h), fun h =>
le_comap_of_map_le ((map_comap_of_surjective f hf I).le.trans h)⟩
end Surjective
section Bijective
/-- Special case of the correspondence theorem for isomorphic rings -/
def relIsoOfBijective (hf : Function.Bijective f) : Ideal S ≃o Ideal R where
toFun := comap f
invFun := map f
left_inv := (relIsoOfSurjective f hf.right).left_inv
right_inv J :=
Subtype.ext_iff.1
((relIsoOfSurjective f hf.right).right_inv ⟨J, comap_bot_le_of_injective f hf.left⟩)
map_rel_iff' {_ _} := (relIsoOfSurjective f hf.right).map_rel_iff'
theorem comap_le_iff_le_map (hf : Function.Bijective f) {I : Ideal R} {K : Ideal S} :
comap f K ≤ I ↔ K ≤ map f I :=
⟨fun h => le_map_of_comap_le_of_surjective f hf.right h, fun h =>
(relIsoOfBijective f hf).right_inv I ▸ comap_mono h⟩
theorem map.isMaximal (hf : Function.Bijective f) {I : Ideal R} (H : IsMaximal I) :
IsMaximal (map f I) := by
refine
or_iff_not_imp_left.1 (map_eq_top_or_isMaximal_of_surjective f hf.right H) fun h => H.1.1 ?_
calc
I = comap f (map f I) := ((relIsoOfBijective f hf).right_inv I).symm
_ = comap f ⊤ := by rw [h]
_ = ⊤ := by rw [comap_top]
end Bijective
theorem RingEquiv.bot_maximal_iff (e : R ≃+* S) :
(⊥ : Ideal R).IsMaximal ↔ (⊥ : Ideal S).IsMaximal :=
⟨fun h => map_bot (f := e.toRingHom) ▸ map.isMaximal e.toRingHom e.bijective h, fun h =>
map_bot (f := e.symm.toRingHom) ▸ map.isMaximal e.symm.toRingHom e.symm.bijective h⟩
end Ring
section CommRing
variable {F : Type*} [CommRing R] [CommRing S]
variable [FunLike F R S] [rc : RingHomClass F R S]
variable (f : F)
variable {I J : Ideal R} {K L : Ideal S}
variable (I J K L)
theorem map_mul : map f (I * J) = map f I * map f J :=
le_antisymm
(map_le_iff_le_comap.2 <|
mul_le.2 fun r hri s hsj =>
show (f (r * s)) ∈ map f I * map f J by
rw [_root_.map_mul]; exact mul_mem_mul (mem_map_of_mem f hri) (mem_map_of_mem f hsj))
(span_mul_span (↑f '' ↑I) (↑f '' ↑J) ▸ (span_le.2 <|
Set.iUnion₂_subset fun i ⟨r, hri, hfri⟩ =>
Set.iUnion₂_subset fun j ⟨s, hsj, hfsj⟩ =>
Set.singleton_subset_iff.2 <|
hfri ▸ hfsj ▸ by rw [← _root_.map_mul]; exact mem_map_of_mem f (mul_mem_mul hri hsj)))
/-- The pushforward `Ideal.map` as a monoid-with-zero homomorphism. -/
@[simps]
def mapHom : Ideal R →*₀ Ideal S where
toFun := map f
map_mul' I J := Ideal.map_mul f I J
map_one' := by simp only [one_eq_top]; exact Ideal.map_top f
map_zero' := Ideal.map_bot
protected theorem map_pow (n : ℕ) : map f (I ^ n) = map f I ^ n :=
map_pow (mapHom f) I n
theorem comap_radical : comap f (radical K) = radical (comap f K) := by
ext
simp [radical]
variable {K}
theorem IsRadical.comap (hK : K.IsRadical) : (comap f K).IsRadical := by
rw [← hK.radical, comap_radical]
apply radical_isRadical
variable {I J L}
theorem map_radical_le : map f (radical I) ≤ radical (map f I) :=
map_le_iff_le_comap.2 fun r ⟨n, hrni⟩ => ⟨n, map_pow f r n ▸ mem_map_of_mem f hrni⟩
theorem le_comap_mul : comap f K * comap f L ≤ comap f (K * L) :=
map_le_iff_le_comap.1 <|
(map_mul f (comap f K) (comap f L)).symm ▸
mul_mono (map_le_iff_le_comap.2 <| le_rfl) (map_le_iff_le_comap.2 <| le_rfl)
theorem le_comap_pow (n : ℕ) : K.comap f ^ n ≤ (K ^ n).comap f := by
induction' n with n n_ih
· rw [pow_zero, pow_zero, Ideal.one_eq_top, Ideal.one_eq_top]
exact rfl.le
· rw [pow_succ, pow_succ]
exact (Ideal.mul_mono_left n_ih).trans (Ideal.le_comap_mul f)
end CommRing
end MapAndComap
end Ideal
namespace RingHom
variable {R : Type u} {S : Type v} {T : Type w}
section Semiring
variable {F : Type*} {G : Type*} [Semiring R] [Semiring S] [Semiring T]
variable [FunLike F R S] [rcf : RingHomClass F R S] [FunLike G T S] [rcg : RingHomClass G T S]
variable (f : F) (g : G)
/-- Kernel of a ring homomorphism as an ideal of the domain. -/
def ker : Ideal R :=
Ideal.comap f ⊥
/-- An element is in the kernel if and only if it maps to zero. -/
theorem mem_ker {r} : r ∈ ker f ↔ f r = 0 := by rw [ker, Ideal.mem_comap, Submodule.mem_bot]
theorem ker_eq : (ker f : Set R) = Set.preimage f {0} :=
rfl
theorem ker_eq_comap_bot (f : F) : ker f = Ideal.comap f ⊥ :=
rfl
theorem comap_ker (f : S →+* R) (g : T →+* S) : f.ker.comap g = ker (f.comp g) := by
rw [RingHom.ker_eq_comap_bot, Ideal.comap_comap, RingHom.ker_eq_comap_bot]
/-- If the target is not the zero ring, then one is not in the kernel. -/
theorem not_one_mem_ker [Nontrivial S] (f : F) : (1 : R) ∉ ker f := by
rw [mem_ker, map_one]
exact one_ne_zero
theorem ker_ne_top [Nontrivial S] (f : F) : ker f ≠ ⊤ :=
(Ideal.ne_top_iff_one _).mpr <| not_one_mem_ker f
lemma _root_.Pi.ker_ringHom {ι : Type*} {R : ι → Type*} [∀ i, Semiring (R i)]
(φ : ∀ i, S →+* R i) : ker (Pi.ringHom φ) = ⨅ i, ker (φ i) := by
ext x
simp [mem_ker, Ideal.mem_iInf, Function.funext_iff]
@[simp]
theorem ker_rangeSRestrict (f : R →+* S) : ker f.rangeSRestrict = ker f :=
Ideal.ext fun _ ↦ Subtype.ext_iff
end Semiring
section Ring
variable {F : Type*} [Ring R] [Semiring S] [FunLike F R S] [rc : RingHomClass F R S] (f : F)
theorem injective_iff_ker_eq_bot : Function.Injective f ↔ ker f = ⊥ := by
rw [SetLike.ext'_iff, ker_eq, Set.ext_iff]
exact injective_iff_map_eq_zero' f
theorem ker_eq_bot_iff_eq_zero : ker f = ⊥ ↔ ∀ x, f x = 0 → x = 0 := by
rw [← injective_iff_map_eq_zero f, injective_iff_ker_eq_bot]
@[simp]
theorem ker_coe_equiv (f : R ≃+* S) : ker (f : R →+* S) = ⊥ := by
simpa only [← injective_iff_ker_eq_bot] using EquivLike.injective f
@[simp]
theorem ker_equiv {F' : Type*} [EquivLike F' R S] [RingEquivClass F' R S] (f : F') : ker f = ⊥ := by
simpa only [← injective_iff_ker_eq_bot] using EquivLike.injective f
end Ring
section RingRing
variable {F : Type*} [Ring R] [Ring S] [FunLike F R S] [rc : RingHomClass F R S] (f : F)
theorem sub_mem_ker_iff {x y} : x - y ∈ ker f ↔ f x = f y := by rw [mem_ker, map_sub, sub_eq_zero]
@[simp]
theorem ker_rangeRestrict (f : R →+* S) : ker f.rangeRestrict = ker f :=
Ideal.ext fun _ ↦ Subtype.ext_iff
end RingRing
/-- The kernel of a homomorphism to a domain is a prime ideal. -/
theorem ker_isPrime {F : Type*} [Ring R] [Ring S] [IsDomain S]
[FunLike F R S] [RingHomClass F R S] (f : F) :
(ker f).IsPrime :=
⟨by
rw [Ne, Ideal.eq_top_iff_one]
exact not_one_mem_ker f,
fun {x y} => by
simpa only [mem_ker, map_mul] using @eq_zero_or_eq_zero_of_mul_eq_zero S _ _ _ _ _⟩
/-- The kernel of a homomorphism to a field is a maximal ideal. -/
theorem ker_isMaximal_of_surjective {R K F : Type*} [Ring R] [Field K]
[FunLike F R K] [RingHomClass F R K] (f : F)
(hf : Function.Surjective f) : (ker f).IsMaximal := by
refine
Ideal.isMaximal_iff.mpr
⟨fun h1 => one_ne_zero' K <| map_one f ▸ (mem_ker f).mp h1, fun J x hJ hxf hxJ => ?_⟩
obtain ⟨y, hy⟩ := hf (f x)⁻¹
have H : 1 = y * x - (y * x - 1) := (sub_sub_cancel _ _).symm
rw [H]
refine J.sub_mem (J.mul_mem_left _ hxJ) (hJ ?_)
rw [mem_ker]
simp only [hy, map_sub, map_one, map_mul, inv_mul_cancel (mt (mem_ker f).mpr hxf), sub_self]
end RingHom
namespace Ideal
variable {R : Type*} {S : Type*} {F : Type*}
section Semiring
variable [Semiring R] [Semiring S] [FunLike F R S] [rc : RingHomClass F R S]
theorem map_eq_bot_iff_le_ker {I : Ideal R} (f : F) : I.map f = ⊥ ↔ I ≤ RingHom.ker f := by
rw [RingHom.ker, eq_bot_iff, map_le_iff_le_comap]
theorem ker_le_comap {K : Ideal S} (f : F) : RingHom.ker f ≤ comap f K := fun _ hx =>
mem_comap.2 (((RingHom.mem_ker f).1 hx).symm ▸ K.zero_mem)
theorem map_isPrime_of_equiv {F' : Type*} [EquivLike F' R S] [RingEquivClass F' R S]
(f : F') {I : Ideal R} [IsPrime I] : IsPrime (map f I) := by
have h : I.map f = I.map ((f : R ≃+* S) : R →+* S) := rfl
rw [h, map_comap_of_equiv I (f : R ≃+* S)]
exact Ideal.IsPrime.comap (RingEquiv.symm (f : R ≃+* S))
end Semiring
section Ring
variable [Ring R] [Ring S] [FunLike F R S] [rc : RingHomClass F R S]
theorem map_sInf {A : Set (Ideal R)} {f : F} (hf : Function.Surjective f) :
(∀ J ∈ A, RingHom.ker f ≤ J) → map f (sInf A) = sInf (map f '' A) := by
refine fun h => le_antisymm (le_sInf ?_) ?_
· intro j hj y hy
cases' (mem_map_iff_of_surjective f hf).1 hy with x hx
cases' (Set.mem_image _ _ _).mp hj with J hJ
rw [← hJ.right, ← hx.right]
exact mem_map_of_mem f (sInf_le_of_le hJ.left (le_of_eq rfl) hx.left)
· intro y hy
cases' hf y with x hx
refine hx ▸ mem_map_of_mem f ?_
have : ∀ I ∈ A, y ∈ map f I := by simpa using hy
rw [Submodule.mem_sInf]
intro J hJ
rcases (mem_map_iff_of_surjective f hf).1 (this J hJ) with ⟨x', hx', rfl⟩
have : x - x' ∈ J := by
apply h J hJ
rw [RingHom.mem_ker, map_sub, hx, sub_self]
simpa only [sub_add_cancel] using J.add_mem this hx'
theorem map_isPrime_of_surjective {f : F} (hf : Function.Surjective f) {I : Ideal R} [H : IsPrime I]
(hk : RingHom.ker f ≤ I) : IsPrime (map f I) := by
refine ⟨fun h => H.ne_top (eq_top_iff.2 ?_), fun {x y} => ?_⟩
· replace h := congr_arg (comap f) h
rw [comap_map_of_surjective _ hf, comap_top] at h
exact h ▸ sup_le (le_of_eq rfl) hk
· refine fun hxy => (hf x).recOn fun a ha => (hf y).recOn fun b hb => ?_
rw [← ha, ← hb, ← _root_.map_mul f, mem_map_iff_of_surjective _ hf] at hxy
rcases hxy with ⟨c, hc, hc'⟩
rw [← sub_eq_zero, ← map_sub] at hc'
have : a * b ∈ I := by
convert I.sub_mem hc (hk (hc' : c - a * b ∈ RingHom.ker f)) using 1
abel
exact
(H.mem_or_mem this).imp (fun h => ha ▸ mem_map_of_mem f h) fun h => hb ▸ mem_map_of_mem f h
theorem map_eq_bot_iff_of_injective {I : Ideal R} {f : F} (hf : Function.Injective f) :
I.map f = ⊥ ↔ I = ⊥ := by
rw [map_eq_bot_iff_le_ker, (RingHom.injective_iff_ker_eq_bot f).mp hf, le_bot_iff]
end Ring
section CommRing
variable [CommRing R] [CommRing S]
theorem map_eq_iff_sup_ker_eq_of_surjective {I J : Ideal R} (f : R →+* S)
(hf : Function.Surjective f) : map f I = map f J ↔ I ⊔ RingHom.ker f = J ⊔ RingHom.ker f := by
rw [← (comap_injective_of_surjective f hf).eq_iff, comap_map_of_surjective f hf,
comap_map_of_surjective f hf, RingHom.ker_eq_comap_bot]
theorem map_radical_of_surjective {f : R →+* S} (hf : Function.Surjective f) {I : Ideal R}
(h : RingHom.ker f ≤ I) : map f I.radical = (map f I).radical := by
rw [radical_eq_sInf, radical_eq_sInf]
have : ∀ J ∈ {J : Ideal R | I ≤ J ∧ J.IsPrime}, RingHom.ker f ≤ J := fun J hJ => h.trans hJ.left
convert map_sInf hf this
refine funext fun j => propext ⟨?_, ?_⟩
· rintro ⟨hj, hj'⟩
haveI : j.IsPrime := hj'
exact
⟨comap f j, ⟨⟨map_le_iff_le_comap.1 hj, comap_isPrime f j⟩, map_comap_of_surjective f hf j⟩⟩
· rintro ⟨J, ⟨hJ, hJ'⟩⟩
haveI : J.IsPrime := hJ.right
exact ⟨hJ' ▸ map_mono hJ.left, hJ' ▸ map_isPrime_of_surjective hf (le_trans h hJ.left)⟩
end CommRing
end Ideal
namespace RingHom
variable {A B C : Type*} [Ring A] [Ring B] [Ring C]
variable (f : A →+* B) (f_inv : B → A)
/-- Auxiliary definition used to define `liftOfRightInverse` -/
def liftOfRightInverseAux (hf : Function.RightInverse f_inv f) (g : A →+* C)
(hg : RingHom.ker f ≤ RingHom.ker g) :
B →+* C :=
{ AddMonoidHom.liftOfRightInverse f.toAddMonoidHom f_inv hf ⟨g.toAddMonoidHom, hg⟩ with
toFun := fun b => g (f_inv b)
map_one' := by
rw [← map_one g, ← sub_eq_zero, ← map_sub g, ← mem_ker g]
apply hg
rw [mem_ker f, map_sub f, sub_eq_zero, map_one f]
exact hf 1
map_mul' := by
intro x y
rw [← map_mul g, ← sub_eq_zero, ← map_sub g, ← mem_ker g]
apply hg
rw [mem_ker f, map_sub f, sub_eq_zero, map_mul f]
simp only [hf _] }
@[simp]
theorem liftOfRightInverseAux_comp_apply (hf : Function.RightInverse f_inv f) (g : A →+* C)
(hg : RingHom.ker f ≤ RingHom.ker g) (a : A) :
(f.liftOfRightInverseAux f_inv hf g hg) (f a) = g a :=
f.toAddMonoidHom.liftOfRightInverse_comp_apply f_inv hf ⟨g.toAddMonoidHom, hg⟩ a
/-- `liftOfRightInverse f hf g hg` is the unique ring homomorphism `φ`
* such that `φ.comp f = g` (`RingHom.liftOfRightInverse_comp`),
* where `f : A →+* B` has a right_inverse `f_inv` (`hf`),
* and `g : B →+* C` satisfies `hg : f.ker ≤ g.ker`.
See `RingHom.eq_liftOfRightInverse` for the uniqueness lemma.
```
A .
| \
f | \ g
| \
v \⌟
B ----> C
∃!φ
```
-/
def liftOfRightInverse (hf : Function.RightInverse f_inv f) :
{ g : A →+* C // RingHom.ker f ≤ RingHom.ker g } ≃ (B →+* C) where
toFun g := f.liftOfRightInverseAux f_inv hf g.1 g.2
invFun φ := ⟨φ.comp f, fun x hx => (mem_ker _).mpr <| by simp [(mem_ker _).mp hx]⟩
left_inv g := by
ext
simp only [comp_apply, liftOfRightInverseAux_comp_apply, Subtype.coe_mk]
right_inv φ := by
ext b
simp [liftOfRightInverseAux, hf b]
/-- A non-computable version of `RingHom.liftOfRightInverse` for when no computable right
inverse is available, that uses `Function.surjInv`. -/
@[simp]
noncomputable abbrev liftOfSurjective (hf : Function.Surjective f) :
{ g : A →+* C // RingHom.ker f ≤ RingHom.ker g } ≃ (B →+* C) :=
f.liftOfRightInverse (Function.surjInv hf) (Function.rightInverse_surjInv hf)
theorem liftOfRightInverse_comp_apply (hf : Function.RightInverse f_inv f)
(g : { g : A →+* C // RingHom.ker f ≤ RingHom.ker g }) (x : A) :
(f.liftOfRightInverse f_inv hf g) (f x) = g.1 x :=
f.liftOfRightInverseAux_comp_apply f_inv hf g.1 g.2 x
theorem liftOfRightInverse_comp (hf : Function.RightInverse f_inv f)
(g : { g : A →+* C // RingHom.ker f ≤ RingHom.ker g }) :
(f.liftOfRightInverse f_inv hf g).comp f = g :=
RingHom.ext <| f.liftOfRightInverse_comp_apply f_inv hf g
theorem eq_liftOfRightInverse (hf : Function.RightInverse f_inv f) (g : A →+* C)
(hg : RingHom.ker f ≤ RingHom.ker g) (h : B →+* C) (hh : h.comp f = g) :
h = f.liftOfRightInverse f_inv hf ⟨g, hg⟩ := by
simp_rw [← hh]
exact ((f.liftOfRightInverse f_inv hf).apply_symm_apply _).symm
end RingHom
namespace AlgHom
variable {R A B : Type*} [CommSemiring R] [Semiring A] [Semiring B]
[Algebra R A] [Algebra R B] (f : A →ₐ[R] B)
lemma coe_ker : RingHom.ker f = RingHom.ker (f : A →+* B) := rfl
lemma coe_ideal_map (I : Ideal A) :
Ideal.map f I = Ideal.map (f : A →+* B) I := rfl
end AlgHom
namespace Algebra
variable {R : Type*} [CommSemiring R] (S : Type*) [Semiring S] [Algebra R S]
/-- The induced linear map from `I` to the span of `I` in an `R`-algebra `S`. -/
@[simps!]
def idealMap (I : Ideal R) : I →ₗ[R] I.map (algebraMap R S) :=
(Algebra.linearMap R S).restrict (q := (I.map (algebraMap R S)).restrictScalars R)
(fun _ ↦ Ideal.mem_map_of_mem _)
end Algebra
|
RingTheory\Ideal\MinimalPrime.lean | /-
Copyright (c) 2022 Andrew Yang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Andrew Yang
-/
import Mathlib.RingTheory.Ideal.IsPrimary
import Mathlib.RingTheory.Localization.AtPrime
import Mathlib.Order.Minimal
/-!
# Minimal primes
We provide various results concerning the minimal primes above an ideal
## Main results
- `Ideal.minimalPrimes`: `I.minimalPrimes` is the set of ideals that are minimal primes over `I`.
- `minimalPrimes`: `minimalPrimes R` is the set of minimal primes of `R`.
- `Ideal.exists_minimalPrimes_le`: Every prime ideal over `I` contains a minimal prime over `I`.
- `Ideal.radical_minimalPrimes`: The minimal primes over `I.radical` are precisely
the minimal primes over `I`.
- `Ideal.sInf_minimalPrimes`: The intersection of minimal primes over `I` is `I.radical`.
- `Ideal.exists_minimalPrimes_comap_eq` If `p` is a minimal prime over `f ⁻¹ I`, then it is the
preimage of some minimal prime over `I`.
- `Ideal.minimalPrimes_eq_comap`: The minimal primes over `I` are precisely the preimages of
minimal primes of `R ⧸ I`.
- `Localization.AtPrime.prime_unique_of_minimal`: When localizing at a minimal prime ideal `I`,
the resulting ring only has a single prime ideal.
-/
section
variable {R S : Type*} [CommSemiring R] [CommSemiring S] (I J : Ideal R)
/-- `I.minimalPrimes` is the set of ideals that are minimal primes over `I`. -/
protected def Ideal.minimalPrimes : Set (Ideal R) :=
{p | Minimal (fun q ↦ q.IsPrime ∧ I ≤ q) p}
variable (R) in
/-- `minimalPrimes R` is the set of minimal primes of `R`.
This is defined as `Ideal.minimalPrimes ⊥`. -/
def minimalPrimes : Set (Ideal R) :=
Ideal.minimalPrimes ⊥
lemma minimalPrimes_eq_minimals : minimalPrimes R = {x | Minimal Ideal.IsPrime x} :=
congr_arg Minimal (by simp)
variable {I J}
theorem Ideal.exists_minimalPrimes_le [J.IsPrime] (e : I ≤ J) : ∃ p ∈ I.minimalPrimes, p ≤ J := by
suffices
∃ m ∈ { p : (Ideal R)ᵒᵈ | Ideal.IsPrime p ∧ I ≤ OrderDual.ofDual p },
OrderDual.toDual J ≤ m ∧ ∀ z ∈ { p : (Ideal R)ᵒᵈ | Ideal.IsPrime p ∧ I ≤ p }, m ≤ z → z = m by
obtain ⟨p, h₁, h₂, h₃⟩ := this
simp_rw [← @eq_comm _ p] at h₃
exact ⟨p, ⟨h₁, fun a b c => le_of_eq (h₃ a b c)⟩, h₂⟩
apply zorn_nonempty_partialOrder₀
swap
· refine ⟨show J.IsPrime by infer_instance, e⟩
rintro (c : Set (Ideal R)) hc hc' J' hJ'
refine
⟨OrderDual.toDual (sInf c),
⟨Ideal.sInf_isPrime_of_isChain ⟨J', hJ'⟩ hc'.symm fun x hx => (hc hx).1, ?_⟩, ?_⟩
· rw [OrderDual.ofDual_toDual, le_sInf_iff]
exact fun _ hx => (hc hx).2
· rintro z hz
rw [OrderDual.le_toDual]
exact sInf_le hz
@[simp]
theorem Ideal.radical_minimalPrimes : I.radical.minimalPrimes = I.minimalPrimes := by
rw [Ideal.minimalPrimes, Ideal.minimalPrimes]
ext p
refine ⟨?_, ?_⟩ <;> rintro ⟨⟨a, ha⟩, b⟩
· refine ⟨⟨a, a.radical_le_iff.1 ha⟩, ?_⟩
simp only [Set.mem_setOf_eq, and_imp] at *
exact fun _ h2 h3 h4 => b h2 (h2.radical_le_iff.2 h3) h4
· refine ⟨⟨a, a.radical_le_iff.2 ha⟩, ?_⟩
simp only [Set.mem_setOf_eq, and_imp] at *
exact fun _ h2 h3 h4 => b h2 (h2.radical_le_iff.1 h3) h4
@[simp]
theorem Ideal.sInf_minimalPrimes : sInf I.minimalPrimes = I.radical := by
rw [I.radical_eq_sInf]
apply le_antisymm
· intro x hx
rw [Ideal.mem_sInf] at hx ⊢
rintro J ⟨e, hJ⟩
obtain ⟨p, hp, hp'⟩ := Ideal.exists_minimalPrimes_le e
exact hp' (hx hp)
· apply sInf_le_sInf _
intro I hI
exact hI.1.symm
theorem Ideal.exists_comap_eq_of_mem_minimalPrimes_of_injective {f : R →+* S}
(hf : Function.Injective f) (p) (H : p ∈ minimalPrimes R) :
∃ p' : Ideal S, p'.IsPrime ∧ p'.comap f = p := by
have := H.1.1
have : Nontrivial (Localization (Submonoid.map f p.primeCompl)) := by
refine ⟨⟨1, 0, ?_⟩⟩
convert (IsLocalization.map_injective_of_injective p.primeCompl (Localization.AtPrime p)
(Localization <| p.primeCompl.map f) hf).ne one_ne_zero
· rw [map_one]
· rw [map_zero]
obtain ⟨M, hM⟩ := Ideal.exists_maximal (Localization (Submonoid.map f p.primeCompl))
refine ⟨M.comap (algebraMap S <| Localization (Submonoid.map f p.primeCompl)), inferInstance, ?_⟩
rw [Ideal.comap_comap, ← @IsLocalization.map_comp _ _ _ _ _ _ _ _ Localization.isLocalization
_ _ _ _ p.primeCompl.le_comap_map _ Localization.isLocalization,
← Ideal.comap_comap]
suffices _ ≤ p by exact this.antisymm (H.2 ⟨inferInstance, bot_le⟩ this)
intro x hx
by_contra h
apply hM.ne_top
apply M.eq_top_of_isUnit_mem hx
apply IsUnit.map
apply IsLocalization.map_units _ (show p.primeCompl from ⟨x, h⟩)
end
section
variable {R S : Type*} [CommRing R] [CommRing S] {I J : Ideal R}
theorem Ideal.exists_comap_eq_of_mem_minimalPrimes {I : Ideal S} (f : R →+* S) (p)
(H : p ∈ (I.comap f).minimalPrimes) : ∃ p' : Ideal S, p'.IsPrime ∧ I ≤ p' ∧ p'.comap f = p := by
have := H.1.1
let f' := (Ideal.Quotient.mk I).comp f
have e : RingHom.ker f' = I.comap f := by
ext1
exact Submodule.Quotient.mk_eq_zero _
have : RingHom.ker (Ideal.Quotient.mk <| RingHom.ker f') ≤ p := by
rw [Ideal.mk_ker, e]
exact H.1.2
suffices _ by
have ⟨p', hp₁, hp₂⟩ := Ideal.exists_comap_eq_of_mem_minimalPrimes_of_injective
(RingHom.kerLift_injective f') (p.map <| Ideal.Quotient.mk <| RingHom.ker f') this
refine ⟨p'.comap <| Ideal.Quotient.mk I, Ideal.IsPrime.comap _, ?_, ?_⟩
· exact Ideal.mk_ker.symm.trans_le (Ideal.comap_mono bot_le)
· convert congr_arg (Ideal.comap <| Ideal.Quotient.mk <| RingHom.ker f') hp₂
rwa [Ideal.comap_map_of_surjective (Ideal.Quotient.mk <| RingHom.ker f')
Ideal.Quotient.mk_surjective, eq_comm, sup_eq_left]
refine ⟨⟨?_, bot_le⟩, ?_⟩
· apply Ideal.map_isPrime_of_surjective _ this
exact Ideal.Quotient.mk_surjective
· rintro q ⟨hq, -⟩ hq'
rw [← Ideal.map_comap_of_surjective
(Ideal.Quotient.mk (RingHom.ker ((Ideal.Quotient.mk I).comp f)))
Ideal.Quotient.mk_surjective q]
apply Ideal.map_mono
apply H.2
· refine ⟨inferInstance, (Ideal.mk_ker.trans e).symm.trans_le (Ideal.comap_mono bot_le)⟩
· refine (Ideal.comap_mono hq').trans ?_
rw [Ideal.comap_map_of_surjective]
exacts [sup_le rfl.le this, Ideal.Quotient.mk_surjective]
theorem Ideal.exists_minimalPrimes_comap_eq {I : Ideal S} (f : R →+* S) (p)
(H : p ∈ (I.comap f).minimalPrimes) : ∃ p' ∈ I.minimalPrimes, Ideal.comap f p' = p := by
obtain ⟨p', h₁, h₂, h₃⟩ := Ideal.exists_comap_eq_of_mem_minimalPrimes f p H
obtain ⟨q, hq, hq'⟩ := Ideal.exists_minimalPrimes_le h₂
refine ⟨q, hq, Eq.symm ?_⟩
have := hq.1.1
have := (Ideal.comap_mono hq').trans_eq h₃
exact (H.2 ⟨inferInstance, Ideal.comap_mono hq.1.2⟩ this).antisymm this
theorem Ideal.minimal_primes_comap_of_surjective {f : R →+* S} (hf : Function.Surjective f)
{I J : Ideal S} (h : J ∈ I.minimalPrimes) : J.comap f ∈ (I.comap f).minimalPrimes := by
have := h.1.1
refine ⟨⟨inferInstance, Ideal.comap_mono h.1.2⟩, ?_⟩
rintro K ⟨hK, e₁⟩ e₂
have : RingHom.ker f ≤ K := (Ideal.comap_mono bot_le).trans e₁
rw [← sup_eq_left.mpr this, RingHom.ker_eq_comap_bot, ← Ideal.comap_map_of_surjective f hf]
apply Ideal.comap_mono _
apply h.2 _ _
· exact ⟨Ideal.map_isPrime_of_surjective hf this, Ideal.le_map_of_comap_le_of_surjective f hf e₁⟩
· exact Ideal.map_le_of_le_comap e₂
theorem Ideal.comap_minimalPrimes_eq_of_surjective {f : R →+* S} (hf : Function.Surjective f)
(I : Ideal S) : (I.comap f).minimalPrimes = Ideal.comap f '' I.minimalPrimes := by
ext J
constructor
· intro H
obtain ⟨p, h, rfl⟩ := Ideal.exists_minimalPrimes_comap_eq f J H
exact ⟨p, h, rfl⟩
· rintro ⟨J, hJ, rfl⟩
exact Ideal.minimal_primes_comap_of_surjective hf hJ
theorem Ideal.minimalPrimes_eq_comap :
I.minimalPrimes = Ideal.comap (Ideal.Quotient.mk I) '' minimalPrimes (R ⧸ I) := by
rw [minimalPrimes, ← Ideal.comap_minimalPrimes_eq_of_surjective Ideal.Quotient.mk_surjective,
← RingHom.ker_eq_comap_bot, Ideal.mk_ker]
theorem Ideal.minimalPrimes_eq_subsingleton (hI : I.IsPrimary) : I.minimalPrimes = {I.radical} := by
ext J
constructor
· exact fun H =>
let e := H.1.1.radical_le_iff.mpr H.1.2
(H.2 ⟨Ideal.isPrime_radical hI, Ideal.le_radical⟩ e).antisymm e
· rintro (rfl : J = I.radical)
exact ⟨⟨Ideal.isPrime_radical hI, Ideal.le_radical⟩, fun _ H _ => H.1.radical_le_iff.mpr H.2⟩
theorem Ideal.minimalPrimes_eq_subsingleton_self [I.IsPrime] : I.minimalPrimes = {I} := by
ext J
constructor
· exact fun H => (H.2 ⟨inferInstance, rfl.le⟩ H.1.2).antisymm H.1.2
· rintro (rfl : J = I)
exact ⟨⟨inferInstance, rfl.le⟩, fun _ h _ => h.2⟩
end
namespace Localization.AtPrime
variable {R : Type*} [CommSemiring R] {I : Ideal R} [hI : I.IsPrime] (hMin : I ∈ minimalPrimes R)
theorem _root_.IsLocalization.AtPrime.prime_unique_of_minimal {S} [CommSemiring S] [Algebra R S]
[IsLocalization.AtPrime S I] {J K : Ideal S} [J.IsPrime] [K.IsPrime] : J = K :=
haveI : Subsingleton {i : Ideal R // i.IsPrime ∧ i ≤ I} := ⟨fun i₁ i₂ ↦ Subtype.ext <| by
rw [minimalPrimes_eq_minimals, Set.mem_setOf] at hMin
rw [hMin.eq_of_le i₁.2.1 i₁.2.2, hMin.eq_of_le i₂.2.1 i₂.2.2]⟩
Subtype.ext_iff.mp <| (IsLocalization.AtPrime.orderIsoOfPrime S I).injective
(a₁ := ⟨J, ‹_›⟩) (a₂ := ⟨K, ‹_›⟩) (Subsingleton.elim _ _)
theorem prime_unique_of_minimal (J : Ideal (Localization I.primeCompl)) [J.IsPrime] :
J = LocalRing.maximalIdeal (Localization I.primeCompl) :=
IsLocalization.AtPrime.prime_unique_of_minimal hMin
theorem nilpotent_iff_mem_maximal_of_minimal {x : _} :
IsNilpotent x ↔ x ∈ LocalRing.maximalIdeal (Localization I.primeCompl) := by
rw [nilpotent_iff_mem_prime]
exact ⟨(· (LocalRing.maximalIdeal _) (Ideal.IsMaximal.isPrime' _)), fun _ J _ =>
by simpa [prime_unique_of_minimal hMin J]⟩
theorem nilpotent_iff_not_unit_of_minimal {x : Localization I.primeCompl} :
IsNilpotent x ↔ x ∈ nonunits _ := by
simpa only [← LocalRing.mem_maximalIdeal] using nilpotent_iff_mem_maximal_of_minimal hMin
end Localization.AtPrime
|
RingTheory\Ideal\Norm.lean | /-
Copyright (c) 2022 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen, Alex J. Best
-/
import Mathlib.Algebra.CharP.Quotient
import Mathlib.Algebra.GroupWithZero.NonZeroDivisors
import Mathlib.Data.Finsupp.Fintype
import Mathlib.Data.Int.AbsoluteValue
import Mathlib.Data.Int.Associated
import Mathlib.LinearAlgebra.FreeModule.Determinant
import Mathlib.LinearAlgebra.FreeModule.IdealQuotient
import Mathlib.RingTheory.DedekindDomain.PID
import Mathlib.RingTheory.Ideal.Basis
import Mathlib.RingTheory.LocalProperties
import Mathlib.RingTheory.Localization.NormTrace
/-!
# Ideal norms
This file defines the absolute ideal norm `Ideal.absNorm (I : Ideal R) : ℕ` as the cardinality of
the quotient `R ⧸ I` (setting it to 0 if the cardinality is infinite),
and the relative ideal norm `Ideal.spanNorm R (I : Ideal S) : Ideal S` as the ideal spanned by
the norms of elements in `I`.
## Main definitions
* `Submodule.cardQuot (S : Submodule R M)`: the cardinality of the quotient `M ⧸ S`, in `ℕ`.
This maps `⊥` to `0` and `⊤` to `1`.
* `Ideal.absNorm (I : Ideal R)`: the absolute ideal norm, defined as
the cardinality of the quotient `R ⧸ I`, as a bundled monoid-with-zero homomorphism.
* `Ideal.spanNorm R (I : Ideal S)`: the ideal spanned by the norms of elements in `I`.
This is used to define `Ideal.relNorm`.
* `Ideal.relNorm R (I : Ideal S)`: the relative ideal norm as a bundled monoid-with-zero morphism,
defined as the ideal spanned by the norms of elements in `I`.
## Main results
* `map_mul Ideal.absNorm`: multiplicativity of the ideal norm is bundled in
the definition of `Ideal.absNorm`
* `Ideal.natAbs_det_basis_change`: the ideal norm is given by the determinant
of the basis change matrix
* `Ideal.absNorm_span_singleton`: the ideal norm of a principal ideal is the
norm of its generator
* `map_mul Ideal.relNorm`: multiplicativity of the relative ideal norm
-/
open scoped nonZeroDivisors
section abs_norm
namespace Submodule
variable {R M : Type*} [Ring R] [AddCommGroup M] [Module R M]
section
/-- The cardinality of `(M ⧸ S)`, if `(M ⧸ S)` is finite, and `0` otherwise.
This is used to define the absolute ideal norm `Ideal.absNorm`.
-/
noncomputable def cardQuot (S : Submodule R M) : ℕ :=
AddSubgroup.index S.toAddSubgroup
theorem cardQuot_apply (S : Submodule R M) : cardQuot S = Nat.card (M ⧸ S) := by
rfl
variable (R M)
@[simp]
theorem cardQuot_bot [Infinite M] : cardQuot (⊥ : Submodule R M) = 0 :=
AddSubgroup.index_bot.trans Nat.card_eq_zero_of_infinite
@[simp]
theorem cardQuot_top : cardQuot (⊤ : Submodule R M) = 1 :=
AddSubgroup.index_top
variable {R M}
@[simp]
theorem cardQuot_eq_one_iff {P : Submodule R M} : cardQuot P = 1 ↔ P = ⊤ :=
AddSubgroup.index_eq_one.trans (by simp [SetLike.ext_iff])
end
end Submodule
section RingOfIntegers
variable {S : Type*} [CommRing S]
open Submodule
/-- Multiplicity of the ideal norm, for coprime ideals.
This is essentially just a repackaging of the Chinese Remainder Theorem.
-/
theorem cardQuot_mul_of_coprime
{I J : Ideal S} (coprime : IsCoprime I J) : cardQuot (I * J) = cardQuot I * cardQuot J := by
rw [cardQuot_apply, cardQuot_apply, cardQuot_apply,
Nat.card_congr (Ideal.quotientMulEquivQuotientProd I J coprime).toEquiv,
Nat.card_prod]
/-- If the `d` from `Ideal.exists_mul_add_mem_pow_succ` is unique, up to `P`,
then so are the `c`s, up to `P ^ (i + 1)`.
Inspired by [Neukirch], proposition 6.1 -/
theorem Ideal.mul_add_mem_pow_succ_inj (P : Ideal S) {i : ℕ} (a d d' e e' : S) (a_mem : a ∈ P ^ i)
(e_mem : e ∈ P ^ (i + 1)) (e'_mem : e' ∈ P ^ (i + 1)) (h : d - d' ∈ P) :
a * d + e - (a * d' + e') ∈ P ^ (i + 1) := by
have : a * d - a * d' ∈ P ^ (i + 1) := by
simp only [← mul_sub]
exact Ideal.mul_mem_mul a_mem h
convert Ideal.add_mem _ this (Ideal.sub_mem _ e_mem e'_mem) using 1
ring
section PPrime
variable {P : Ideal S} [P_prime : P.IsPrime]
/-- If `a ∈ P^i \ P^(i+1)` and `c ∈ P^i`, then `a * d + e = c` for `e ∈ P^(i+1)`.
`Ideal.mul_add_mem_pow_succ_unique` shows the choice of `d` is unique, up to `P`.
Inspired by [Neukirch], proposition 6.1 -/
theorem Ideal.exists_mul_add_mem_pow_succ [IsDedekindDomain S] (hP : P ≠ ⊥)
{i : ℕ} (a c : S) (a_mem : a ∈ P ^ i)
(a_not_mem : a ∉ P ^ (i + 1)) (c_mem : c ∈ P ^ i) :
∃ d : S, ∃ e ∈ P ^ (i + 1), a * d + e = c := by
suffices eq_b : P ^ i = Ideal.span {a} ⊔ P ^ (i + 1) by
rw [eq_b] at c_mem
simp only [mul_comm a]
exact Ideal.mem_span_singleton_sup.mp c_mem
refine (Ideal.eq_prime_pow_of_succ_lt_of_le hP (lt_of_le_of_ne le_sup_right ?_)
(sup_le (Ideal.span_le.mpr (Set.singleton_subset_iff.mpr a_mem))
(Ideal.pow_succ_lt_pow hP i).le)).symm
contrapose! a_not_mem with this
rw [this]
exact mem_sup.mpr ⟨a, mem_span_singleton_self a, 0, by simp, by simp⟩
theorem Ideal.mem_prime_of_mul_mem_pow [IsDedekindDomain S] {P : Ideal S} [P_prime : P.IsPrime]
(hP : P ≠ ⊥) {i : ℕ} {a b : S} (a_not_mem : a ∉ P ^ (i + 1)) (ab_mem : a * b ∈ P ^ (i + 1)) :
b ∈ P := by
simp only [← Ideal.span_singleton_le_iff_mem, ← Ideal.dvd_iff_le, pow_succ, ←
Ideal.span_singleton_mul_span_singleton] at a_not_mem ab_mem ⊢
exact (prime_pow_succ_dvd_mul (Ideal.prime_of_isPrime hP P_prime) ab_mem).resolve_left a_not_mem
/-- The choice of `d` in `Ideal.exists_mul_add_mem_pow_succ` is unique, up to `P`.
Inspired by [Neukirch], proposition 6.1 -/
theorem Ideal.mul_add_mem_pow_succ_unique [IsDedekindDomain S] (hP : P ≠ ⊥)
{i : ℕ} (a d d' e e' : S)
(a_not_mem : a ∉ P ^ (i + 1)) (e_mem : e ∈ P ^ (i + 1)) (e'_mem : e' ∈ P ^ (i + 1))
(h : a * d + e - (a * d' + e') ∈ P ^ (i + 1)) : d - d' ∈ P := by
have h' : a * (d - d') ∈ P ^ (i + 1) := by
convert Ideal.add_mem _ h (Ideal.sub_mem _ e'_mem e_mem) using 1
ring
exact Ideal.mem_prime_of_mul_mem_pow hP a_not_mem h'
/-- Multiplicity of the ideal norm, for powers of prime ideals. -/
theorem cardQuot_pow_of_prime [IsDedekindDomain S] (hP : P ≠ ⊥) {i : ℕ} :
cardQuot (P ^ i) = cardQuot P ^ i := by
induction' i with i ih
· simp
have : P ^ (i + 1) < P ^ i := Ideal.pow_succ_lt_pow hP i
suffices hquot : map (P ^ i.succ).mkQ (P ^ i) ≃ S ⧸ P by
rw [pow_succ' (cardQuot P), ← ih, cardQuot_apply (P ^ i.succ), ←
card_quotient_mul_card_quotient (P ^ i) (P ^ i.succ) this.le, cardQuot_apply (P ^ i),
cardQuot_apply P, Nat.card_congr hquot]
choose a a_mem a_not_mem using SetLike.exists_of_lt this
choose f g hg hf using fun c (hc : c ∈ P ^ i) =>
Ideal.exists_mul_add_mem_pow_succ hP a c a_mem a_not_mem hc
choose k hk_mem hk_eq using fun c' (hc' : c' ∈ map (mkQ (P ^ i.succ)) (P ^ i)) =>
Submodule.mem_map.mp hc'
refine Equiv.ofBijective (fun c' => Quotient.mk'' (f (k c' c'.prop) (hk_mem c' c'.prop))) ⟨?_, ?_⟩
· rintro ⟨c₁', hc₁'⟩ ⟨c₂', hc₂'⟩ h
rw [Subtype.mk_eq_mk, ← hk_eq _ hc₁', ← hk_eq _ hc₂', mkQ_apply, mkQ_apply,
Submodule.Quotient.eq, ← hf _ (hk_mem _ hc₁'), ← hf _ (hk_mem _ hc₂')]
refine Ideal.mul_add_mem_pow_succ_inj _ _ _ _ _ _ a_mem (hg _ _) (hg _ _) ?_
simpa only [Submodule.Quotient.mk''_eq_mk, Submodule.Quotient.mk''_eq_mk,
Submodule.Quotient.eq] using h
· intro d'
refine Quotient.inductionOn' d' fun d => ?_
have hd' := (mem_map (f := mkQ (P ^ i.succ))).mpr ⟨a * d, Ideal.mul_mem_right d _ a_mem, rfl⟩
refine ⟨⟨_, hd'⟩, ?_⟩
simp only [Submodule.Quotient.mk''_eq_mk, Ideal.Quotient.mk_eq_mk, Ideal.Quotient.eq,
Subtype.coe_mk]
refine
Ideal.mul_add_mem_pow_succ_unique hP a _ _ _ _ a_not_mem (hg _ (hk_mem _ hd')) (zero_mem _) ?_
rw [hf, add_zero]
exact (Submodule.Quotient.eq _).mp (hk_eq _ hd')
end PPrime
/-- Multiplicativity of the ideal norm in number rings. -/
theorem cardQuot_mul [IsDedekindDomain S] [Module.Free ℤ S] (I J : Ideal S) :
cardQuot (I * J) = cardQuot I * cardQuot J := by
let b := Module.Free.chooseBasis ℤ S
haveI : Infinite S := Infinite.of_surjective _ b.repr.toEquiv.surjective
exact UniqueFactorizationMonoid.multiplicative_of_coprime cardQuot I J (cardQuot_bot _ _)
(fun {I J} hI => by simp [Ideal.isUnit_iff.mp hI, Ideal.mul_top])
(fun {I} i hI =>
have : Ideal.IsPrime I := Ideal.isPrime_of_prime hI
cardQuot_pow_of_prime hI.ne_zero)
fun {I J} hIJ => cardQuot_mul_of_coprime <| Ideal.isCoprime_iff_sup_eq.mpr
(Ideal.isUnit_iff.mp
(hIJ (Ideal.dvd_iff_le.mpr le_sup_left) (Ideal.dvd_iff_le.mpr le_sup_right)))
/-- The absolute norm of the ideal `I : Ideal R` is the cardinality of the quotient `R ⧸ I`. -/
noncomputable def Ideal.absNorm [Nontrivial S] [IsDedekindDomain S] [Module.Free ℤ S] :
Ideal S →*₀ ℕ where
toFun := Submodule.cardQuot
map_mul' I J := by dsimp only; rw [cardQuot_mul]
map_one' := by dsimp only; rw [Ideal.one_eq_top, cardQuot_top]
map_zero' := by
have : Infinite S := Module.Free.infinite ℤ S
rw [Ideal.zero_eq_bot, cardQuot_bot]
namespace Ideal
variable [Nontrivial S] [IsDedekindDomain S] [Module.Free ℤ S]
theorem absNorm_apply (I : Ideal S) : absNorm I = cardQuot I := rfl
@[simp]
theorem absNorm_bot : absNorm (⊥ : Ideal S) = 0 := by rw [← Ideal.zero_eq_bot, _root_.map_zero]
@[simp]
theorem absNorm_top : absNorm (⊤ : Ideal S) = 1 := by rw [← Ideal.one_eq_top, _root_.map_one]
@[simp]
theorem absNorm_eq_one_iff {I : Ideal S} : absNorm I = 1 ↔ I = ⊤ := by
rw [absNorm_apply, cardQuot_eq_one_iff]
theorem absNorm_ne_zero_iff (I : Ideal S) : Ideal.absNorm I ≠ 0 ↔ Finite (S ⧸ I) :=
⟨fun h => Nat.finite_of_card_ne_zero h, fun h =>
(@AddSubgroup.finiteIndex_of_finite_quotient _ _ _ h).finiteIndex⟩
theorem absNorm_dvd_absNorm_of_le {I J : Ideal S} (h : J ≤ I) : Ideal.absNorm I ∣ Ideal.absNorm J :=
map_dvd absNorm (dvd_iff_le.mpr h)
theorem irreducible_of_irreducible_absNorm {I : Ideal S} (hI : Irreducible (Ideal.absNorm I)) :
Irreducible I :=
irreducible_iff.mpr
⟨fun h =>
hI.not_unit (by simpa only [Ideal.isUnit_iff, Nat.isUnit_iff, absNorm_eq_one_iff] using h),
by
rintro a b rfl
simpa only [Ideal.isUnit_iff, Nat.isUnit_iff, absNorm_eq_one_iff] using
hI.isUnit_or_isUnit (_root_.map_mul absNorm a b)⟩
theorem isPrime_of_irreducible_absNorm {I : Ideal S} (hI : Irreducible (Ideal.absNorm I)) :
I.IsPrime :=
isPrime_of_prime
(UniqueFactorizationMonoid.irreducible_iff_prime.mp (irreducible_of_irreducible_absNorm hI))
theorem prime_of_irreducible_absNorm_span {a : S} (ha : a ≠ 0)
(hI : Irreducible (Ideal.absNorm (Ideal.span ({a} : Set S)))) : Prime a :=
(Ideal.span_singleton_prime ha).mp (isPrime_of_irreducible_absNorm hI)
theorem absNorm_mem (I : Ideal S) : ↑(Ideal.absNorm I) ∈ I := by
rw [absNorm_apply, cardQuot, ← Ideal.Quotient.eq_zero_iff_mem, map_natCast,
Quotient.index_eq_zero]
theorem span_singleton_absNorm_le (I : Ideal S) : Ideal.span {(Ideal.absNorm I : S)} ≤ I := by
simp only [Ideal.span_le, Set.singleton_subset_iff, SetLike.mem_coe, Ideal.absNorm_mem I]
theorem span_singleton_absNorm {I : Ideal S} (hI : (Ideal.absNorm I).Prime) :
Ideal.span (singleton (Ideal.absNorm I : ℤ)) = I.comap (algebraMap ℤ S) := by
have : Ideal.IsPrime (Ideal.span (singleton (Ideal.absNorm I : ℤ))) := by
rwa [Ideal.span_singleton_prime (Int.ofNat_ne_zero.mpr hI.ne_zero), ← Nat.prime_iff_prime_int]
apply (this.isMaximal _).eq_of_le
· exact ((isPrime_of_irreducible_absNorm
((Nat.irreducible_iff_nat_prime _).mpr hI)).comap (algebraMap ℤ S)).ne_top
· rw [span_singleton_le_iff_mem, mem_comap, algebraMap_int_eq, map_natCast]
exact absNorm_mem I
· rw [Ne, span_singleton_eq_bot]
exact Int.ofNat_ne_zero.mpr hI.ne_zero
variable [Module.Finite ℤ S]
/-- Let `e : S ≃ I` be an additive isomorphism (therefore a `ℤ`-linear equiv).
Then an alternative way to compute the norm of `I` is given by taking the determinant of `e`.
See `natAbs_det_basis_change` for a more familiar formulation of this result. -/
theorem natAbs_det_equiv (I : Ideal S) {E : Type*} [EquivLike E S I] [AddEquivClass E S I] (e : E) :
Int.natAbs
(LinearMap.det
((Submodule.subtype I).restrictScalars ℤ ∘ₗ AddMonoidHom.toIntLinearMap (e : S →+ I))) =
Ideal.absNorm I := by
-- `S ⧸ I` might be infinite if `I = ⊥`, but then `e` can't be an equiv.
by_cases hI : I = ⊥
· subst hI
have : (1 : S) ≠ 0 := one_ne_zero
have : (1 : S) = 0 := EquivLike.injective e (Subsingleton.elim _ _)
contradiction
let ι := Module.Free.ChooseBasisIndex ℤ S
let b := Module.Free.chooseBasis ℤ S
cases isEmpty_or_nonempty ι
· nontriviality S
exact (not_nontrivial_iff_subsingleton.mpr
(Function.Surjective.subsingleton b.repr.toEquiv.symm.surjective) (by infer_instance)).elim
-- Thus `(S ⧸ I)` is isomorphic to a product of `ZMod`s, so it is a fintype.
letI := Ideal.fintypeQuotientOfFreeOfNeBot I hI
-- Use the Smith normal form to choose a nice basis for `I`.
letI := Classical.decEq ι
let a := I.smithCoeffs b hI
let b' := I.ringBasis b hI
let ab := I.selfBasis b hI
have ab_eq := I.selfBasis_def b hI
let e' : S ≃ₗ[ℤ] I := b'.equiv ab (Equiv.refl _)
let f : S →ₗ[ℤ] S := (I.subtype.restrictScalars ℤ).comp (e' : S →ₗ[ℤ] I)
let f_apply : ∀ x, f x = b'.equiv ab (Equiv.refl _) x := fun x => rfl
suffices (LinearMap.det f).natAbs = Ideal.absNorm I by
calc
_ = (LinearMap.det ((Submodule.subtype I).restrictScalars ℤ ∘ₗ
(AddEquiv.toIntLinearEquiv e : S ≃ₗ[ℤ] I))).natAbs := rfl
_ = (LinearMap.det ((Submodule.subtype I).restrictScalars ℤ ∘ₗ _)).natAbs :=
Int.natAbs_eq_iff_associated.mpr (LinearMap.associated_det_comp_equiv _ _ _)
_ = absNorm I := this
have ha : ∀ i, f (b' i) = a i • b' i := by
intro i; rw [f_apply, b'.equiv_apply, Equiv.refl_apply, ab_eq]
-- `det f` is equal to `∏ i, a i`,
letI := Classical.decEq ι
calc
Int.natAbs (LinearMap.det f) = Int.natAbs (LinearMap.toMatrix b' b' f).det := by
rw [LinearMap.det_toMatrix]
_ = Int.natAbs (Matrix.diagonal a).det := ?_
_ = Int.natAbs (∏ i, a i) := by rw [Matrix.det_diagonal]
_ = ∏ i, Int.natAbs (a i) := map_prod Int.natAbsHom a Finset.univ
_ = Nat.card (S ⧸ I) := ?_
_ = absNorm I := (Submodule.cardQuot_apply _).symm
-- since `LinearMap.toMatrix b' b' f` is the diagonal matrix with `a` along the diagonal.
· congr 2; ext i j
rw [LinearMap.toMatrix_apply, ha, LinearEquiv.map_smul, Basis.repr_self, Finsupp.smul_single,
smul_eq_mul, mul_one]
by_cases h : i = j
· rw [h, Matrix.diagonal_apply_eq, Finsupp.single_eq_same]
· rw [Matrix.diagonal_apply_ne _ h, Finsupp.single_eq_of_ne (Ne.symm h)]
-- Now we map everything through the linear equiv `S ≃ₗ (ι → ℤ)`,
-- which maps `(S ⧸ I)` to `Π i, ZMod (a i).nat_abs`.
haveI : ∀ i, NeZero (a i).natAbs := fun i =>
⟨Int.natAbs_ne_zero.mpr (Ideal.smithCoeffs_ne_zero b I hI i)⟩
simp_rw [Nat.card_congr (Ideal.quotientEquivPiZMod I b hI).toEquiv, Nat.card_pi,
Nat.card_zmod]
/-- Let `b` be a basis for `S` over `ℤ` and `bI` a basis for `I` over `ℤ` of the same dimension.
Then an alternative way to compute the norm of `I` is given by taking the determinant of `bI`
over `b`. -/
theorem natAbs_det_basis_change {ι : Type*} [Fintype ι] [DecidableEq ι] (b : Basis ι ℤ S)
(I : Ideal S) (bI : Basis ι ℤ I) : (b.det ((↑) ∘ bI)).natAbs = Ideal.absNorm I := by
let e := b.equiv bI (Equiv.refl _)
calc
(b.det ((Submodule.subtype I).restrictScalars ℤ ∘ bI)).natAbs =
(LinearMap.det ((Submodule.subtype I).restrictScalars ℤ ∘ₗ (e : S →ₗ[ℤ] I))).natAbs := by
rw [Basis.det_comp_basis]
_ = _ := natAbs_det_equiv I e
@[simp]
theorem absNorm_span_singleton (r : S) :
absNorm (span ({r} : Set S)) = (Algebra.norm ℤ r).natAbs := by
rw [Algebra.norm_apply]
by_cases hr : r = 0
· simp only [hr, Ideal.span_zero, Algebra.coe_lmul_eq_mul, eq_self_iff_true, Ideal.absNorm_bot,
LinearMap.det_zero'', Set.singleton_zero, _root_.map_zero, Int.natAbs_zero]
letI := Ideal.fintypeQuotientOfFreeOfNeBot (span {r}) (mt span_singleton_eq_bot.mp hr)
let b := Module.Free.chooseBasis ℤ S
rw [← natAbs_det_equiv _ (b.equiv (basisSpanSingleton b hr) (Equiv.refl _))]
congr
refine b.ext fun i => ?_
simp
theorem absNorm_dvd_norm_of_mem {I : Ideal S} {x : S} (h : x ∈ I) :
↑(Ideal.absNorm I) ∣ Algebra.norm ℤ x := by
rw [← Int.dvd_natAbs, ← absNorm_span_singleton x, Int.natCast_dvd_natCast]
exact absNorm_dvd_absNorm_of_le ((span_singleton_le_iff_mem _).mpr h)
@[simp]
theorem absNorm_span_insert (r : S) (s : Set S) :
absNorm (span (insert r s)) ∣ gcd (absNorm (span s)) (Algebra.norm ℤ r).natAbs :=
(dvd_gcd_iff _ _ _).mpr
⟨absNorm_dvd_absNorm_of_le (span_mono (Set.subset_insert _ _)),
_root_.trans
(absNorm_dvd_absNorm_of_le (span_mono (Set.singleton_subset_iff.mpr (Set.mem_insert _ _))))
(by rw [absNorm_span_singleton])⟩
theorem absNorm_eq_zero_iff {I : Ideal S} : Ideal.absNorm I = 0 ↔ I = ⊥ := by
constructor
· intro hI
rw [← le_bot_iff]
intros x hx
rw [mem_bot, ← Algebra.norm_eq_zero_iff (R := ℤ), ← Int.natAbs_eq_zero,
← Ideal.absNorm_span_singleton, ← zero_dvd_iff, ← hI]
apply Ideal.absNorm_dvd_absNorm_of_le
rwa [Ideal.span_singleton_le_iff_mem]
· rintro rfl
exact absNorm_bot
theorem absNorm_ne_zero_of_nonZeroDivisors (I : (Ideal S)⁰) : Ideal.absNorm (I : Ideal S) ≠ 0 :=
Ideal.absNorm_eq_zero_iff.not.mpr <| nonZeroDivisors.coe_ne_zero _
theorem finite_setOf_absNorm_eq [CharZero S] {n : ℕ} (hn : 0 < n) :
{I : Ideal S | Ideal.absNorm I = n}.Finite := by
let f := fun I : Ideal S => Ideal.map (Ideal.Quotient.mk (@Ideal.span S _ {↑n})) I
refine @Set.Finite.of_finite_image _ _ _ f ?_ ?_
· suffices Finite (S ⧸ @Ideal.span S _ {↑n}) by
let g := ((↑) : Ideal (S ⧸ @Ideal.span S _ {↑n}) → Set (S ⧸ @Ideal.span S _ {↑n}))
refine @Set.Finite.of_finite_image _ _ _ g ?_ SetLike.coe_injective.injOn
exact Set.Finite.subset (@Set.finite_univ _ (@Set.finite' _ this)) (Set.subset_univ _)
rw [← absNorm_ne_zero_iff, absNorm_span_singleton]
simpa only [Ne, Int.natAbs_eq_zero, Algebra.norm_eq_zero_iff, Nat.cast_eq_zero] using
ne_of_gt hn
· intro I hI J hJ h
rw [← comap_map_mk (span_singleton_absNorm_le I), ← hI.symm, ←
comap_map_mk (span_singleton_absNorm_le J), ← hJ.symm]
congr
theorem norm_dvd_iff {x : S} (hx : Prime (Algebra.norm ℤ x)) {y : ℤ} :
Algebra.norm ℤ x ∣ y ↔ x ∣ y := by
rw [← Ideal.mem_span_singleton (y := x), ← eq_intCast (algebraMap ℤ S), ← Ideal.mem_comap,
← Ideal.span_singleton_absNorm, Ideal.mem_span_singleton, Ideal.absNorm_span_singleton,
Int.natAbs_dvd]
rwa [Ideal.absNorm_span_singleton, ← Int.prime_iff_natAbs_prime]
end Ideal
end RingOfIntegers
end abs_norm
section SpanNorm
namespace Ideal
open Submodule
variable (R : Type*) [CommRing R] {S : Type*} [CommRing S] [Algebra R S]
/-- `Ideal.spanNorm R (I : Ideal S)` is the ideal generated by mapping `Algebra.norm R` over `I`.
See also `Ideal.relNorm`.
-/
def spanNorm (I : Ideal S) : Ideal R :=
Ideal.span (Algebra.norm R '' (I : Set S))
@[simp]
theorem spanNorm_bot [Nontrivial S] [Module.Free R S] [Module.Finite R S] :
spanNorm R (⊥ : Ideal S) = ⊥ := span_eq_bot.mpr fun x hx => by simpa using hx
variable {R}
@[simp]
theorem spanNorm_eq_bot_iff [IsDomain R] [IsDomain S] [Module.Free R S] [Module.Finite R S]
{I : Ideal S} : spanNorm R I = ⊥ ↔ I = ⊥ := by
simp only [spanNorm, Ideal.span_eq_bot, Set.mem_image, SetLike.mem_coe, forall_exists_index,
and_imp, forall_apply_eq_imp_iff₂,
Algebra.norm_eq_zero_iff_of_basis (Module.Free.chooseBasis R S), @eq_bot_iff _ _ _ I,
SetLike.le_def]
rfl
variable (R)
theorem norm_mem_spanNorm {I : Ideal S} (x : S) (hx : x ∈ I) : Algebra.norm R x ∈ I.spanNorm R :=
subset_span (Set.mem_image_of_mem _ hx)
@[simp]
theorem spanNorm_singleton {r : S} : spanNorm R (span ({r} : Set S)) = span {Algebra.norm R r} :=
le_antisymm
(span_le.mpr fun x hx =>
mem_span_singleton.mpr
(by
obtain ⟨x, hx', rfl⟩ := (Set.mem_image _ _ _).mp hx
exact map_dvd _ (mem_span_singleton.mp hx')))
((span_singleton_le_iff_mem _).mpr (norm_mem_spanNorm _ _ (mem_span_singleton_self _)))
@[simp]
theorem spanNorm_top : spanNorm R (⊤ : Ideal S) = ⊤ := by
-- Porting note: was
-- simp [← Ideal.span_singleton_one]
rw [← Ideal.span_singleton_one, spanNorm_singleton]
simp
theorem map_spanNorm (I : Ideal S) {T : Type*} [CommRing T] (f : R →+* T) :
map f (spanNorm R I) = span (f ∘ Algebra.norm R '' (I : Set S)) := by
rw [spanNorm, map_span, Set.image_image]
-- Porting note: `Function.comp` reducibility
rfl
@[mono]
theorem spanNorm_mono {I J : Ideal S} (h : I ≤ J) : spanNorm R I ≤ spanNorm R J :=
Ideal.span_mono (Set.monotone_image h)
theorem spanNorm_localization (I : Ideal S) [Module.Finite R S] [Module.Free R S] (M : Submonoid R)
{Rₘ : Type*} (Sₘ : Type*) [CommRing Rₘ] [Algebra R Rₘ] [CommRing Sₘ] [Algebra S Sₘ]
[Algebra Rₘ Sₘ] [Algebra R Sₘ] [IsScalarTower R Rₘ Sₘ] [IsScalarTower R S Sₘ]
[IsLocalization M Rₘ] [IsLocalization (Algebra.algebraMapSubmonoid S M) Sₘ] :
spanNorm Rₘ (I.map (algebraMap S Sₘ)) = (spanNorm R I).map (algebraMap R Rₘ) := by
cases subsingleton_or_nontrivial R
· haveI := IsLocalization.unique R Rₘ M
simp [eq_iff_true_of_subsingleton]
let b := Module.Free.chooseBasis R S
rw [map_spanNorm]
refine span_eq_span (Set.image_subset_iff.mpr ?_) (Set.image_subset_iff.mpr ?_)
· rintro a' ha'
simp only [Set.mem_preimage, submodule_span_eq, ← map_spanNorm, SetLike.mem_coe,
IsLocalization.mem_map_algebraMap_iff (Algebra.algebraMapSubmonoid S M) Sₘ,
IsLocalization.mem_map_algebraMap_iff M Rₘ, Prod.exists] at ha' ⊢
obtain ⟨⟨a, ha⟩, ⟨_, ⟨s, hs, rfl⟩⟩, has⟩ := ha'
refine ⟨⟨Algebra.norm R a, norm_mem_spanNorm _ _ ha⟩,
⟨s ^ Fintype.card (Module.Free.ChooseBasisIndex R S), pow_mem hs _⟩, ?_⟩
simp only [Submodule.coe_mk, Subtype.coe_mk, map_pow] at has ⊢
apply_fun Algebra.norm Rₘ at has
rwa [_root_.map_mul, ← IsScalarTower.algebraMap_apply, IsScalarTower.algebraMap_apply R Rₘ,
Algebra.norm_algebraMap_of_basis (b.localizationLocalization Rₘ M Sₘ),
Algebra.norm_localization R M a] at has
· intro a ha
rw [Set.mem_preimage, Function.comp_apply, ← Algebra.norm_localization (Sₘ := Sₘ) R M a]
exact subset_span (Set.mem_image_of_mem _ (mem_map_of_mem _ ha))
theorem spanNorm_mul_spanNorm_le (I J : Ideal S) :
spanNorm R I * spanNorm R J ≤ spanNorm R (I * J) := by
rw [spanNorm, spanNorm, spanNorm, Ideal.span_mul_span', ← Set.image_mul]
refine Ideal.span_mono (Set.monotone_image ?_)
rintro _ ⟨x, hxI, y, hyJ, rfl⟩
exact Ideal.mul_mem_mul hxI hyJ
/-- This condition `eq_bot_or_top` is equivalent to being a field.
However, `Ideal.spanNorm_mul_of_field` is harder to apply since we'd need to upgrade a `CommRing R`
instance to a `Field R` instance. -/
theorem spanNorm_mul_of_bot_or_top [IsDomain R] [IsDomain S] [Module.Free R S] [Module.Finite R S]
(eq_bot_or_top : ∀ I : Ideal R, I = ⊥ ∨ I = ⊤) (I J : Ideal S) :
spanNorm R (I * J) = spanNorm R I * spanNorm R J := by
refine le_antisymm ?_ (spanNorm_mul_spanNorm_le R _ _)
cases' eq_bot_or_top (spanNorm R I) with hI hI
· rw [hI, spanNorm_eq_bot_iff.mp hI, bot_mul, spanNorm_bot]
exact bot_le
rw [hI, Ideal.top_mul]
cases' eq_bot_or_top (spanNorm R J) with hJ hJ
· rw [hJ, spanNorm_eq_bot_iff.mp hJ, mul_bot, spanNorm_bot]
rw [hJ]
exact le_top
@[simp]
theorem spanNorm_mul_of_field {K : Type*} [Field K] [Algebra K S] [IsDomain S] [Module.Finite K S]
(I J : Ideal S) : spanNorm K (I * J) = spanNorm K I * spanNorm K J :=
spanNorm_mul_of_bot_or_top K eq_bot_or_top I J
variable [IsDomain R] [IsDomain S] [IsDedekindDomain R] [IsDedekindDomain S]
variable [Module.Finite R S] [Module.Free R S]
/-- Multiplicativity of `Ideal.spanNorm`. simp-normal form is `map_mul (Ideal.relNorm R)`. -/
theorem spanNorm_mul (I J : Ideal S) : spanNorm R (I * J) = spanNorm R I * spanNorm R J := by
nontriviality R
cases subsingleton_or_nontrivial S
· have : ∀ I : Ideal S, I = ⊤ := fun I => Subsingleton.elim I ⊤
simp [this I, this J, this (I * J)]
refine eq_of_localization_maximal ?_
intro P hP
by_cases hP0 : P = ⊥
· subst hP0
rw [spanNorm_mul_of_bot_or_top]
intro I
refine or_iff_not_imp_right.mpr fun hI => ?_
exact (hP.eq_of_le hI bot_le).symm
let P' := Algebra.algebraMapSubmonoid S P.primeCompl
letI : Algebra (Localization.AtPrime P) (Localization P') := localizationAlgebra P.primeCompl S
haveI : IsScalarTower R (Localization.AtPrime P) (Localization P') :=
IsScalarTower.of_algebraMap_eq (fun x =>
(IsLocalization.map_eq (T := P') (Q := Localization P') P.primeCompl.le_comap_map x).symm)
have h : P' ≤ S⁰ :=
map_le_nonZeroDivisors_of_injective _ (NoZeroSMulDivisors.algebraMap_injective _ _)
P.primeCompl_le_nonZeroDivisors
haveI : IsDomain (Localization P') := IsLocalization.isDomain_localization h
haveI : IsDedekindDomain (Localization P') := IsLocalization.isDedekindDomain S h _
letI := Classical.decEq (Ideal (Localization P'))
haveI : IsPrincipalIdealRing (Localization P') :=
IsDedekindDomain.isPrincipalIdealRing_localization_over_prime S P hP0
rw [Ideal.map_mul, ← spanNorm_localization R I P.primeCompl (Localization P'),
← spanNorm_localization R J P.primeCompl (Localization P'),
← spanNorm_localization R (I * J) P.primeCompl (Localization P'), Ideal.map_mul,
← (I.map _).span_singleton_generator, ← (J.map _).span_singleton_generator,
span_singleton_mul_span_singleton, spanNorm_singleton, spanNorm_singleton,
spanNorm_singleton, span_singleton_mul_span_singleton, _root_.map_mul]
/-- The relative norm `Ideal.relNorm R (I : Ideal S)`, where `R` and `S` are Dedekind domains,
and `S` is an extension of `R` that is finite and free as a module. -/
def relNorm : Ideal S →*₀ Ideal R where
toFun := spanNorm R
map_zero' := spanNorm_bot R
map_one' := by dsimp only; rw [one_eq_top, spanNorm_top R, one_eq_top]
map_mul' := spanNorm_mul R
theorem relNorm_apply (I : Ideal S) : relNorm R I = span (Algebra.norm R '' (I : Set S) : Set R) :=
rfl
@[simp]
theorem spanNorm_eq (I : Ideal S) : spanNorm R I = relNorm R I := rfl
@[simp]
theorem relNorm_bot : relNorm R (⊥ : Ideal S) = ⊥ := by
simpa only [zero_eq_bot] using _root_.map_zero (relNorm R : Ideal S →*₀ _)
@[simp]
theorem relNorm_top : relNorm R (⊤ : Ideal S) = ⊤ := by
simpa only [one_eq_top] using map_one (relNorm R : Ideal S →*₀ _)
variable {R}
@[simp]
theorem relNorm_eq_bot_iff {I : Ideal S} : relNorm R I = ⊥ ↔ I = ⊥ :=
spanNorm_eq_bot_iff
variable (R)
theorem norm_mem_relNorm (I : Ideal S) {x : S} (hx : x ∈ I) : Algebra.norm R x ∈ relNorm R I :=
norm_mem_spanNorm R x hx
@[simp]
theorem relNorm_singleton (r : S) : relNorm R (span ({r} : Set S)) = span {Algebra.norm R r} :=
spanNorm_singleton R
theorem map_relNorm (I : Ideal S) {T : Type*} [CommRing T] (f : R →+* T) :
map f (relNorm R I) = span (f ∘ Algebra.norm R '' (I : Set S)) :=
map_spanNorm R I f
@[mono]
theorem relNorm_mono {I J : Ideal S} (h : I ≤ J) : relNorm R I ≤ relNorm R J :=
spanNorm_mono R h
end Ideal
end SpanNorm
|
RingTheory\Ideal\Operations.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.Algebra.Algebra.Operations
import Mathlib.Data.Fintype.Lattice
import Mathlib.RingTheory.Coprime.Lemmas
/-!
# More operations on modules and ideals
-/
assert_not_exists Basis -- See `RingTheory.Ideal.Basis`
assert_not_exists Submodule.hasQuotient -- See `RingTheory.Ideal.QuotientOperations`
universe u v w x
open Pointwise
namespace Submodule
variable {R : Type u} {M : Type v} {M' F G : Type*}
section CommSemiring
variable [CommSemiring R] [AddCommMonoid M] [Module R M] [AddCommMonoid M'] [Module R M']
open Pointwise
instance hasSMul' : SMul (Ideal R) (Submodule R M) :=
⟨Submodule.map₂ (LinearMap.lsmul R M)⟩
/-- This duplicates the global `smul_eq_mul`, but doesn't have to unfold anywhere near as much to
apply. -/
protected theorem _root_.Ideal.smul_eq_mul (I J : Ideal R) : I • J = I * J :=
rfl
variable (R M) in
/-- `Module.annihilator R M` is the ideal of all elements `r : R` such that `r • M = 0`. -/
def _root_.Module.annihilator : Ideal R := LinearMap.ker (LinearMap.lsmul R M)
theorem _root_.Module.mem_annihilator {r} : r ∈ Module.annihilator R M ↔ ∀ m : M, r • m = 0 :=
⟨fun h ↦ (congr($h ·)), (LinearMap.ext ·)⟩
theorem _root_.LinearMap.annihilator_le_of_injective (f : M →ₗ[R] M') (hf : Function.Injective f) :
Module.annihilator R M' ≤ Module.annihilator R M := fun x h ↦ by
rw [Module.mem_annihilator] at h ⊢; exact fun m ↦ hf (by rw [map_smul, h, f.map_zero])
theorem _root_.LinearMap.annihilator_le_of_surjective (f : M →ₗ[R] M')
(hf : Function.Surjective f) : Module.annihilator R M ≤ Module.annihilator R M' := fun x h ↦ by
rw [Module.mem_annihilator] at h ⊢
intro m; obtain ⟨m, rfl⟩ := hf m
rw [← map_smul, h, f.map_zero]
theorem _root_.LinearEquiv.annihilator_eq (e : M ≃ₗ[R] M') :
Module.annihilator R M = Module.annihilator R M' :=
(e.annihilator_le_of_surjective e.surjective).antisymm (e.annihilator_le_of_injective e.injective)
/-- `N.annihilator` is the ideal of all elements `r : R` such that `r • N = 0`. -/
abbrev annihilator (N : Submodule R M) : Ideal R :=
Module.annihilator R N
theorem annihilator_top : (⊤ : Submodule R M).annihilator = Module.annihilator R M :=
topEquiv.annihilator_eq
variable {I J : Ideal R} {N P : Submodule R M}
theorem mem_annihilator {r} : r ∈ N.annihilator ↔ ∀ n ∈ N, r • n = (0 : M) := by
simp_rw [annihilator, Module.mem_annihilator, Subtype.forall, Subtype.ext_iff]; rfl
theorem mem_annihilator' {r} : r ∈ N.annihilator ↔ N ≤ comap (r • (LinearMap.id : M →ₗ[R] M)) ⊥ :=
mem_annihilator.trans ⟨fun H n hn => (mem_bot R).2 <| H n hn, fun H _ hn => (mem_bot R).1 <| H hn⟩
theorem mem_annihilator_span (s : Set M) (r : R) :
r ∈ (Submodule.span R s).annihilator ↔ ∀ n : s, r • (n : M) = 0 := by
rw [Submodule.mem_annihilator]
constructor
· intro h n
exact h _ (Submodule.subset_span n.prop)
· intro h n hn
refine Submodule.span_induction hn ?_ ?_ ?_ ?_
· intro x hx
exact h ⟨x, hx⟩
· exact smul_zero _
· intro x y hx hy
rw [smul_add, hx, hy, zero_add]
· intro a x hx
rw [smul_comm, hx, smul_zero]
theorem mem_annihilator_span_singleton (g : M) (r : R) :
r ∈ (Submodule.span R ({g} : Set M)).annihilator ↔ r • g = 0 := by simp [mem_annihilator_span]
theorem annihilator_bot : (⊥ : Submodule R M).annihilator = ⊤ :=
(Ideal.eq_top_iff_one _).2 <| mem_annihilator'.2 bot_le
theorem annihilator_eq_top_iff : N.annihilator = ⊤ ↔ N = ⊥ :=
⟨fun H =>
eq_bot_iff.2 fun (n : M) hn =>
(mem_bot R).2 <| one_smul R n ▸ mem_annihilator.1 ((Ideal.eq_top_iff_one _).1 H) n hn,
fun H => H.symm ▸ annihilator_bot⟩
theorem annihilator_mono (h : N ≤ P) : P.annihilator ≤ N.annihilator := fun _ hrp =>
mem_annihilator.2 fun n hn => mem_annihilator.1 hrp n <| h hn
theorem annihilator_iSup (ι : Sort w) (f : ι → Submodule R M) :
annihilator (⨆ i, f i) = ⨅ i, annihilator (f i) :=
le_antisymm (le_iInf fun _ => annihilator_mono <| le_iSup _ _) fun _ H =>
mem_annihilator'.2 <|
iSup_le fun i =>
have := (mem_iInf _).1 H i
mem_annihilator'.1 this
theorem smul_mem_smul {r} {n} (hr : r ∈ I) (hn : n ∈ N) : r • n ∈ I • N :=
apply_mem_map₂ _ hr hn
theorem smul_le {P : Submodule R M} : I • N ≤ P ↔ ∀ r ∈ I, ∀ n ∈ N, r • n ∈ P :=
map₂_le
@[simp, norm_cast]
lemma coe_set_smul : (I : Set R) • N = I • N :=
Submodule.set_smul_eq_of_le _ _ _
(fun _ _ hr hx => smul_mem_smul hr hx)
(smul_le.mpr fun _ hr _ hx => mem_set_smul_of_mem_mem hr hx)
@[elab_as_elim]
theorem smul_induction_on {p : M → Prop} {x} (H : x ∈ I • N) (smul : ∀ r ∈ I, ∀ n ∈ N, p (r • n))
(add : ∀ x y, p x → p y → p (x + y)) : p x := by
have H0 : p 0 := by simpa only [zero_smul] using smul 0 I.zero_mem 0 N.zero_mem
refine Submodule.iSup_induction (x := x) _ H ?_ H0 add
rintro ⟨i, hi⟩ m ⟨j, hj, hj'⟩
rw [← hj']
exact smul _ hi _ hj
/-- Dependent version of `Submodule.smul_induction_on`. -/
@[elab_as_elim]
theorem smul_induction_on' {x : M} (hx : x ∈ I • N) {p : ∀ x, x ∈ I • N → Prop}
(smul : ∀ (r : R) (hr : r ∈ I) (n : M) (hn : n ∈ N), p (r • n) (smul_mem_smul hr hn))
(add : ∀ x hx y hy, p x hx → p y hy → p (x + y) (Submodule.add_mem _ ‹_› ‹_›)) : p x hx := by
refine Exists.elim ?_ fun (h : x ∈ I • N) (H : p x h) => H
exact
smul_induction_on hx (fun a ha x hx => ⟨_, smul _ ha _ hx⟩) fun x y ⟨_, hx⟩ ⟨_, hy⟩ =>
⟨_, add _ _ _ _ hx hy⟩
theorem mem_smul_span_singleton {I : Ideal R} {m : M} {x : M} :
x ∈ I • span R ({m} : Set M) ↔ ∃ y ∈ I, y • m = x :=
⟨fun hx =>
smul_induction_on hx
(fun r hri n hnm =>
let ⟨s, hs⟩ := mem_span_singleton.1 hnm
⟨r * s, I.mul_mem_right _ hri, hs ▸ mul_smul r s m⟩)
fun m1 m2 ⟨y1, hyi1, hy1⟩ ⟨y2, hyi2, hy2⟩ =>
⟨y1 + y2, I.add_mem hyi1 hyi2, by rw [add_smul, hy1, hy2]⟩,
fun ⟨y, hyi, hy⟩ => hy ▸ smul_mem_smul hyi (subset_span <| Set.mem_singleton m)⟩
theorem smul_le_right : I • N ≤ N :=
smul_le.2 fun r _ _ => N.smul_mem r
theorem smul_mono (hij : I ≤ J) (hnp : N ≤ P) : I • N ≤ J • P :=
map₂_le_map₂ hij hnp
theorem smul_mono_left (h : I ≤ J) : I • N ≤ J • N :=
map₂_le_map₂_left h
instance : CovariantClass (Ideal R) (Submodule R M) HSMul.hSMul LE.le :=
⟨fun _ _ => map₂_le_map₂_right⟩
@[deprecated smul_mono_right (since := "2024-03-31")]
protected theorem smul_mono_right (h : N ≤ P) : I • N ≤ I • P :=
_root_.smul_mono_right I h
theorem map_le_smul_top (I : Ideal R) (f : R →ₗ[R] M) :
Submodule.map f I ≤ I • (⊤ : Submodule R M) := by
rintro _ ⟨y, hy, rfl⟩
rw [← mul_one y, ← smul_eq_mul, f.map_smul]
exact smul_mem_smul hy mem_top
@[simp]
theorem annihilator_smul (N : Submodule R M) : annihilator N • N = ⊥ :=
eq_bot_iff.2 (smul_le.2 fun _ => mem_annihilator.1)
@[simp]
theorem annihilator_mul (I : Ideal R) : annihilator I * I = ⊥ :=
annihilator_smul I
@[simp]
theorem mul_annihilator (I : Ideal R) : I * annihilator I = ⊥ := by rw [mul_comm, annihilator_mul]
variable (I J N P)
@[simp]
theorem smul_bot : I • (⊥ : Submodule R M) = ⊥ :=
map₂_bot_right _ _
@[simp]
theorem bot_smul : (⊥ : Ideal R) • N = ⊥ :=
map₂_bot_left _ _
@[simp]
theorem top_smul : (⊤ : Ideal R) • N = N :=
le_antisymm smul_le_right fun r hri => one_smul R r ▸ smul_mem_smul mem_top hri
theorem smul_sup : I • (N ⊔ P) = I • N ⊔ I • P :=
map₂_sup_right _ _ _ _
theorem sup_smul : (I ⊔ J) • N = I • N ⊔ J • N :=
map₂_sup_left _ _ _ _
protected theorem smul_assoc : (I • J) • N = I • J • N :=
le_antisymm
(smul_le.2 fun _ hrsij t htn =>
smul_induction_on hrsij
(fun r hr s hs =>
(@smul_eq_mul R _ r s).symm ▸ smul_smul r s t ▸ smul_mem_smul hr (smul_mem_smul hs htn))
fun x y => (add_smul x y t).symm ▸ Submodule.add_mem _)
(smul_le.2 fun r hr _ hsn =>
suffices J • N ≤ Submodule.comap (r • (LinearMap.id : M →ₗ[R] M)) ((I • J) • N) from this hsn
smul_le.2 fun s hs n hn =>
show r • s • n ∈ (I • J) • N from mul_smul r s n ▸ smul_mem_smul (smul_mem_smul hr hs) hn)
@[deprecated smul_inf_le (since := "2024-03-31")]
protected theorem smul_inf_le (M₁ M₂ : Submodule R M) :
I • (M₁ ⊓ M₂) ≤ I • M₁ ⊓ I • M₂ := smul_inf_le _ _ _
theorem smul_iSup {ι : Sort*} {I : Ideal R} {t : ι → Submodule R M} : I • iSup t = ⨆ i, I • t i :=
map₂_iSup_right _ _ _
@[deprecated smul_iInf_le (since := "2024-03-31")]
protected theorem smul_iInf_le {ι : Sort*} {I : Ideal R} {t : ι → Submodule R M} :
I • iInf t ≤ ⨅ i, I • t i :=
smul_iInf_le
variable (S : Set R) (T : Set M)
theorem span_smul_span : Ideal.span S • span R T = span R (⋃ (s ∈ S) (t ∈ T), {s • t}) :=
(map₂_span_span _ _ _ _).trans <| congr_arg _ <| Set.image2_eq_iUnion _ _ _
theorem ideal_span_singleton_smul (r : R) (N : Submodule R M) :
(Ideal.span {r} : Ideal R) • N = r • N := by
have : span R (⋃ (t : M) (_ : t ∈ N), {r • t}) = r • N := by
convert span_eq (r • N)
exact (Set.image_eq_iUnion _ (N : Set M)).symm
conv_lhs => rw [← span_eq N, span_smul_span]
simpa
theorem mem_of_span_top_of_smul_mem (M' : Submodule R M) (s : Set R) (hs : Ideal.span s = ⊤) (x : M)
(H : ∀ r : s, (r : R) • x ∈ M') : x ∈ M' := by
suffices (⊤ : Ideal R) • span R ({x} : Set M) ≤ M' by
rw [top_smul] at this
exact this (subset_span (Set.mem_singleton x))
rw [← hs, span_smul_span, span_le]
simpa using H
/-- Given `s`, a generating set of `R`, to check that an `x : M` falls in a
submodule `M'` of `x`, we only need to show that `r ^ n • x ∈ M'` for some `n` for each `r : s`. -/
theorem mem_of_span_eq_top_of_smul_pow_mem (M' : Submodule R M) (s : Set R) (hs : Ideal.span s = ⊤)
(x : M) (H : ∀ r : s, ∃ n : ℕ, ((r : R) ^ n : R) • x ∈ M') : x ∈ M' := by
obtain ⟨s', hs₁, hs₂⟩ := (Ideal.span_eq_top_iff_finite _).mp hs
replace H : ∀ r : s', ∃ n : ℕ, ((r : R) ^ n : R) • x ∈ M' := fun r => H ⟨_, hs₁ r.2⟩
choose n₁ n₂ using H
let N := s'.attach.sup n₁
have hs' := Ideal.span_pow_eq_top (s' : Set R) hs₂ N
apply M'.mem_of_span_top_of_smul_mem _ hs'
rintro ⟨_, r, hr, rfl⟩
convert M'.smul_mem (r ^ (N - n₁ ⟨r, hr⟩)) (n₂ ⟨r, hr⟩) using 1
simp only [Subtype.coe_mk, smul_smul, ← pow_add]
rw [tsub_add_cancel_of_le (Finset.le_sup (s'.mem_attach _) : n₁ ⟨r, hr⟩ ≤ N)]
variable {M' : Type w} [AddCommMonoid M'] [Module R M']
@[simp]
theorem map_smul'' (f : M →ₗ[R] M') : (I • N).map f = I • N.map f :=
le_antisymm
(map_le_iff_le_comap.2 <|
smul_le.2 fun r hr n hn =>
show f (r • n) ∈ I • N.map f from
(f.map_smul r n).symm ▸ smul_mem_smul hr (mem_map_of_mem hn)) <|
smul_le.2 fun r hr _ hn =>
let ⟨p, hp, hfp⟩ := mem_map.1 hn
hfp ▸ f.map_smul r p ▸ mem_map_of_mem (smul_mem_smul hr hp)
open Pointwise in
@[simp]
theorem map_pointwise_smul (r : R) (N : Submodule R M) (f : M →ₗ[R] M') :
(r • N).map f = r • N.map f := by
simp_rw [← ideal_span_singleton_smul, map_smul'']
variable {I}
theorem mem_smul_span {s : Set M} {x : M} :
x ∈ I • Submodule.span R s ↔ x ∈ Submodule.span R (⋃ (a ∈ I) (b ∈ s), ({a • b} : Set M)) := by
rw [← I.span_eq, Submodule.span_smul_span, I.span_eq]
rfl
variable (I)
/-- If `x` is an `I`-multiple of the submodule spanned by `f '' s`,
then we can write `x` as an `I`-linear combination of the elements of `f '' s`. -/
theorem mem_ideal_smul_span_iff_exists_sum {ι : Type*} (f : ι → M) (x : M) :
x ∈ I • span R (Set.range f) ↔
∃ (a : ι →₀ R) (_ : ∀ i, a i ∈ I), (a.sum fun i c => c • f i) = x := by
constructor; swap
· rintro ⟨a, ha, rfl⟩
exact Submodule.sum_mem _ fun c _ => smul_mem_smul (ha c) <| subset_span <| Set.mem_range_self _
refine fun hx => span_induction (mem_smul_span.mp hx) ?_ ?_ ?_ ?_
· simp only [Set.mem_iUnion, Set.mem_range, Set.mem_singleton_iff]
rintro x ⟨y, hy, x, ⟨i, rfl⟩, rfl⟩
refine ⟨Finsupp.single i y, fun j => ?_, ?_⟩
· letI := Classical.decEq ι
rw [Finsupp.single_apply]
split_ifs
· assumption
· exact I.zero_mem
refine @Finsupp.sum_single_index ι R M _ _ i _ (fun i y => y • f i) ?_
simp
· exact ⟨0, fun _ => I.zero_mem, Finsupp.sum_zero_index⟩
· rintro x y ⟨ax, hax, rfl⟩ ⟨ay, hay, rfl⟩
refine ⟨ax + ay, fun i => I.add_mem (hax i) (hay i), Finsupp.sum_add_index' ?_ ?_⟩ <;>
intros <;> simp only [zero_smul, add_smul]
· rintro c x ⟨a, ha, rfl⟩
refine ⟨c • a, fun i => I.mul_mem_left c (ha i), ?_⟩
rw [Finsupp.sum_smul_index, Finsupp.smul_sum] <;> intros <;> simp only [zero_smul, mul_smul]
theorem mem_ideal_smul_span_iff_exists_sum' {ι : Type*} (s : Set ι) (f : ι → M) (x : M) :
x ∈ I • span R (f '' s) ↔
∃ (a : s →₀ R) (_ : ∀ i, a i ∈ I), (a.sum fun i c => c • f i) = x := by
rw [← Submodule.mem_ideal_smul_span_iff_exists_sum, ← Set.image_eq_range]
theorem mem_smul_top_iff (N : Submodule R M) (x : N) :
x ∈ I • (⊤ : Submodule R N) ↔ (x : M) ∈ I • N := by
change _ ↔ N.subtype x ∈ I • N
have : Submodule.map N.subtype (I • ⊤) = I • N := by
rw [Submodule.map_smul'', Submodule.map_top, Submodule.range_subtype]
rw [← this]
exact (Function.Injective.mem_set_image N.injective_subtype).symm
@[simp]
theorem smul_comap_le_comap_smul (f : M →ₗ[R] M') (S : Submodule R M') (I : Ideal R) :
I • S.comap f ≤ (I • S).comap f := by
refine Submodule.smul_le.mpr fun r hr x hx => ?_
rw [Submodule.mem_comap] at hx ⊢
rw [f.map_smul]
exact Submodule.smul_mem_smul hr hx
end CommSemiring
end Submodule
namespace Ideal
section Add
variable {R : Type u} [Semiring R]
@[simp]
theorem add_eq_sup {I J : Ideal R} : I + J = I ⊔ J :=
rfl
-- dsimp loops when applying this lemma to its LHS,
-- probably https://github.com/leanprover/lean4/pull/2867
@[simp, nolint simpNF]
theorem zero_eq_bot : (0 : Ideal R) = ⊥ :=
rfl
@[simp]
theorem sum_eq_sup {ι : Type*} (s : Finset ι) (f : ι → Ideal R) : s.sum f = s.sup f :=
rfl
end Add
section MulAndRadical
variable {R : Type u} {ι : Type*} [CommSemiring R]
variable {I J K L : Ideal R}
instance : Mul (Ideal R) :=
⟨(· • ·)⟩
@[simp]
theorem one_eq_top : (1 : Ideal R) = ⊤ := by erw [Submodule.one_eq_range, LinearMap.range_id]
theorem add_eq_one_iff : I + J = 1 ↔ ∃ i ∈ I, ∃ j ∈ J, i + j = 1 := by
rw [one_eq_top, eq_top_iff_one, add_eq_sup, Submodule.mem_sup]
theorem mul_mem_mul {r s} (hr : r ∈ I) (hs : s ∈ J) : r * s ∈ I * J :=
Submodule.smul_mem_smul hr hs
theorem mul_mem_mul_rev {r s} (hr : r ∈ I) (hs : s ∈ J) : s * r ∈ I * J :=
mul_comm r s ▸ mul_mem_mul hr hs
theorem pow_mem_pow {x : R} (hx : x ∈ I) (n : ℕ) : x ^ n ∈ I ^ n :=
Submodule.pow_mem_pow _ hx _
theorem prod_mem_prod {ι : Type*} {s : Finset ι} {I : ι → Ideal R} {x : ι → R} :
(∀ i ∈ s, x i ∈ I i) → (∏ i ∈ s, x i) ∈ ∏ i ∈ s, I i := by
classical
refine Finset.induction_on s ?_ ?_
· intro
rw [Finset.prod_empty, Finset.prod_empty, one_eq_top]
exact Submodule.mem_top
· intro a s ha IH h
rw [Finset.prod_insert ha, Finset.prod_insert ha]
exact
mul_mem_mul (h a <| Finset.mem_insert_self a s)
(IH fun i hi => h i <| Finset.mem_insert_of_mem hi)
theorem mul_le : I * J ≤ K ↔ ∀ r ∈ I, ∀ s ∈ J, r * s ∈ K :=
Submodule.smul_le
theorem mul_le_left : I * J ≤ J :=
Ideal.mul_le.2 fun _ _ _ => J.mul_mem_left _
theorem mul_le_right : I * J ≤ I :=
Ideal.mul_le.2 fun _ hr _ _ => I.mul_mem_right _ hr
@[simp]
theorem sup_mul_right_self : I ⊔ I * J = I :=
sup_eq_left.2 Ideal.mul_le_right
@[simp]
theorem sup_mul_left_self : I ⊔ J * I = I :=
sup_eq_left.2 Ideal.mul_le_left
@[simp]
theorem mul_right_self_sup : I * J ⊔ I = I :=
sup_eq_right.2 Ideal.mul_le_right
@[simp]
theorem mul_left_self_sup : J * I ⊔ I = I :=
sup_eq_right.2 Ideal.mul_le_left
variable (I J K)
protected theorem mul_comm : I * J = J * I :=
le_antisymm (mul_le.2 fun _ hrI _ hsJ => mul_mem_mul_rev hsJ hrI)
(mul_le.2 fun _ hrJ _ hsI => mul_mem_mul_rev hsI hrJ)
protected theorem mul_assoc : I * J * K = I * (J * K) :=
Submodule.smul_assoc I J K
theorem span_mul_span (S T : Set R) : span S * span T = span (⋃ (s ∈ S) (t ∈ T), {s * t}) :=
Submodule.span_smul_span S T
variable {I J K}
theorem span_mul_span' (S T : Set R) : span S * span T = span (S * T) := by
unfold span
rw [Submodule.span_mul_span]
theorem span_singleton_mul_span_singleton (r s : R) :
span {r} * span {s} = (span {r * s} : Ideal R) := by
unfold span
rw [Submodule.span_mul_span, Set.singleton_mul_singleton]
theorem span_singleton_pow (s : R) (n : ℕ) : span {s} ^ n = (span {s ^ n} : Ideal R) := by
induction' n with n ih; · simp [Set.singleton_one]
simp only [pow_succ, ih, span_singleton_mul_span_singleton]
theorem mem_mul_span_singleton {x y : R} {I : Ideal R} : x ∈ I * span {y} ↔ ∃ z ∈ I, z * y = x :=
Submodule.mem_smul_span_singleton
theorem mem_span_singleton_mul {x y : R} {I : Ideal R} : x ∈ span {y} * I ↔ ∃ z ∈ I, y * z = x := by
simp only [mul_comm, mem_mul_span_singleton]
theorem le_span_singleton_mul_iff {x : R} {I J : Ideal R} :
I ≤ span {x} * J ↔ ∀ zI ∈ I, ∃ zJ ∈ J, x * zJ = zI :=
show (∀ {zI} (_ : zI ∈ I), zI ∈ span {x} * J) ↔ ∀ zI ∈ I, ∃ zJ ∈ J, x * zJ = zI by
simp only [mem_span_singleton_mul]
theorem span_singleton_mul_le_iff {x : R} {I J : Ideal R} :
span {x} * I ≤ J ↔ ∀ z ∈ I, x * z ∈ J := by
simp only [mul_le, mem_span_singleton_mul, mem_span_singleton]
constructor
· intro h zI hzI
exact h x (dvd_refl x) zI hzI
· rintro h _ ⟨z, rfl⟩ zI hzI
rw [mul_comm x z, mul_assoc]
exact J.mul_mem_left _ (h zI hzI)
theorem span_singleton_mul_le_span_singleton_mul {x y : R} {I J : Ideal R} :
span {x} * I ≤ span {y} * J ↔ ∀ zI ∈ I, ∃ zJ ∈ J, x * zI = y * zJ := by
simp only [span_singleton_mul_le_iff, mem_span_singleton_mul, eq_comm]
theorem span_singleton_mul_right_mono [IsDomain R] {x : R} (hx : x ≠ 0) :
span {x} * I ≤ span {x} * J ↔ I ≤ J := by
simp_rw [span_singleton_mul_le_span_singleton_mul, mul_right_inj' hx,
exists_eq_right', SetLike.le_def]
theorem span_singleton_mul_left_mono [IsDomain R] {x : R} (hx : x ≠ 0) :
I * span {x} ≤ J * span {x} ↔ I ≤ J := by
simpa only [mul_comm I, mul_comm J] using span_singleton_mul_right_mono hx
theorem span_singleton_mul_right_inj [IsDomain R] {x : R} (hx : x ≠ 0) :
span {x} * I = span {x} * J ↔ I = J := by
simp only [le_antisymm_iff, span_singleton_mul_right_mono hx]
theorem span_singleton_mul_left_inj [IsDomain R] {x : R} (hx : x ≠ 0) :
I * span {x} = J * span {x} ↔ I = J := by
simp only [le_antisymm_iff, span_singleton_mul_left_mono hx]
theorem span_singleton_mul_right_injective [IsDomain R] {x : R} (hx : x ≠ 0) :
Function.Injective ((span {x} : Ideal R) * ·) := fun _ _ =>
(span_singleton_mul_right_inj hx).mp
theorem span_singleton_mul_left_injective [IsDomain R] {x : R} (hx : x ≠ 0) :
Function.Injective fun I : Ideal R => I * span {x} := fun _ _ =>
(span_singleton_mul_left_inj hx).mp
theorem eq_span_singleton_mul {x : R} (I J : Ideal R) :
I = span {x} * J ↔ (∀ zI ∈ I, ∃ zJ ∈ J, x * zJ = zI) ∧ ∀ z ∈ J, x * z ∈ I := by
simp only [le_antisymm_iff, le_span_singleton_mul_iff, span_singleton_mul_le_iff]
theorem span_singleton_mul_eq_span_singleton_mul {x y : R} (I J : Ideal R) :
span {x} * I = span {y} * J ↔
(∀ zI ∈ I, ∃ zJ ∈ J, x * zI = y * zJ) ∧ ∀ zJ ∈ J, ∃ zI ∈ I, x * zI = y * zJ := by
simp only [le_antisymm_iff, span_singleton_mul_le_span_singleton_mul, eq_comm]
theorem prod_span {ι : Type*} (s : Finset ι) (I : ι → Set R) :
(∏ i ∈ s, Ideal.span (I i)) = Ideal.span (∏ i ∈ s, I i) :=
Submodule.prod_span s I
theorem prod_span_singleton {ι : Type*} (s : Finset ι) (I : ι → R) :
(∏ i ∈ s, Ideal.span ({I i} : Set R)) = Ideal.span {∏ i ∈ s, I i} :=
Submodule.prod_span_singleton s I
@[simp]
theorem multiset_prod_span_singleton (m : Multiset R) :
(m.map fun x => Ideal.span {x}).prod = Ideal.span ({Multiset.prod m} : Set R) :=
Multiset.induction_on m (by simp) fun a m ih => by
simp only [Multiset.map_cons, Multiset.prod_cons, ih, ← Ideal.span_singleton_mul_span_singleton]
theorem finset_inf_span_singleton {ι : Type*} (s : Finset ι) (I : ι → R)
(hI : Set.Pairwise (↑s) (IsCoprime on I)) :
(s.inf fun i => Ideal.span ({I i} : Set R)) = Ideal.span {∏ i ∈ s, I i} := by
ext x
simp only [Submodule.mem_finset_inf, Ideal.mem_span_singleton]
exact ⟨Finset.prod_dvd_of_coprime hI, fun h i hi => (Finset.dvd_prod_of_mem _ hi).trans h⟩
theorem iInf_span_singleton {ι : Type*} [Fintype ι] {I : ι → R}
(hI : ∀ (i j) (_ : i ≠ j), IsCoprime (I i) (I j)) :
⨅ i, span ({I i} : Set R) = span {∏ i, I i} := by
rw [← Finset.inf_univ_eq_iInf, finset_inf_span_singleton]
rwa [Finset.coe_univ, Set.pairwise_univ]
theorem iInf_span_singleton_natCast {R : Type*} [CommRing R] {ι : Type*} [Fintype ι]
{I : ι → ℕ} (hI : Pairwise fun i j => (I i).Coprime (I j)) :
⨅ (i : ι), span {(I i : R)} = span {((∏ i : ι, I i : ℕ) : R)} := by
rw [iInf_span_singleton, Nat.cast_prod]
exact fun i j h ↦ (hI h).cast
theorem sup_eq_top_iff_isCoprime {R : Type*} [CommSemiring R] (x y : R) :
span ({x} : Set R) ⊔ span {y} = ⊤ ↔ IsCoprime x y := by
rw [eq_top_iff_one, Submodule.mem_sup]
constructor
· rintro ⟨u, hu, v, hv, h1⟩
rw [mem_span_singleton'] at hu hv
rw [← hu.choose_spec, ← hv.choose_spec] at h1
exact ⟨_, _, h1⟩
· exact fun ⟨u, v, h1⟩ =>
⟨_, mem_span_singleton'.mpr ⟨_, rfl⟩, _, mem_span_singleton'.mpr ⟨_, rfl⟩, h1⟩
theorem mul_le_inf : I * J ≤ I ⊓ J :=
mul_le.2 fun r hri s hsj => ⟨I.mul_mem_right s hri, J.mul_mem_left r hsj⟩
theorem multiset_prod_le_inf {s : Multiset (Ideal R)} : s.prod ≤ s.inf := by
classical
refine s.induction_on ?_ ?_
· rw [Multiset.inf_zero]
exact le_top
intro a s ih
rw [Multiset.prod_cons, Multiset.inf_cons]
exact le_trans mul_le_inf (inf_le_inf le_rfl ih)
theorem prod_le_inf {s : Finset ι} {f : ι → Ideal R} : s.prod f ≤ s.inf f :=
multiset_prod_le_inf
theorem mul_eq_inf_of_coprime (h : I ⊔ J = ⊤) : I * J = I ⊓ J :=
le_antisymm mul_le_inf fun r ⟨hri, hrj⟩ =>
let ⟨s, hsi, t, htj, hst⟩ := Submodule.mem_sup.1 ((eq_top_iff_one _).1 h)
mul_one r ▸
hst ▸
(mul_add r s t).symm ▸ Ideal.add_mem (I * J) (mul_mem_mul_rev hsi hrj) (mul_mem_mul hri htj)
theorem sup_mul_eq_of_coprime_left (h : I ⊔ J = ⊤) : I ⊔ J * K = I ⊔ K :=
le_antisymm (sup_le_sup_left mul_le_left _) fun i hi => by
rw [eq_top_iff_one] at h; rw [Submodule.mem_sup] at h hi ⊢
obtain ⟨i1, hi1, j, hj, h⟩ := h; obtain ⟨i', hi', k, hk, hi⟩ := hi
refine ⟨_, add_mem hi' (mul_mem_right k _ hi1), _, mul_mem_mul hj hk, ?_⟩
rw [add_assoc, ← add_mul, h, one_mul, hi]
theorem sup_mul_eq_of_coprime_right (h : I ⊔ K = ⊤) : I ⊔ J * K = I ⊔ J := by
rw [mul_comm]
exact sup_mul_eq_of_coprime_left h
theorem mul_sup_eq_of_coprime_left (h : I ⊔ J = ⊤) : I * K ⊔ J = K ⊔ J := by
rw [sup_comm] at h
rw [sup_comm, sup_mul_eq_of_coprime_left h, sup_comm]
theorem mul_sup_eq_of_coprime_right (h : K ⊔ J = ⊤) : I * K ⊔ J = I ⊔ J := by
rw [sup_comm] at h
rw [sup_comm, sup_mul_eq_of_coprime_right h, sup_comm]
theorem sup_prod_eq_top {s : Finset ι} {J : ι → Ideal R} (h : ∀ i, i ∈ s → I ⊔ J i = ⊤) :
(I ⊔ ∏ i ∈ s, J i) = ⊤ :=
Finset.prod_induction _ (fun J => I ⊔ J = ⊤)
(fun J K hJ hK => (sup_mul_eq_of_coprime_left hJ).trans hK)
(by simp_rw [one_eq_top, sup_top_eq]) h
theorem sup_iInf_eq_top {s : Finset ι} {J : ι → Ideal R} (h : ∀ i, i ∈ s → I ⊔ J i = ⊤) :
(I ⊔ ⨅ i ∈ s, J i) = ⊤ :=
eq_top_iff.mpr <|
le_of_eq_of_le (sup_prod_eq_top h).symm <|
sup_le_sup_left (le_of_le_of_eq prod_le_inf <| Finset.inf_eq_iInf _ _) _
theorem prod_sup_eq_top {s : Finset ι} {J : ι → Ideal R} (h : ∀ i, i ∈ s → J i ⊔ I = ⊤) :
(∏ i ∈ s, J i) ⊔ I = ⊤ := by rw [sup_comm, sup_prod_eq_top]; intro i hi; rw [sup_comm, h i hi]
theorem iInf_sup_eq_top {s : Finset ι} {J : ι → Ideal R} (h : ∀ i, i ∈ s → J i ⊔ I = ⊤) :
(⨅ i ∈ s, J i) ⊔ I = ⊤ := by rw [sup_comm, sup_iInf_eq_top]; intro i hi; rw [sup_comm, h i hi]
theorem sup_pow_eq_top {n : ℕ} (h : I ⊔ J = ⊤) : I ⊔ J ^ n = ⊤ := by
rw [← Finset.card_range n, ← Finset.prod_const]
exact sup_prod_eq_top fun _ _ => h
theorem pow_sup_eq_top {n : ℕ} (h : I ⊔ J = ⊤) : I ^ n ⊔ J = ⊤ := by
rw [← Finset.card_range n, ← Finset.prod_const]
exact prod_sup_eq_top fun _ _ => h
theorem pow_sup_pow_eq_top {m n : ℕ} (h : I ⊔ J = ⊤) : I ^ m ⊔ J ^ n = ⊤ :=
sup_pow_eq_top (pow_sup_eq_top h)
variable (I)
-- @[simp] -- Porting note (#10618): simp can prove this
theorem mul_bot : I * ⊥ = ⊥ := by simp
-- @[simp] -- Porting note (#10618): simp can prove thisrove this
theorem bot_mul : ⊥ * I = ⊥ := by simp
@[simp]
theorem mul_top : I * ⊤ = I :=
Ideal.mul_comm ⊤ I ▸ Submodule.top_smul I
@[simp]
theorem top_mul : ⊤ * I = I :=
Submodule.top_smul I
variable {I}
theorem mul_mono (hik : I ≤ K) (hjl : J ≤ L) : I * J ≤ K * L :=
Submodule.smul_mono hik hjl
theorem mul_mono_left (h : I ≤ J) : I * K ≤ J * K :=
Submodule.smul_mono_left h
theorem mul_mono_right (h : J ≤ K) : I * J ≤ I * K :=
smul_mono_right _ h
variable (I J K)
theorem mul_sup : I * (J ⊔ K) = I * J ⊔ I * K :=
Submodule.smul_sup I J K
theorem sup_mul : (I ⊔ J) * K = I * K ⊔ J * K :=
Submodule.sup_smul I J K
variable {I J K}
theorem pow_le_pow_right {m n : ℕ} (h : m ≤ n) : I ^ n ≤ I ^ m := by
cases' Nat.exists_eq_add_of_le h with k hk
rw [hk, pow_add]
exact le_trans mul_le_inf inf_le_left
theorem pow_le_self {n : ℕ} (hn : n ≠ 0) : I ^ n ≤ I :=
calc
I ^ n ≤ I ^ 1 := pow_le_pow_right (Nat.pos_of_ne_zero hn)
_ = I := pow_one _
theorem pow_right_mono {I J : Ideal R} (e : I ≤ J) (n : ℕ) : I ^ n ≤ J ^ n := by
induction' n with _ hn
· rw [pow_zero, pow_zero]
· rw [pow_succ, pow_succ]
exact Ideal.mul_mono hn e
@[simp]
theorem mul_eq_bot {R : Type*} [CommSemiring R] [NoZeroDivisors R] {I J : Ideal R} :
I * J = ⊥ ↔ I = ⊥ ∨ J = ⊥ :=
⟨fun hij =>
or_iff_not_imp_left.mpr fun I_ne_bot =>
J.eq_bot_iff.mpr fun j hj =>
let ⟨i, hi, ne0⟩ := I.ne_bot_iff.mp I_ne_bot
Or.resolve_left (mul_eq_zero.mp ((I * J).eq_bot_iff.mp hij _ (mul_mem_mul hi hj))) ne0,
fun h => by cases' h with h h <;> rw [← Ideal.mul_bot, h, Ideal.mul_comm]⟩
instance {R : Type*} [CommSemiring R] [NoZeroDivisors R] : NoZeroDivisors (Ideal R) where
eq_zero_or_eq_zero_of_mul_eq_zero := mul_eq_bot.1
instance {R : Type*} [CommSemiring R] {S : Type*} [CommRing S] [Algebra R S]
[NoZeroSMulDivisors R S] {I : Ideal S} : NoZeroSMulDivisors R I :=
Submodule.noZeroSMulDivisors (Submodule.restrictScalars R I)
/-- A product of ideals in an integral domain is zero if and only if one of the terms is zero. -/
@[simp]
lemma multiset_prod_eq_bot {R : Type*} [CommRing R] [IsDomain R] {s : Multiset (Ideal R)} :
s.prod = ⊥ ↔ ⊥ ∈ s :=
Multiset.prod_eq_zero_iff
/-- A product of ideals in an integral domain is zero if and only if one of the terms is zero. -/
@[deprecated multiset_prod_eq_bot (since := "2023-12-26")]
theorem prod_eq_bot {R : Type*} [CommRing R] [IsDomain R] {s : Multiset (Ideal R)} :
s.prod = ⊥ ↔ ∃ I ∈ s, I = ⊥ := by
simp
theorem span_pair_mul_span_pair (w x y z : R) :
(span {w, x} : Ideal R) * span {y, z} = span {w * y, w * z, x * y, x * z} := by
simp_rw [span_insert, sup_mul, mul_sup, span_singleton_mul_span_singleton, sup_assoc]
theorem isCoprime_iff_codisjoint : IsCoprime I J ↔ Codisjoint I J := by
rw [IsCoprime, codisjoint_iff]
constructor
· rintro ⟨x, y, hxy⟩
rw [eq_top_iff_one]
apply (show x * I + y * J ≤ I ⊔ J from
sup_le (mul_le_left.trans le_sup_left) (mul_le_left.trans le_sup_right))
rw [hxy]
simp only [one_eq_top, Submodule.mem_top]
· intro h
refine ⟨1, 1, ?_⟩
simpa only [one_eq_top, top_mul, Submodule.add_eq_sup]
theorem isCoprime_iff_add : IsCoprime I J ↔ I + J = 1 := by
rw [isCoprime_iff_codisjoint, codisjoint_iff, add_eq_sup, one_eq_top]
theorem isCoprime_iff_exists : IsCoprime I J ↔ ∃ i ∈ I, ∃ j ∈ J, i + j = 1 := by
rw [← add_eq_one_iff, isCoprime_iff_add]
theorem isCoprime_iff_sup_eq : IsCoprime I J ↔ I ⊔ J = ⊤ := by
rw [isCoprime_iff_codisjoint, codisjoint_iff]
open List in
theorem isCoprime_tfae : TFAE [IsCoprime I J, Codisjoint I J, I + J = 1,
∃ i ∈ I, ∃ j ∈ J, i + j = 1, I ⊔ J = ⊤] := by
rw [← isCoprime_iff_codisjoint, ← isCoprime_iff_add, ← isCoprime_iff_exists,
← isCoprime_iff_sup_eq]
simp
theorem _root_.IsCoprime.codisjoint (h : IsCoprime I J) : Codisjoint I J :=
isCoprime_iff_codisjoint.mp h
theorem _root_.IsCoprime.add_eq (h : IsCoprime I J) : I + J = 1 := isCoprime_iff_add.mp h
theorem _root_.IsCoprime.exists (h : IsCoprime I J) : ∃ i ∈ I, ∃ j ∈ J, i + j = 1 :=
isCoprime_iff_exists.mp h
theorem _root_.IsCoprime.sup_eq (h : IsCoprime I J) : I ⊔ J = ⊤ := isCoprime_iff_sup_eq.mp h
theorem inf_eq_mul_of_isCoprime (coprime : IsCoprime I J) : I ⊓ J = I * J :=
(Ideal.mul_eq_inf_of_coprime coprime.sup_eq).symm
@[deprecated (since := "2024-05-28")]
alias inf_eq_mul_of_coprime := inf_eq_mul_of_isCoprime
theorem isCoprime_span_singleton_iff (x y : R) :
IsCoprime (span <| singleton x) (span <| singleton y) ↔ IsCoprime x y := by
simp_rw [isCoprime_iff_codisjoint, codisjoint_iff, eq_top_iff_one, mem_span_singleton_sup,
mem_span_singleton]
constructor
· rintro ⟨a, _, ⟨b, rfl⟩, e⟩; exact ⟨a, b, mul_comm b y ▸ e⟩
· rintro ⟨a, b, e⟩; exact ⟨a, _, ⟨b, rfl⟩, mul_comm y b ▸ e⟩
theorem isCoprime_biInf {J : ι → Ideal R} {s : Finset ι}
(hf : ∀ j ∈ s, IsCoprime I (J j)) : IsCoprime I (⨅ j ∈ s, J j) := by
classical
simp_rw [isCoprime_iff_add] at *
induction s using Finset.induction with
| empty =>
simp
| @insert i s _ hs =>
rw [Finset.iInf_insert, inf_comm, one_eq_top, eq_top_iff, ← one_eq_top]
set K := ⨅ j ∈ s, J j
calc
1 = I + K := (hs fun j hj ↦ hf j (Finset.mem_insert_of_mem hj)).symm
_ = I + K*(I + J i) := by rw [hf i (Finset.mem_insert_self i s), mul_one]
_ = (1+K)*I + K*J i := by ring
_ ≤ I + K ⊓ J i := add_le_add mul_le_left mul_le_inf
/-- The radical of an ideal `I` consists of the elements `r` such that `r ^ n ∈ I` for some `n`. -/
def radical (I : Ideal R) : Ideal R where
carrier := { r | ∃ n : ℕ, r ^ n ∈ I }
zero_mem' := ⟨1, (pow_one (0 : R)).symm ▸ I.zero_mem⟩
add_mem' := fun {_ _} ⟨m, hxmi⟩ ⟨n, hyni⟩ =>
⟨m + n - 1, add_pow_add_pred_mem_of_pow_mem I hxmi hyni⟩
smul_mem' {r s} := fun ⟨n, h⟩ ↦ ⟨n, (mul_pow r s n).symm ▸ I.mul_mem_left (r ^ n) h⟩
theorem mem_radical_iff {r : R} : r ∈ I.radical ↔ ∃ n : ℕ, r ^ n ∈ I := Iff.rfl
/-- An ideal is radical if it contains its radical. -/
def IsRadical (I : Ideal R) : Prop :=
I.radical ≤ I
theorem le_radical : I ≤ radical I := fun r hri => ⟨1, (pow_one r).symm ▸ hri⟩
/-- An ideal is radical iff it is equal to its radical. -/
theorem radical_eq_iff : I.radical = I ↔ I.IsRadical := by
rw [le_antisymm_iff, and_iff_left le_radical, IsRadical]
alias ⟨_, IsRadical.radical⟩ := radical_eq_iff
theorem isRadical_iff_pow_one_lt (k : ℕ) (hk : 1 < k) : I.IsRadical ↔ ∀ r, r ^ k ∈ I → r ∈ I :=
⟨fun h _r hr ↦ h ⟨k, hr⟩, fun h x ⟨n, hx⟩ ↦
k.pow_imp_self_of_one_lt hk _ (fun _ _ ↦ .inr ∘ I.smul_mem _) h n x hx⟩
variable (R)
theorem radical_top : (radical ⊤ : Ideal R) = ⊤ :=
(eq_top_iff_one _).2 ⟨0, Submodule.mem_top⟩
variable {R}
theorem radical_mono (H : I ≤ J) : radical I ≤ radical J := fun _ ⟨n, hrni⟩ => ⟨n, H hrni⟩
variable (I)
theorem radical_isRadical : (radical I).IsRadical := fun r ⟨n, k, hrnki⟩ =>
⟨n * k, (pow_mul r n k).symm ▸ hrnki⟩
@[simp]
theorem radical_idem : radical (radical I) = radical I :=
(radical_isRadical I).radical
variable {I}
theorem IsRadical.radical_le_iff (hJ : J.IsRadical) : I.radical ≤ J ↔ I ≤ J :=
⟨le_trans le_radical, fun h => hJ.radical ▸ radical_mono h⟩
theorem radical_le_radical_iff : radical I ≤ radical J ↔ I ≤ radical J :=
(radical_isRadical J).radical_le_iff
theorem radical_eq_top : radical I = ⊤ ↔ I = ⊤ :=
⟨fun h =>
(eq_top_iff_one _).2 <|
let ⟨n, hn⟩ := (eq_top_iff_one _).1 h
@one_pow R _ n ▸ hn,
fun h => h.symm ▸ radical_top R⟩
theorem IsPrime.isRadical (H : IsPrime I) : I.IsRadical := fun _ ⟨n, hrni⟩ =>
H.mem_of_pow_mem n hrni
theorem IsPrime.radical (H : IsPrime I) : radical I = I :=
IsRadical.radical H.isRadical
theorem mem_radical_of_pow_mem {I : Ideal R} {x : R} {m : ℕ} (hx : x ^ m ∈ radical I) :
x ∈ radical I :=
radical_idem I ▸ ⟨m, hx⟩
theorem disjoint_powers_iff_not_mem (y : R) (hI : I.IsRadical) :
Disjoint (Submonoid.powers y : Set R) ↑I ↔ y ∉ I.1 := by
refine ⟨fun h => Set.disjoint_left.1 h (Submonoid.mem_powers _),
fun h => disjoint_iff.mpr (eq_bot_iff.mpr ?_)⟩
rintro x ⟨⟨n, rfl⟩, hx'⟩
exact h (hI <| mem_radical_of_pow_mem <| le_radical hx')
variable (I J)
theorem radical_sup : radical (I ⊔ J) = radical (radical I ⊔ radical J) :=
le_antisymm (radical_mono <| sup_le_sup le_radical le_radical) <|
radical_le_radical_iff.2 <| sup_le (radical_mono le_sup_left) (radical_mono le_sup_right)
theorem radical_inf : radical (I ⊓ J) = radical I ⊓ radical J :=
le_antisymm (le_inf (radical_mono inf_le_left) (radical_mono inf_le_right))
fun r ⟨⟨m, hrm⟩, ⟨n, hrn⟩⟩ =>
⟨m + n, (pow_add r m n).symm ▸ I.mul_mem_right _ hrm,
(pow_add r m n).symm ▸ J.mul_mem_left _ hrn⟩
variable {I J} in
theorem IsRadical.inf (hI : IsRadical I) (hJ : IsRadical J) : IsRadical (I ⊓ J) := by
rw [IsRadical, radical_inf]; exact inf_le_inf hI hJ
/-- The reverse inclusion does not hold for e.g. `I := fun n : ℕ ↦ Ideal.span {(2 ^ n : ℤ)}`. -/
theorem radical_iInf_le {ι} (I : ι → Ideal R) : radical (⨅ i, I i) ≤ ⨅ i, radical (I i) :=
le_iInf fun _ ↦ radical_mono (iInf_le _ _)
theorem isRadical_iInf {ι} (I : ι → Ideal R) (hI : ∀ i, IsRadical (I i)) : IsRadical (⨅ i, I i) :=
(radical_iInf_le I).trans (iInf_mono hI)
theorem radical_mul : radical (I * J) = radical I ⊓ radical J := by
refine le_antisymm ?_ fun r ⟨⟨m, hrm⟩, ⟨n, hrn⟩⟩ =>
⟨m + n, (pow_add r m n).symm ▸ mul_mem_mul hrm hrn⟩
have := radical_mono <| @mul_le_inf _ _ I J
simp_rw [radical_inf I J] at this
assumption
variable {I J}
theorem IsPrime.radical_le_iff (hJ : IsPrime J) : I.radical ≤ J ↔ I ≤ J :=
IsRadical.radical_le_iff hJ.isRadical
theorem radical_eq_sInf (I : Ideal R) : radical I = sInf { J : Ideal R | I ≤ J ∧ IsPrime J } :=
le_antisymm (le_sInf fun J hJ ↦ hJ.2.radical_le_iff.2 hJ.1) fun r hr ↦
by_contradiction fun hri ↦
let ⟨m, (hrm : r ∉ radical m), him, hm⟩ :=
zorn_nonempty_partialOrder₀ { K : Ideal R | r ∉ radical K }
(fun c hc hcc y hyc =>
⟨sSup c, fun ⟨n, hrnc⟩ =>
let ⟨y, hyc, hrny⟩ := (Submodule.mem_sSup_of_directed ⟨y, hyc⟩ hcc.directedOn).1 hrnc
hc hyc ⟨n, hrny⟩,
fun z => le_sSup⟩)
I hri
have : ∀ x ∉ m, r ∈ radical (m ⊔ span {x}) := fun x hxm =>
by_contradiction fun hrmx =>
hxm <|
hm (m ⊔ span {x}) hrmx le_sup_left ▸
(le_sup_right : _ ≤ m ⊔ span {x}) (subset_span <| Set.mem_singleton _)
have : IsPrime m :=
⟨by rintro rfl; rw [radical_top] at hrm; exact hrm trivial, fun {x y} hxym =>
or_iff_not_imp_left.2 fun hxm =>
by_contradiction fun hym =>
let ⟨n, hrn⟩ := this _ hxm
let ⟨p, hpm, q, hq, hpqrn⟩ := Submodule.mem_sup.1 hrn
let ⟨c, hcxq⟩ := mem_span_singleton'.1 hq
let ⟨k, hrk⟩ := this _ hym
let ⟨f, hfm, g, hg, hfgrk⟩ := Submodule.mem_sup.1 hrk
let ⟨d, hdyg⟩ := mem_span_singleton'.1 hg
hrm
⟨n + k, by
rw [pow_add, ← hpqrn, ← hcxq, ← hfgrk, ← hdyg, add_mul, mul_add (c * x),
mul_assoc c x (d * y), mul_left_comm x, ← mul_assoc]
refine
m.add_mem (m.mul_mem_right _ hpm)
(m.add_mem (m.mul_mem_left _ hfm) (m.mul_mem_left _ hxym))⟩⟩
hrm <|
this.radical.symm ▸ (sInf_le ⟨him, this⟩ : sInf { J : Ideal R | I ≤ J ∧ IsPrime J } ≤ m) hr
theorem isRadical_bot_of_noZeroDivisors {R} [CommSemiring R] [NoZeroDivisors R] :
(⊥ : Ideal R).IsRadical := fun _ hx => hx.recOn fun _ hn => pow_eq_zero hn
@[simp]
theorem radical_bot_of_noZeroDivisors {R : Type u} [CommSemiring R] [NoZeroDivisors R] :
radical (⊥ : Ideal R) = ⊥ :=
eq_bot_iff.2 isRadical_bot_of_noZeroDivisors
instance : IdemCommSemiring (Ideal R) :=
inferInstance
variable (R)
theorem top_pow (n : ℕ) : (⊤ ^ n : Ideal R) = ⊤ :=
Nat.recOn n one_eq_top fun n ih => by rw [pow_succ, ih, top_mul]
variable {R}
variable (I)
lemma radical_pow : ∀ {n}, n ≠ 0 → radical (I ^ n) = radical I
| 1, _ => by simp
| n + 2, _ => by rw [pow_succ, radical_mul, radical_pow n.succ_ne_zero, inf_idem]
theorem IsPrime.mul_le {I J P : Ideal R} (hp : IsPrime P) : I * J ≤ P ↔ I ≤ P ∨ J ≤ P := by
rw [or_comm, Ideal.mul_le]
simp_rw [hp.mul_mem_iff_mem_or_mem, SetLike.le_def, ← forall_or_left, or_comm, forall_or_left]
theorem IsPrime.inf_le {I J P : Ideal R} (hp : IsPrime P) : I ⊓ J ≤ P ↔ I ≤ P ∨ J ≤ P :=
⟨fun h ↦ hp.mul_le.1 <| mul_le_inf.trans h, fun h ↦ h.elim inf_le_left.trans inf_le_right.trans⟩
theorem IsPrime.multiset_prod_le {s : Multiset (Ideal R)} {P : Ideal R} (hp : IsPrime P) :
s.prod ≤ P ↔ ∃ I ∈ s, I ≤ P :=
s.induction_on (by simp [hp.ne_top]) fun I s ih ↦ by simp [hp.mul_le, ih]
theorem IsPrime.multiset_prod_map_le {s : Multiset ι} (f : ι → Ideal R) {P : Ideal R}
(hp : IsPrime P) : (s.map f).prod ≤ P ↔ ∃ i ∈ s, f i ≤ P := by
simp_rw [hp.multiset_prod_le, Multiset.mem_map, exists_exists_and_eq_and]
theorem IsPrime.multiset_prod_mem_iff_exists_mem {I : Ideal R} (hI : I.IsPrime) (s : Multiset R) :
s.prod ∈ I ↔ ∃ p ∈ s, p ∈ I := by
simpa [span_singleton_le_iff_mem] using (hI.multiset_prod_map_le (span {·}))
theorem IsPrime.prod_le {s : Finset ι} {f : ι → Ideal R} {P : Ideal R} (hp : IsPrime P) :
s.prod f ≤ P ↔ ∃ i ∈ s, f i ≤ P :=
hp.multiset_prod_map_le f
theorem IsPrime.prod_mem_iff_exists_mem {I : Ideal R} (hI : I.IsPrime) (s : Finset R) :
s.prod (fun x ↦ x) ∈ I ↔ ∃ p ∈ s, p ∈ I := by
rw [Finset.prod_eq_multiset_prod, Multiset.map_id']
exact hI.multiset_prod_mem_iff_exists_mem s.val
theorem IsPrime.inf_le' {s : Finset ι} {f : ι → Ideal R} {P : Ideal R} (hp : IsPrime P) :
s.inf f ≤ P ↔ ∃ i ∈ s, f i ≤ P :=
⟨fun h ↦ hp.prod_le.1 <| prod_le_inf.trans h, fun ⟨_, his, hip⟩ ↦ (Finset.inf_le his).trans hip⟩
-- Porting note: needed to add explicit coercions (· : Set R).
theorem subset_union {R : Type u} [Ring R] {I J K : Ideal R} :
(I : Set R) ⊆ J ∪ K ↔ I ≤ J ∨ I ≤ K :=
AddSubgroupClass.subset_union
theorem subset_union_prime' {R : Type u} [CommRing R] {s : Finset ι} {f : ι → Ideal R} {a b : ι}
(hp : ∀ i ∈ s, IsPrime (f i)) {I : Ideal R} :
((I : Set R) ⊆ f a ∪ f b ∪ ⋃ i ∈ (↑s : Set ι), f i) ↔ I ≤ f a ∨ I ≤ f b ∨ ∃ i ∈ s, I ≤ f i := by
suffices
((I : Set R) ⊆ f a ∪ f b ∪ ⋃ i ∈ (↑s : Set ι), f i) → I ≤ f a ∨ I ≤ f b ∨ ∃ i ∈ s, I ≤ f i from
⟨this, fun h =>
Or.casesOn h
(fun h =>
Set.Subset.trans h <|
Set.Subset.trans Set.subset_union_left Set.subset_union_left)
fun h =>
Or.casesOn h
(fun h =>
Set.Subset.trans h <|
Set.Subset.trans Set.subset_union_right Set.subset_union_left)
fun ⟨i, his, hi⟩ => by
refine Set.Subset.trans hi <| Set.Subset.trans ?_ Set.subset_union_right
exact Set.subset_biUnion_of_mem (u := fun x ↦ (f x : Set R)) (Finset.mem_coe.2 his)⟩
generalize hn : s.card = n; intro h
induction' n with n ih generalizing a b s
· clear hp
rw [Finset.card_eq_zero] at hn
subst hn
rw [Finset.coe_empty, Set.biUnion_empty, Set.union_empty, subset_union] at h
simpa only [exists_prop, Finset.not_mem_empty, false_and_iff, exists_false, or_false_iff]
classical
replace hn : ∃ (i : ι) (t : Finset ι), i ∉ t ∧ insert i t = s ∧ t.card = n :=
Finset.card_eq_succ.1 hn
rcases hn with ⟨i, t, hit, rfl, hn⟩
replace hp : IsPrime (f i) ∧ ∀ x ∈ t, IsPrime (f x) := (t.forall_mem_insert _ _).1 hp
by_cases Ht : ∃ j ∈ t, f j ≤ f i
· obtain ⟨j, hjt, hfji⟩ : ∃ j ∈ t, f j ≤ f i := Ht
obtain ⟨u, hju, rfl⟩ : ∃ u, j ∉ u ∧ insert j u = t :=
⟨t.erase j, t.not_mem_erase j, Finset.insert_erase hjt⟩
have hp' : ∀ k ∈ insert i u, IsPrime (f k) := by
rw [Finset.forall_mem_insert] at hp ⊢
exact ⟨hp.1, hp.2.2⟩
have hiu : i ∉ u := mt Finset.mem_insert_of_mem hit
have hn' : (insert i u).card = n := by
rwa [Finset.card_insert_of_not_mem] at hn ⊢
exacts [hiu, hju]
have h' : (I : Set R) ⊆ f a ∪ f b ∪ ⋃ k ∈ (↑(insert i u) : Set ι), f k := by
rw [Finset.coe_insert] at h ⊢
rw [Finset.coe_insert] at h
simp only [Set.biUnion_insert] at h ⊢
rw [← Set.union_assoc (f i : Set R)] at h
erw [Set.union_eq_self_of_subset_right hfji] at h
exact h
specialize ih hp' hn' h'
refine ih.imp id (Or.imp id (Exists.imp fun k => ?_))
exact And.imp (fun hk => Finset.insert_subset_insert i (Finset.subset_insert j u) hk) id
by_cases Ha : f a ≤ f i
· have h' : (I : Set R) ⊆ f i ∪ f b ∪ ⋃ j ∈ (↑t : Set ι), f j := by
rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_assoc,
Set.union_right_comm (f a : Set R)] at h
erw [Set.union_eq_self_of_subset_left Ha] at h
exact h
specialize ih hp.2 hn h'
right
rcases ih with (ih | ih | ⟨k, hkt, ih⟩)
· exact Or.inr ⟨i, Finset.mem_insert_self i t, ih⟩
· exact Or.inl ih
· exact Or.inr ⟨k, Finset.mem_insert_of_mem hkt, ih⟩
by_cases Hb : f b ≤ f i
· have h' : (I : Set R) ⊆ f a ∪ f i ∪ ⋃ j ∈ (↑t : Set ι), f j := by
rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_assoc,
Set.union_assoc (f a : Set R)] at h
erw [Set.union_eq_self_of_subset_left Hb] at h
exact h
specialize ih hp.2 hn h'
rcases ih with (ih | ih | ⟨k, hkt, ih⟩)
· exact Or.inl ih
· exact Or.inr (Or.inr ⟨i, Finset.mem_insert_self i t, ih⟩)
· exact Or.inr (Or.inr ⟨k, Finset.mem_insert_of_mem hkt, ih⟩)
by_cases Hi : I ≤ f i
· exact Or.inr (Or.inr ⟨i, Finset.mem_insert_self i t, Hi⟩)
have : ¬I ⊓ f a ⊓ f b ⊓ t.inf f ≤ f i := by
simp only [hp.1.inf_le, hp.1.inf_le', not_or]
exact ⟨⟨⟨Hi, Ha⟩, Hb⟩, Ht⟩
rcases Set.not_subset.1 this with ⟨r, ⟨⟨⟨hrI, hra⟩, hrb⟩, hr⟩, hri⟩
by_cases HI : (I : Set R) ⊆ f a ∪ f b ∪ ⋃ j ∈ (↑t : Set ι), f j
· specialize ih hp.2 hn HI
rcases ih with (ih | ih | ⟨k, hkt, ih⟩)
· left
exact ih
· right
left
exact ih
· right
right
exact ⟨k, Finset.mem_insert_of_mem hkt, ih⟩
exfalso
rcases Set.not_subset.1 HI with ⟨s, hsI, hs⟩
rw [Finset.coe_insert, Set.biUnion_insert] at h
have hsi : s ∈ f i := ((h hsI).resolve_left (mt Or.inl hs)).resolve_right (mt Or.inr hs)
rcases h (I.add_mem hrI hsI) with (⟨ha | hb⟩ | hi | ht)
· exact hs (Or.inl <| Or.inl <| add_sub_cancel_left r s ▸ (f a).sub_mem ha hra)
· exact hs (Or.inl <| Or.inr <| add_sub_cancel_left r s ▸ (f b).sub_mem hb hrb)
· exact hri (add_sub_cancel_right r s ▸ (f i).sub_mem hi hsi)
· rw [Set.mem_iUnion₂] at ht
rcases ht with ⟨j, hjt, hj⟩
simp only [Finset.inf_eq_iInf, SetLike.mem_coe, Submodule.mem_iInf] at hr
exact hs $ Or.inr $ Set.mem_biUnion hjt <|
add_sub_cancel_left r s ▸ (f j).sub_mem hj <| hr j hjt
/-- Prime avoidance. Atiyah-Macdonald 1.11, Eisenbud 3.3, Stacks 00DS, Matsumura Ex.1.6. -/
theorem subset_union_prime {R : Type u} [CommRing R] {s : Finset ι} {f : ι → Ideal R} (a b : ι)
(hp : ∀ i ∈ s, i ≠ a → i ≠ b → IsPrime (f i)) {I : Ideal R} :
((I : Set R) ⊆ ⋃ i ∈ (↑s : Set ι), f i) ↔ ∃ i ∈ s, I ≤ f i :=
suffices ((I : Set R) ⊆ ⋃ i ∈ (↑s : Set ι), f i) → ∃ i, i ∈ s ∧ I ≤ f i by
have aux := fun h => (bex_def.2 <| this h)
simp_rw [exists_prop] at aux
refine ⟨aux, fun ⟨i, his, hi⟩ ↦ Set.Subset.trans hi ?_⟩
apply Set.subset_biUnion_of_mem (show i ∈ (↑s : Set ι) from his)
fun h : (I : Set R) ⊆ ⋃ i ∈ (↑s : Set ι), f i => by
classical
by_cases has : a ∈ s
· obtain ⟨t, hat, rfl⟩ : ∃ t, a ∉ t ∧ insert a t = s :=
⟨s.erase a, Finset.not_mem_erase a s, Finset.insert_erase has⟩
by_cases hbt : b ∈ t
· obtain ⟨u, hbu, rfl⟩ : ∃ u, b ∉ u ∧ insert b u = t :=
⟨t.erase b, Finset.not_mem_erase b t, Finset.insert_erase hbt⟩
have hp' : ∀ i ∈ u, IsPrime (f i) := by
intro i hiu
refine hp i (Finset.mem_insert_of_mem (Finset.mem_insert_of_mem hiu)) ?_ ?_ <;>
rintro rfl <;>
solve_by_elim only [Finset.mem_insert_of_mem, *]
rw [Finset.coe_insert, Finset.coe_insert, Set.biUnion_insert, Set.biUnion_insert, ←
Set.union_assoc, subset_union_prime' hp'] at h
rwa [Finset.exists_mem_insert, Finset.exists_mem_insert]
· have hp' : ∀ j ∈ t, IsPrime (f j) := by
intro j hj
refine hp j (Finset.mem_insert_of_mem hj) ?_ ?_ <;> rintro rfl <;>
solve_by_elim only [Finset.mem_insert_of_mem, *]
rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_self (f a : Set R),
subset_union_prime' hp', ← or_assoc, or_self_iff] at h
rwa [Finset.exists_mem_insert]
· by_cases hbs : b ∈ s
· obtain ⟨t, hbt, rfl⟩ : ∃ t, b ∉ t ∧ insert b t = s :=
⟨s.erase b, Finset.not_mem_erase b s, Finset.insert_erase hbs⟩
have hp' : ∀ j ∈ t, IsPrime (f j) := by
intro j hj
refine hp j (Finset.mem_insert_of_mem hj) ?_ ?_ <;> rintro rfl <;>
solve_by_elim only [Finset.mem_insert_of_mem, *]
rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_self (f b : Set R),
subset_union_prime' hp', ← or_assoc, or_self_iff] at h
rwa [Finset.exists_mem_insert]
rcases s.eq_empty_or_nonempty with hse | hsne
· subst hse
rw [Finset.coe_empty, Set.biUnion_empty, Set.subset_empty_iff] at h
have : (I : Set R) ≠ ∅ := Set.Nonempty.ne_empty (Set.nonempty_of_mem I.zero_mem)
exact absurd h this
· cases' hsne with i his
obtain ⟨t, _, rfl⟩ : ∃ t, i ∉ t ∧ insert i t = s :=
⟨s.erase i, Finset.not_mem_erase i s, Finset.insert_erase his⟩
have hp' : ∀ j ∈ t, IsPrime (f j) := by
intro j hj
refine hp j (Finset.mem_insert_of_mem hj) ?_ ?_ <;> rintro rfl <;>
solve_by_elim only [Finset.mem_insert_of_mem, *]
rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_self (f i : Set R),
subset_union_prime' hp', ← or_assoc, or_self_iff] at h
rwa [Finset.exists_mem_insert]
section Dvd
/-- If `I` divides `J`, then `I` contains `J`.
In a Dedekind domain, to divide and contain are equivalent, see `Ideal.dvd_iff_le`.
-/
theorem le_of_dvd {I J : Ideal R} : I ∣ J → J ≤ I
| ⟨_, h⟩ => h.symm ▸ le_trans mul_le_inf inf_le_left
@[simp]
theorem isUnit_iff {I : Ideal R} : IsUnit I ↔ I = ⊤ :=
isUnit_iff_dvd_one.trans
((@one_eq_top R _).symm ▸
⟨fun h => eq_top_iff.mpr (Ideal.le_of_dvd h), fun h => ⟨⊤, by rw [mul_top, h]⟩⟩)
instance uniqueUnits : Unique (Ideal R)ˣ where
default := 1
uniq u := Units.ext (show (u : Ideal R) = 1 by rw [isUnit_iff.mp u.isUnit, one_eq_top])
end Dvd
end MulAndRadical
section Total
variable (ι : Type*)
variable (M : Type*) [AddCommGroup M] {R : Type*} [CommRing R] [Module R M] (I : Ideal R)
variable (v : ι → M) (hv : Submodule.span R (Set.range v) = ⊤)
/-- A variant of `Finsupp.total` that takes in vectors valued in `I`. -/
noncomputable def finsuppTotal : (ι →₀ I) →ₗ[R] M :=
(Finsupp.total ι M R v).comp (Finsupp.mapRange.linearMap I.subtype)
variable {ι M v}
theorem finsuppTotal_apply (f : ι →₀ I) :
finsuppTotal ι M I v f = f.sum fun i x => (x : R) • v i := by
dsimp [finsuppTotal]
rw [Finsupp.total_apply, Finsupp.sum_mapRange_index]
exact fun _ => zero_smul _ _
theorem finsuppTotal_apply_eq_of_fintype [Fintype ι] (f : ι →₀ I) :
finsuppTotal ι M I v f = ∑ i, (f i : R) • v i := by
rw [finsuppTotal_apply, Finsupp.sum_fintype]
exact fun _ => zero_smul _ _
theorem range_finsuppTotal :
LinearMap.range (finsuppTotal ι M I v) = I • Submodule.span R (Set.range v) := by
ext
rw [Submodule.mem_ideal_smul_span_iff_exists_sum]
refine ⟨fun ⟨f, h⟩ => ⟨Finsupp.mapRange.linearMap I.subtype f, fun i => (f i).2, h⟩, ?_⟩
rintro ⟨a, ha, rfl⟩
classical
refine ⟨a.mapRange (fun r => if h : r ∈ I then ⟨r, h⟩ else 0) (by simp), ?_⟩
rw [finsuppTotal_apply, Finsupp.sum_mapRange_index]
· apply Finsupp.sum_congr
intro i _
rw [dif_pos (ha i)]
· exact fun _ => zero_smul _ _
end Total
end Ideal
section span_range
variable {α R : Type*} [Semiring R]
theorem Finsupp.mem_ideal_span_range_iff_exists_finsupp {x : R} {v : α → R} :
x ∈ Ideal.span (Set.range v) ↔ ∃ c : α →₀ R, (c.sum fun i a => a * v i) = x :=
Finsupp.mem_span_range_iff_exists_finsupp
/-- An element `x` lies in the span of `v` iff it can be written as sum `∑ cᵢ • vᵢ = x`.
-/
theorem mem_ideal_span_range_iff_exists_fun [Fintype α] {x : R} {v : α → R} :
x ∈ Ideal.span (Set.range v) ↔ ∃ c : α → R, ∑ i, c i * v i = x :=
mem_span_range_iff_exists_fun _
end span_range
theorem Associates.mk_ne_zero' {R : Type*} [CommSemiring R] {r : R} :
Associates.mk (Ideal.span {r} : Ideal R) ≠ 0 ↔ r ≠ 0 := by
rw [Associates.mk_ne_zero, Ideal.zero_eq_bot, Ne, Ideal.span_singleton_eq_bot]
open scoped nonZeroDivisors in
theorem Ideal.span_singleton_nonZeroDivisors {R : Type*} [CommSemiring R] [NoZeroDivisors R]
{r : R} : span {r} ∈ (Ideal R)⁰ ↔ r ∈ R⁰ := by
cases subsingleton_or_nontrivial R
· exact ⟨fun _ _ _ ↦ Subsingleton.eq_zero _, fun _ _ _ ↦ Subsingleton.eq_zero _⟩
· rw [mem_nonZeroDivisors_iff_ne_zero, mem_nonZeroDivisors_iff_ne_zero, ne_eq, zero_eq_bot,
span_singleton_eq_bot]
namespace Submodule
variable {R : Type u} {M : Type v}
variable [CommSemiring R] [AddCommMonoid M] [Module R M]
-- TODO: show `[Algebra R A] : Algebra (Ideal R) A` too
instance moduleSubmodule : Module (Ideal R) (Submodule R M) where
smul_add := smul_sup
add_smul := sup_smul
mul_smul := Submodule.smul_assoc
one_smul := by simp
zero_smul := bot_smul
smul_zero := smul_bot
lemma span_smul_eq
(s : Set R) (N : Submodule R M) :
Ideal.span s • N = s • N := by
rw [← coe_set_smul, coe_span_smul]
@[simp]
theorem set_smul_top_eq_span (s : Set R) :
s • ⊤ = Ideal.span s :=
Eq.trans (span_smul_eq s ⊤).symm <|
Eq.trans (smul_eq_mul (Ideal R)) (Ideal.mul_top (.span s))
end Submodule
|
RingTheory\Ideal\Over.lean | /-
Copyright (c) 2020 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen
-/
import Mathlib.RingTheory.Algebraic
import Mathlib.RingTheory.Localization.AtPrime
import Mathlib.RingTheory.Localization.Integral
/-!
# Ideals over/under ideals
This file concerns ideals lying over other ideals.
Let `f : R →+* S` be a ring homomorphism (typically a ring extension), `I` an ideal of `R` and
`J` an ideal of `S`. We say `J` lies over `I` (and `I` under `J`) if `I` is the `f`-preimage of `J`.
This is expressed here by writing `I = J.comap f`.
## Implementation notes
The proofs of the `comap_ne_bot` and `comap_lt_comap` families use an approach
specific for their situation: we construct an element in `I.comap f` from the
coefficients of a minimal polynomial.
Once mathlib has more material on the localization at a prime ideal, the results
can be proven using more general going-up/going-down theory.
-/
variable {R : Type*} [CommRing R]
namespace Ideal
open Polynomial
open Polynomial
open Submodule
section CommRing
variable {S : Type*} [CommRing S] {f : R →+* S} {I J : Ideal S}
theorem coeff_zero_mem_comap_of_root_mem_of_eval_mem {r : S} (hr : r ∈ I) {p : R[X]}
(hp : p.eval₂ f r ∈ I) : p.coeff 0 ∈ I.comap f := by
rw [← p.divX_mul_X_add, eval₂_add, eval₂_C, eval₂_mul, eval₂_X] at hp
refine mem_comap.mpr ((I.add_mem_iff_right ?_).mp hp)
exact I.mul_mem_left _ hr
theorem coeff_zero_mem_comap_of_root_mem {r : S} (hr : r ∈ I) {p : R[X]} (hp : p.eval₂ f r = 0) :
p.coeff 0 ∈ I.comap f :=
coeff_zero_mem_comap_of_root_mem_of_eval_mem hr (hp.symm ▸ I.zero_mem)
theorem exists_coeff_ne_zero_mem_comap_of_non_zero_divisor_root_mem {r : S}
(r_non_zero_divisor : ∀ {x}, x * r = 0 → x = 0) (hr : r ∈ I) {p : R[X]} :
p ≠ 0 → p.eval₂ f r = 0 → ∃ i, p.coeff i ≠ 0 ∧ p.coeff i ∈ I.comap f := by
refine p.recOnHorner ?_ ?_ ?_
· intro h
contradiction
· intro p a coeff_eq_zero a_ne_zero _ _ hp
refine ⟨0, ?_, coeff_zero_mem_comap_of_root_mem hr hp⟩
simp [coeff_eq_zero, a_ne_zero]
· intro p p_nonzero ih _ hp
rw [eval₂_mul, eval₂_X] at hp
obtain ⟨i, hi, mem⟩ := ih p_nonzero (r_non_zero_divisor hp)
refine ⟨i + 1, ?_, ?_⟩
· simp [hi, mem]
· simpa [hi] using mem
/-- Let `P` be an ideal in `R[x]`. The map
`R[x]/P → (R / (P ∩ R))[x] / (P / (P ∩ R))`
is injective.
-/
theorem injective_quotient_le_comap_map (P : Ideal R[X]) :
Function.Injective <|
Ideal.quotientMap
(Ideal.map (Polynomial.mapRingHom (Quotient.mk (P.comap (C : R →+* R[X])))) P)
(Polynomial.mapRingHom (Ideal.Quotient.mk (P.comap (C : R →+* R[X]))))
le_comap_map := by
refine quotientMap_injective' (le_of_eq ?_)
rw [comap_map_of_surjective (mapRingHom (Ideal.Quotient.mk (P.comap (C : R →+* R[X]))))
(map_surjective (Ideal.Quotient.mk (P.comap (C : R →+* R[X]))) Ideal.Quotient.mk_surjective)]
refine le_antisymm (sup_le le_rfl ?_) (le_sup_of_le_left le_rfl)
refine fun p hp =>
polynomial_mem_ideal_of_coeff_mem_ideal P p fun n => Ideal.Quotient.eq_zero_iff_mem.mp ?_
simpa only [coeff_map, coe_mapRingHom] using ext_iff.mp (Ideal.mem_bot.mp (mem_comap.mp hp)) n
/-- The identity in this lemma asserts that the "obvious" square
```
R → (R / (P ∩ R))
↓ ↓
R[x] / P → (R / (P ∩ R))[x] / (P / (P ∩ R))
```
commutes. It is used, for instance, in the proof of `quotient_mk_comp_C_is_integral_of_jacobson`,
in the file `RingTheory.Jacobson`.
-/
theorem quotient_mk_maps_eq (P : Ideal R[X]) :
((Quotient.mk (map (mapRingHom (Quotient.mk (P.comap (C : R →+* R[X])))) P)).comp C).comp
(Quotient.mk (P.comap (C : R →+* R[X]))) =
(Ideal.quotientMap (map (mapRingHom (Quotient.mk (P.comap (C : R →+* R[X])))) P)
(mapRingHom (Quotient.mk (P.comap (C : R →+* R[X])))) le_comap_map).comp
((Quotient.mk P).comp C) := by
refine RingHom.ext fun x => ?_
repeat' rw [RingHom.coe_comp, Function.comp_apply]
rw [quotientMap_mk, coe_mapRingHom, map_C]
/-- This technical lemma asserts the existence of a polynomial `p` in an ideal `P ⊂ R[x]`
that is non-zero in the quotient `R / (P ∩ R) [x]`. The assumptions are equivalent to
`P ≠ 0` and `P ∩ R = (0)`.
-/
theorem exists_nonzero_mem_of_ne_bot {P : Ideal R[X]} (Pb : P ≠ ⊥) (hP : ∀ x : R, C x ∈ P → x = 0) :
∃ p : R[X], p ∈ P ∧ Polynomial.map (Quotient.mk (P.comap (C : R →+* R[X]))) p ≠ 0 := by
obtain ⟨m, hm⟩ := Submodule.nonzero_mem_of_bot_lt (bot_lt_iff_ne_bot.mpr Pb)
refine ⟨m, Submodule.coe_mem m, fun pp0 => hm (Submodule.coe_eq_zero.mp ?_)⟩
refine
(injective_iff_map_eq_zero (Polynomial.mapRingHom (Ideal.Quotient.mk
(P.comap (C : R →+* R[X]))))).mp
?_ _ pp0
refine map_injective _ ((Ideal.Quotient.mk (P.comap C)).injective_iff_ker_eq_bot.mpr ?_)
rw [mk_ker]
exact (Submodule.eq_bot_iff _).mpr fun x hx => hP x (mem_comap.mp hx)
variable {p : Ideal R} {P : Ideal S}
/-- If there is an injective map `R/p → S/P` such that following diagram commutes:
```
R → S
↓ ↓
R/p → S/P
```
then `P` lies over `p`.
-/
theorem comap_eq_of_scalar_tower_quotient [Algebra R S] [Algebra (R ⧸ p) (S ⧸ P)]
[IsScalarTower R (R ⧸ p) (S ⧸ P)] (h : Function.Injective (algebraMap (R ⧸ p) (S ⧸ P))) :
comap (algebraMap R S) P = p := by
ext x
rw [mem_comap, ← Quotient.eq_zero_iff_mem, ← Quotient.eq_zero_iff_mem, Quotient.mk_algebraMap,
IsScalarTower.algebraMap_apply R (R ⧸ p) (S ⧸ P), Quotient.algebraMap_eq]
constructor
· intro hx
exact (injective_iff_map_eq_zero (algebraMap (R ⧸ p) (S ⧸ P))).mp h _ hx
· intro hx
rw [hx, RingHom.map_zero]
/-- If `P` lies over `p`, then `R / p` has a canonical map to `S / P`. -/
def Quotient.algebraQuotientOfLEComap (h : p ≤ comap f P) : Algebra (R ⧸ p) (S ⧸ P) :=
RingHom.toAlgebra <| quotientMap _ f h
/-- `R / p` has a canonical map to `S / pS`. -/
instance Quotient.algebraQuotientMapQuotient : Algebra (R ⧸ p) (S ⧸ map f p) :=
Ideal.Quotient.algebraQuotientOfLEComap le_comap_map
@[simp]
theorem Quotient.algebraMap_quotient_map_quotient (x : R) :
algebraMap (R ⧸ p) (S ⧸ map f p) (Ideal.Quotient.mk p x) =
Ideal.Quotient.mk (map f p) (f x) :=
rfl
@[simp]
theorem Quotient.mk_smul_mk_quotient_map_quotient (x : R) (y : S) :
Quotient.mk p x • Quotient.mk (map f p) y = Quotient.mk (map f p) (f x * y) :=
rfl
instance Quotient.tower_quotient_map_quotient [Algebra R S] :
IsScalarTower R (R ⧸ p) (S ⧸ map (algebraMap R S) p) :=
IsScalarTower.of_algebraMap_eq fun x => by
rw [Quotient.algebraMap_eq, Quotient.algebraMap_quotient_map_quotient,
Quotient.mk_algebraMap]
instance QuotientMapQuotient.isNoetherian [Algebra R S] [IsNoetherian R S] (I : Ideal R) :
IsNoetherian (R ⧸ I) (S ⧸ Ideal.map (algebraMap R S) I) :=
isNoetherian_of_tower R <|
isNoetherian_of_surjective S (Ideal.Quotient.mkₐ R _).toLinearMap <|
LinearMap.range_eq_top.mpr Ideal.Quotient.mk_surjective
end CommRing
section IsDomain
variable {S : Type*} [CommRing S] {f : R →+* S} {I J : Ideal S}
theorem exists_coeff_ne_zero_mem_comap_of_root_mem [IsDomain S] {r : S} (r_ne_zero : r ≠ 0)
(hr : r ∈ I) {p : R[X]} :
p ≠ 0 → p.eval₂ f r = 0 → ∃ i, p.coeff i ≠ 0 ∧ p.coeff i ∈ I.comap f :=
exists_coeff_ne_zero_mem_comap_of_non_zero_divisor_root_mem
(fun {_} h => Or.resolve_right (mul_eq_zero.mp h) r_ne_zero) hr
theorem exists_coeff_mem_comap_sdiff_comap_of_root_mem_sdiff [IsPrime I] (hIJ : I ≤ J) {r : S}
(hr : r ∈ (J : Set S) \ I) {p : R[X]} (p_ne_zero : p.map (Quotient.mk (I.comap f)) ≠ 0)
(hpI : p.eval₂ f r ∈ I) : ∃ i, p.coeff i ∈ (J.comap f : Set R) \ I.comap f := by
obtain ⟨hrJ, hrI⟩ := hr
have rbar_ne_zero : Ideal.Quotient.mk I r ≠ 0 := mt (Quotient.mk_eq_zero I).mp hrI
have rbar_mem_J : Ideal.Quotient.mk I r ∈ J.map (Ideal.Quotient.mk I) := mem_map_of_mem _ hrJ
have quotient_f : ∀ x ∈ I.comap f, (Ideal.Quotient.mk I).comp f x = 0 := by
simp [Quotient.eq_zero_iff_mem]
have rbar_root :
(p.map (Ideal.Quotient.mk (I.comap f))).eval₂ (Quotient.lift (I.comap f) _ quotient_f)
(Ideal.Quotient.mk I r) =
0 := by
convert Quotient.eq_zero_iff_mem.mpr hpI
exact _root_.trans (eval₂_map _ _ _) (hom_eval₂ p f (Ideal.Quotient.mk I) r).symm
obtain ⟨i, ne_zero, mem⟩ :=
exists_coeff_ne_zero_mem_comap_of_root_mem rbar_ne_zero rbar_mem_J p_ne_zero rbar_root
rw [coeff_map] at ne_zero mem
refine ⟨i, (mem_quotient_iff_mem hIJ).mp ?_, mt ?_ ne_zero⟩
· simpa using mem
simp [Quotient.eq_zero_iff_mem]
theorem comap_lt_comap_of_root_mem_sdiff [I.IsPrime] (hIJ : I ≤ J) {r : S}
(hr : r ∈ (J : Set S) \ I) {p : R[X]} (p_ne_zero : p.map (Quotient.mk (I.comap f)) ≠ 0)
(hp : p.eval₂ f r ∈ I) : I.comap f < J.comap f :=
let ⟨i, hJ, hI⟩ := exists_coeff_mem_comap_sdiff_comap_of_root_mem_sdiff hIJ hr p_ne_zero hp
SetLike.lt_iff_le_and_exists.mpr ⟨comap_mono hIJ, p.coeff i, hJ, hI⟩
theorem mem_of_one_mem (h : (1 : S) ∈ I) (x) : x ∈ I :=
(I.eq_top_iff_one.mpr h).symm ▸ mem_top
theorem comap_lt_comap_of_integral_mem_sdiff [Algebra R S] [hI : I.IsPrime] (hIJ : I ≤ J) {x : S}
(mem : x ∈ (J : Set S) \ I) (integral : IsIntegral R x) :
I.comap (algebraMap R S) < J.comap (algebraMap R S) := by
obtain ⟨p, p_monic, hpx⟩ := integral
refine comap_lt_comap_of_root_mem_sdiff hIJ mem (map_monic_ne_zero p_monic) ?_
convert I.zero_mem
theorem comap_ne_bot_of_root_mem [IsDomain S] {r : S} (r_ne_zero : r ≠ 0) (hr : r ∈ I) {p : R[X]}
(p_ne_zero : p ≠ 0) (hp : p.eval₂ f r = 0) : I.comap f ≠ ⊥ := fun h =>
let ⟨_, hi, mem⟩ := exists_coeff_ne_zero_mem_comap_of_root_mem r_ne_zero hr p_ne_zero hp
absurd (mem_bot.mp (eq_bot_iff.mp h mem)) hi
theorem isMaximal_of_isIntegral_of_isMaximal_comap [Algebra R S] [Algebra.IsIntegral R S]
(I : Ideal S) [I.IsPrime] (hI : IsMaximal (I.comap (algebraMap R S))) : IsMaximal I :=
⟨⟨mt comap_eq_top_iff.mpr hI.1.1, fun _ I_lt_J =>
let ⟨I_le_J, x, hxJ, hxI⟩ := SetLike.lt_iff_le_and_exists.mp I_lt_J
comap_eq_top_iff.1 <|
hI.1.2 _ (comap_lt_comap_of_integral_mem_sdiff I_le_J ⟨hxJ, hxI⟩
(Algebra.IsIntegral.isIntegral x))⟩⟩
theorem isMaximal_of_isIntegral_of_isMaximal_comap' (f : R →+* S) (hf : f.IsIntegral) (I : Ideal S)
[I.IsPrime] (hI : IsMaximal (I.comap f)) : IsMaximal I :=
let _ : Algebra R S := f.toAlgebra
have : Algebra.IsIntegral R S := ⟨hf⟩
isMaximal_of_isIntegral_of_isMaximal_comap (R := R) (S := S) I hI
variable [Algebra R S]
theorem comap_ne_bot_of_algebraic_mem [IsDomain S] {x : S} (x_ne_zero : x ≠ 0) (x_mem : x ∈ I)
(hx : IsAlgebraic R x) : I.comap (algebraMap R S) ≠ ⊥ :=
let ⟨_, p_ne_zero, hp⟩ := hx
comap_ne_bot_of_root_mem x_ne_zero x_mem p_ne_zero hp
theorem comap_ne_bot_of_integral_mem [Nontrivial R] [IsDomain S] {x : S} (x_ne_zero : x ≠ 0)
(x_mem : x ∈ I) (hx : IsIntegral R x) : I.comap (algebraMap R S) ≠ ⊥ :=
comap_ne_bot_of_algebraic_mem x_ne_zero x_mem hx.isAlgebraic
theorem eq_bot_of_comap_eq_bot [Nontrivial R] [IsDomain S] [Algebra.IsIntegral R S]
(hI : I.comap (algebraMap R S) = ⊥) : I = ⊥ := by
refine eq_bot_iff.2 fun x hx => ?_
by_cases hx0 : x = 0
· exact hx0.symm ▸ Ideal.zero_mem ⊥
· exact absurd hI (comap_ne_bot_of_integral_mem hx0 hx (Algebra.IsIntegral.isIntegral x))
theorem isMaximal_comap_of_isIntegral_of_isMaximal [Algebra.IsIntegral R S] (I : Ideal S)
[hI : I.IsMaximal] : IsMaximal (I.comap (algebraMap R S)) := by
refine Ideal.Quotient.maximal_of_isField _ ?_
haveI : IsPrime (I.comap (algebraMap R S)) := comap_isPrime _ _
exact isField_of_isIntegral_of_isField
algebraMap_quotient_injective (by rwa [← Quotient.maximal_ideal_iff_isField_quotient])
theorem isMaximal_comap_of_isIntegral_of_isMaximal' {R S : Type*} [CommRing R] [CommRing S]
(f : R →+* S) (hf : f.IsIntegral) (I : Ideal S) [I.IsMaximal] : IsMaximal (I.comap f) :=
let _ : Algebra R S := f.toAlgebra
have : Algebra.IsIntegral R S := ⟨hf⟩
isMaximal_comap_of_isIntegral_of_isMaximal (R := R) (S := S) I
section IsIntegralClosure
variable (S) {A : Type*} [CommRing A]
variable [Algebra R A] [Algebra A S] [IsScalarTower R A S] [IsIntegralClosure A R S]
theorem IsIntegralClosure.comap_lt_comap {I J : Ideal A} [I.IsPrime] (I_lt_J : I < J) :
I.comap (algebraMap R A) < J.comap (algebraMap R A) :=
let ⟨I_le_J, x, hxJ, hxI⟩ := SetLike.lt_iff_le_and_exists.mp I_lt_J
comap_lt_comap_of_integral_mem_sdiff I_le_J ⟨hxJ, hxI⟩ (IsIntegralClosure.isIntegral R S x)
theorem IsIntegralClosure.isMaximal_of_isMaximal_comap (I : Ideal A) [I.IsPrime]
(hI : IsMaximal (I.comap (algebraMap R A))) : IsMaximal I :=
have : Algebra.IsIntegral R A := IsIntegralClosure.isIntegral_algebra R S
isMaximal_of_isIntegral_of_isMaximal_comap I hI
variable [IsDomain A]
theorem IsIntegralClosure.comap_ne_bot [Nontrivial R] {I : Ideal A} (I_ne_bot : I ≠ ⊥) :
I.comap (algebraMap R A) ≠ ⊥ :=
let ⟨x, x_mem, x_ne_zero⟩ := I.ne_bot_iff.mp I_ne_bot
comap_ne_bot_of_integral_mem x_ne_zero x_mem (IsIntegralClosure.isIntegral R S x)
theorem IsIntegralClosure.eq_bot_of_comap_eq_bot [Nontrivial R] {I : Ideal A} :
I.comap (algebraMap R A) = ⊥ → I = ⊥ := by
-- Porting note: `imp_of_not_imp_not` seems not existing
contrapose; exact (IsIntegralClosure.comap_ne_bot S)
end IsIntegralClosure
theorem IntegralClosure.comap_lt_comap {I J : Ideal (integralClosure R S)} [I.IsPrime]
(I_lt_J : I < J) :
I.comap (algebraMap R (integralClosure R S)) < J.comap (algebraMap R (integralClosure R S)) :=
IsIntegralClosure.comap_lt_comap S I_lt_J
theorem IntegralClosure.isMaximal_of_isMaximal_comap (I : Ideal (integralClosure R S)) [I.IsPrime]
(hI : IsMaximal (I.comap (algebraMap R (integralClosure R S)))) : IsMaximal I :=
IsIntegralClosure.isMaximal_of_isMaximal_comap S I hI
section
variable [IsDomain S]
theorem IntegralClosure.comap_ne_bot [Nontrivial R] {I : Ideal (integralClosure R S)}
(I_ne_bot : I ≠ ⊥) : I.comap (algebraMap R (integralClosure R S)) ≠ ⊥ :=
IsIntegralClosure.comap_ne_bot S I_ne_bot
theorem IntegralClosure.eq_bot_of_comap_eq_bot [Nontrivial R] {I : Ideal (integralClosure R S)} :
I.comap (algebraMap R (integralClosure R S)) = ⊥ → I = ⊥ :=
IsIntegralClosure.eq_bot_of_comap_eq_bot S
/-- `comap (algebraMap R S)` is a surjection from the prime spec of `R` to prime spec of `S`.
`hP : (algebraMap R S).ker ≤ P` is a slight generalization of the extension being injective -/
theorem exists_ideal_over_prime_of_isIntegral_of_isDomain [Algebra.IsIntegral R S] (P : Ideal R)
[IsPrime P] (hP : RingHom.ker (algebraMap R S) ≤ P) :
∃ Q : Ideal S, IsPrime Q ∧ Q.comap (algebraMap R S) = P := by
have hP0 : (0 : S) ∉ Algebra.algebraMapSubmonoid S P.primeCompl := by
rintro ⟨x, ⟨hx, x0⟩⟩
exact absurd (hP x0) hx
let Rₚ := Localization P.primeCompl
let Sₚ := Localization (Algebra.algebraMapSubmonoid S P.primeCompl)
letI : IsDomain (Localization (Algebra.algebraMapSubmonoid S P.primeCompl)) :=
IsLocalization.isDomain_localization (le_nonZeroDivisors_of_noZeroDivisors hP0)
obtain ⟨Qₚ : Ideal Sₚ, Qₚ_maximal⟩ := exists_maximal Sₚ
let _ : Algebra Rₚ Sₚ := localizationAlgebra P.primeCompl S
have : Algebra.IsIntegral Rₚ Sₚ := ⟨isIntegral_localization⟩
have Qₚ_max : IsMaximal (comap _ Qₚ) :=
isMaximal_comap_of_isIntegral_of_isMaximal (R := Rₚ) (S := Sₚ) Qₚ
refine ⟨comap (algebraMap S Sₚ) Qₚ, ⟨comap_isPrime _ Qₚ, ?_⟩⟩
convert Localization.AtPrime.comap_maximalIdeal (I := P)
rw [comap_comap, ← LocalRing.eq_maximalIdeal Qₚ_max,
← @IsLocalization.map_comp (P := S) (Q := Sₚ) (g := algebraMap R S)
(M := P.primeCompl) (T := Algebra.algebraMapSubmonoid S P.primeCompl) (S := Rₚ) _
_ _ _ _ _ (fun p hp => Algebra.mem_algebraMapSubmonoid_of_mem ⟨p, hp⟩) _ _]
rfl
end
/-- More general going-up theorem than `exists_ideal_over_prime_of_isIntegral_of_isDomain`.
TODO: Version of going-up theorem with arbitrary length chains (by induction on this)?
Not sure how best to write an ascending chain in Lean -/
theorem exists_ideal_over_prime_of_isIntegral_of_isPrime
[Algebra.IsIntegral R S] (P : Ideal R) [IsPrime P]
(I : Ideal S) [IsPrime I] (hIP : I.comap (algebraMap R S) ≤ P) :
∃ Q ≥ I, IsPrime Q ∧ Q.comap (algebraMap R S) = P := by
obtain ⟨Q' : Ideal (S ⧸ I), ⟨Q'_prime, hQ'⟩⟩ :=
@exists_ideal_over_prime_of_isIntegral_of_isDomain (R ⧸ I.comap (algebraMap R S)) _ (S ⧸ I) _
Ideal.quotientAlgebra _ _
(map (Ideal.Quotient.mk (I.comap (algebraMap R S))) P)
(map_isPrime_of_surjective Quotient.mk_surjective (by simp [hIP]))
(le_trans (le_of_eq ((RingHom.injective_iff_ker_eq_bot _).1 algebraMap_quotient_injective))
bot_le)
refine ⟨Q'.comap _, le_trans (le_of_eq mk_ker.symm) (ker_le_comap _), ⟨comap_isPrime _ Q', ?_⟩⟩
rw [comap_comap]
refine _root_.trans ?_ (_root_.trans (congr_arg (comap (Ideal.Quotient.mk
(comap (algebraMap R S) I))) hQ') ?_)
· rw [comap_comap]
exact congr_arg (comap · Q') (RingHom.ext fun r => rfl)
· refine _root_.trans (comap_map_of_surjective _ Quotient.mk_surjective _) (sup_eq_left.2 ?_)
simpa [← RingHom.ker_eq_comap_bot] using hIP
lemma exists_ideal_comap_le_prime (P : Ideal R) [P.IsPrime]
(I : Ideal S) (hI : I.comap (algebraMap R S) ≤ P) :
∃ Q ≥ I, Q.IsPrime ∧ Q.comap (algebraMap R S) ≤ P := by
let Sₚ := Localization (Algebra.algebraMapSubmonoid S P.primeCompl)
let Iₚ := I.map (algebraMap S Sₚ)
have hI' : Disjoint (Algebra.algebraMapSubmonoid S P.primeCompl : Set S) I := by
rw [Set.disjoint_iff]
rintro _ ⟨⟨x, hx : x ∉ P, rfl⟩, hx'⟩
exact (hx (hI hx')).elim
have : Iₚ ≠ ⊤ := by
rw [Ne, Ideal.eq_top_iff_one, IsLocalization.mem_map_algebraMap_iff
(Algebra.algebraMapSubmonoid S P.primeCompl) Sₚ, not_exists]
simp only [one_mul, IsLocalization.eq_iff_exists (Algebra.algebraMapSubmonoid S P.primeCompl),
not_exists]
exact fun x c ↦ hI'.ne_of_mem (mul_mem c.2 x.2.2) (I.mul_mem_left c x.1.2)
obtain ⟨M, hM, hM'⟩ := Ideal.exists_le_maximal _ this
refine ⟨_, Ideal.map_le_iff_le_comap.mp hM', hM.isPrime.comap _, ?_⟩
intro x hx
by_contra hx'
exact Set.disjoint_left.mp ((IsLocalization.isPrime_iff_isPrime_disjoint
(Algebra.algebraMapSubmonoid S P.primeCompl) Sₚ M).mp hM.isPrime).2 ⟨_, hx', rfl⟩ hx
theorem exists_ideal_over_prime_of_isIntegral [Algebra.IsIntegral R S] (P : Ideal R) [IsPrime P]
(I : Ideal S) (hIP : I.comap (algebraMap R S) ≤ P) :
∃ Q ≥ I, IsPrime Q ∧ Q.comap (algebraMap R S) = P := by
have ⟨P', hP, hP', hP''⟩ := exists_ideal_comap_le_prime P I hIP
obtain ⟨Q, hQ, hQ', hQ''⟩ := exists_ideal_over_prime_of_isIntegral_of_isPrime P P' hP''
exact ⟨Q, hP.trans hQ, hQ', hQ''⟩
/-- `comap (algebraMap R S)` is a surjection from the max spec of `S` to max spec of `R`.
`hP : (algebraMap R S).ker ≤ P` is a slight generalization of the extension being injective -/
theorem exists_ideal_over_maximal_of_isIntegral [Algebra.IsIntegral R S]
(P : Ideal R) [P_max : IsMaximal P] (hP : RingHom.ker (algebraMap R S) ≤ P) :
∃ Q : Ideal S, IsMaximal Q ∧ Q.comap (algebraMap R S) = P := by
obtain ⟨Q, -, Q_prime, hQ⟩ := exists_ideal_over_prime_of_isIntegral P ⊥ hP
exact ⟨Q, isMaximal_of_isIntegral_of_isMaximal_comap _ (hQ.symm ▸ P_max), hQ⟩
lemma map_eq_top_iff_of_ker_le
(f : R →+* S) {I : Ideal R} (hf₁ : RingHom.ker f ≤ I) (hf₂ : f.IsIntegral) :
I.map f = ⊤ ↔ I = ⊤ := by
constructor; swap
· rintro rfl; exact Ideal.map_top _
contrapose
intro h
obtain ⟨m, _, hm⟩ := Ideal.exists_le_maximal I h
let _ := f.toAlgebra
have : Algebra.IsIntegral _ _ := ⟨hf₂⟩
obtain ⟨m', _, rfl⟩ := exists_ideal_over_maximal_of_isIntegral m (hf₁.trans hm)
rw [← map_le_iff_le_comap] at hm
exact (hm.trans_lt (lt_top_iff_ne_top.mpr (IsMaximal.ne_top ‹_›))).ne
lemma map_eq_top_iff
(f : R →+* S) {I : Ideal R} (hf₁ : Function.Injective f) (hf₂ : f.IsIntegral) :
I.map f = ⊤ ↔ I = ⊤ :=
map_eq_top_iff_of_ker_le f (by simp [f.injective_iff_ker_eq_bot.mp hf₁]) hf₂
end IsDomain
end Ideal
|
RingTheory\Ideal\Pointwise.lean | /-
Copyright (c) 2024 Eric Wieser. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Eric Wieser
-/
import Mathlib.RingTheory.Ideal.Maps
/-! # Pointwise instances on `Ideal`s
This file provides the action `Ideal.pointwiseMulAction` which morally matches the action of
`mulActionSet` (though here an extra `Ideal.span` is inserted).
This actions is available in the `Pointwise` locale.
## Implementation notes
This file is similar (but not identical) to `Mathlib/Algebra/Ring/Subsemiring/Pointwise.lean`.
Where possible, try to keep them in sync.
-/
open Set
variable {M R : Type*}
namespace Ideal
section Monoid
variable [Monoid M] [CommRing R] [MulSemiringAction M R]
/-- The action on an ideal corresponding to applying the action to every element.
This is available as an instance in the `Pointwise` locale. -/
protected def pointwiseMulSemiringAction : MulSemiringAction M (Ideal R) where
smul a := Ideal.map (MulSemiringAction.toRingHom _ _ a)
one_smul I :=
congr_arg (I.map ·) (RingHom.ext <| one_smul M) |>.trans I.map_id
mul_smul a₁ a₂ I :=
congr_arg (I.map ·) (RingHom.ext <| mul_smul _ _) |>.trans (I.map_map _ _).symm
smul_one a := by simp only [Ideal.one_eq_top]; exact Ideal.map_top _
smul_mul a I J := Ideal.map_mul (MulSemiringAction.toRingHom _ _ a) I J
smul_add a I J := Ideal.map_sup _ I J
smul_zero a := Ideal.map_bot
scoped[Pointwise] attribute [instance] Ideal.pointwiseMulSemiringAction
open Pointwise
theorem pointwise_smul_def {a : M} (S : Ideal R) :
a • S = S.map (MulSemiringAction.toRingHom _ _ a) :=
rfl
-- note: unlike with `Subring`, `pointwise_smul_toAddSubgroup` wouldn't be true
theorem smul_mem_pointwise_smul (m : M) (r : R) (S : Ideal R) : r ∈ S → m • r ∈ m • S :=
fun h => subset_span <| Set.smul_mem_smul_set h
instance : CovariantClass M (Ideal R) HSMul.hSMul LE.le :=
⟨fun _ _ => map_mono⟩
-- note: unlike with `Subring`, `mem_smul_pointwise_iff_exists` wouldn't be true
@[simp]
theorem smul_bot (a : M) : a • (⊥ : Ideal R) = ⊥ :=
map_bot
theorem smul_sup (a : M) (S T : Ideal R) : a • (S ⊔ T) = a • S ⊔ a • T :=
map_sup _ _ _
theorem smul_closure (a : M) (s : Set R) : a • span s = span (a • s) :=
Ideal.map_span _ _
instance pointwise_central_scalar [MulSemiringAction Mᵐᵒᵖ R] [IsCentralScalar M R] :
IsCentralScalar M (Ideal R) :=
⟨fun _ S => (congr_arg fun f => S.map f) <| RingHom.ext <| op_smul_eq_smul _⟩
end Monoid
section Group
variable [Group M] [CommRing R] [MulSemiringAction M R]
open Pointwise
theorem pointwise_smul_eq_comap {a : M} (S : Ideal R) :
a • S = S.comap (MulSemiringAction.toRingAut _ _ a).symm := by
ext
simp [pointwise_smul_def]
rfl
@[simp]
theorem smul_mem_pointwise_smul_iff {a : M} {S : Ideal R} {x : R} : a • x ∈ a • S ↔ x ∈ S :=
⟨fun h => by simpa using smul_mem_pointwise_smul a⁻¹ _ _ h, smul_mem_pointwise_smul _ _ _⟩
theorem mem_pointwise_smul_iff_inv_smul_mem {a : M} {S : Ideal R} {x : R} :
x ∈ a • S ↔ a⁻¹ • x ∈ S :=
⟨fun h => by simpa using smul_mem_pointwise_smul a⁻¹ _ _ h,
fun h => by simpa using smul_mem_pointwise_smul a _ _ h⟩
@[simp]
theorem pointwise_smul_toAddSubmonoid (a : M) (S : Ideal R)
(ha : Function.Surjective fun r : R => a • r) :
(a • S).toAddSubmonoid = a • S.toAddSubmonoid := by
ext
exact Ideal.mem_map_iff_of_surjective _ <| by exact ha
@[simp]
theorem pointwise_smul_toAddSubGroup (a : M) (S : Ideal R)
(ha : Function.Surjective fun r : R => a • r) :
(a • S).toAddSubgroup = a • S.toAddSubgroup := by
ext
exact Ideal.mem_map_iff_of_surjective _ <| by exact ha
theorem mem_inv_pointwise_smul_iff {a : M} {S : Ideal R} {x : R} : x ∈ a⁻¹ • S ↔ a • x ∈ S := by
rw [mem_pointwise_smul_iff_inv_smul_mem, inv_inv]
@[simp]
theorem pointwise_smul_le_pointwise_smul_iff {a : M} {S T : Ideal R} : a • S ≤ a • T ↔ S ≤ T :=
⟨fun h => by simpa using smul_mono_right a⁻¹ h, fun h => smul_mono_right a h⟩
theorem pointwise_smul_subset_iff {a : M} {S T : Ideal R} : a • S ≤ T ↔ S ≤ a⁻¹ • T := by
rw [← pointwise_smul_le_pointwise_smul_iff (a := a⁻¹), inv_smul_smul]
theorem subset_pointwise_smul_iff {a : M} {S T : Ideal R} : S ≤ a • T ↔ a⁻¹ • S ≤ T := by
rw [← pointwise_smul_le_pointwise_smul_iff (a := a⁻¹), inv_smul_smul]
/-! TODO: add `equivSMul` like we have for subgroup. -/
end Group
end Ideal
|
RingTheory\Ideal\Prod.lean | /-
Copyright (c) 2020 Markus Himmel. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Markus Himmel
-/
import Mathlib.RingTheory.Ideal.Maps
/-!
# Ideals in product rings
For commutative rings `R` and `S` and ideals `I ≤ R`, `J ≤ S`, we define `Ideal.prod I J` as the
product `I × J`, viewed as an ideal of `R × S`. In `ideal_prod_eq` we show that every ideal of
`R × S` is of this form. Furthermore, we show that every prime ideal of `R × S` is of the form
`p × S` or `R × p`, where `p` is a prime ideal.
-/
universe u v
variable {R : Type u} {S : Type v} [Semiring R] [Semiring S] (I I' : Ideal R) (J J' : Ideal S)
namespace Ideal
/-- `I × J` as an ideal of `R × S`. -/
def prod : Ideal (R × S) where
carrier := { x | x.fst ∈ I ∧ x.snd ∈ J }
zero_mem' := by simp
add_mem' := by
rintro ⟨a₁, a₂⟩ ⟨b₁, b₂⟩ ⟨ha₁, ha₂⟩ ⟨hb₁, hb₂⟩
exact ⟨I.add_mem ha₁ hb₁, J.add_mem ha₂ hb₂⟩
smul_mem' := by
rintro ⟨a₁, a₂⟩ ⟨b₁, b₂⟩ ⟨hb₁, hb₂⟩
exact ⟨I.mul_mem_left _ hb₁, J.mul_mem_left _ hb₂⟩
@[simp]
theorem mem_prod {r : R} {s : S} : (⟨r, s⟩ : R × S) ∈ prod I J ↔ r ∈ I ∧ s ∈ J :=
Iff.rfl
@[simp]
theorem prod_top_top : prod (⊤ : Ideal R) (⊤ : Ideal S) = ⊤ :=
Ideal.ext <| by simp
/-- Every ideal of the product ring is of the form `I × J`, where `I` and `J` can be explicitly
given as the image under the projection maps. -/
theorem ideal_prod_eq (I : Ideal (R × S)) :
I = Ideal.prod (map (RingHom.fst R S) I : Ideal R) (map (RingHom.snd R S) I) := by
apply Ideal.ext
rintro ⟨r, s⟩
rw [mem_prod, mem_map_iff_of_surjective (RingHom.fst R S) Prod.fst_surjective,
mem_map_iff_of_surjective (RingHom.snd R S) Prod.snd_surjective]
refine ⟨fun h => ⟨⟨_, ⟨h, rfl⟩⟩, ⟨_, ⟨h, rfl⟩⟩⟩, ?_⟩
rintro ⟨⟨⟨r, s'⟩, ⟨h₁, rfl⟩⟩, ⟨⟨r', s⟩, ⟨h₂, rfl⟩⟩⟩
simpa using I.add_mem (I.mul_mem_left (1, 0) h₁) (I.mul_mem_left (0, 1) h₂)
@[simp]
theorem map_fst_prod (I : Ideal R) (J : Ideal S) : map (RingHom.fst R S) (prod I J) = I := by
ext x
rw [mem_map_iff_of_surjective (RingHom.fst R S) Prod.fst_surjective]
exact
⟨by
rintro ⟨x, ⟨h, rfl⟩⟩
exact h.1, fun h => ⟨⟨x, 0⟩, ⟨⟨h, Ideal.zero_mem _⟩, rfl⟩⟩⟩
@[simp]
theorem map_snd_prod (I : Ideal R) (J : Ideal S) : map (RingHom.snd R S) (prod I J) = J := by
ext x
rw [mem_map_iff_of_surjective (RingHom.snd R S) Prod.snd_surjective]
exact
⟨by
rintro ⟨x, ⟨h, rfl⟩⟩
exact h.2, fun h => ⟨⟨0, x⟩, ⟨⟨Ideal.zero_mem _, h⟩, rfl⟩⟩⟩
@[simp]
theorem map_prodComm_prod :
map ((RingEquiv.prodComm : R × S ≃+* S × R) : R × S →+* S × R) (prod I J) = prod J I := by
refine Trans.trans (ideal_prod_eq _) ?_
simp [map_map]
/-- Ideals of `R × S` are in one-to-one correspondence with pairs of ideals of `R` and ideals of
`S`. -/
def idealProdEquiv : Ideal (R × S) ≃ Ideal R × Ideal S where
toFun I := ⟨map (RingHom.fst R S) I, map (RingHom.snd R S) I⟩
invFun I := prod I.1 I.2
left_inv I := (ideal_prod_eq I).symm
right_inv := fun ⟨I, J⟩ => by simp
@[simp]
theorem idealProdEquiv_symm_apply (I : Ideal R) (J : Ideal S) :
idealProdEquiv.symm ⟨I, J⟩ = prod I J :=
rfl
theorem prod.ext_iff {I I' : Ideal R} {J J' : Ideal S} :
prod I J = prod I' J' ↔ I = I' ∧ J = J' := by
simp only [← idealProdEquiv_symm_apply, idealProdEquiv.symm.injective.eq_iff, Prod.mk.inj_iff]
theorem isPrime_of_isPrime_prod_top {I : Ideal R} (h : (Ideal.prod I (⊤ : Ideal S)).IsPrime) :
I.IsPrime := by
constructor
· contrapose! h
rw [h, prod_top_top, isPrime_iff]
simp [isPrime_iff, h]
· intro x y hxy
have : (⟨x, 1⟩ : R × S) * ⟨y, 1⟩ ∈ prod I ⊤ := by
rw [Prod.mk_mul_mk, mul_one, mem_prod]
exact ⟨hxy, trivial⟩
simpa using h.mem_or_mem this
theorem isPrime_of_isPrime_prod_top' {I : Ideal S} (h : (Ideal.prod (⊤ : Ideal R) I).IsPrime) :
I.IsPrime := by
apply isPrime_of_isPrime_prod_top (S := R)
rw [← map_prodComm_prod]
-- Note: couldn't synthesize the right instances without the `R` and `S` hints
exact map_isPrime_of_equiv (RingEquiv.prodComm (R := R) (S := S))
theorem isPrime_ideal_prod_top {I : Ideal R} [h : I.IsPrime] : (prod I (⊤ : Ideal S)).IsPrime := by
constructor
· rcases h with ⟨h, -⟩
contrapose! h
rw [← prod_top_top, prod.ext_iff] at h
exact h.1
rintro ⟨r₁, s₁⟩ ⟨r₂, s₂⟩ ⟨h₁, _⟩
cases' h.mem_or_mem h₁ with h h
· exact Or.inl ⟨h, trivial⟩
· exact Or.inr ⟨h, trivial⟩
theorem isPrime_ideal_prod_top' {I : Ideal S} [h : I.IsPrime] : (prod (⊤ : Ideal R) I).IsPrime := by
letI : IsPrime (prod I (⊤ : Ideal R)) := isPrime_ideal_prod_top
rw [← map_prodComm_prod]
-- Note: couldn't synthesize the right instances without the `R` and `S` hints
exact map_isPrime_of_equiv (RingEquiv.prodComm (R := S) (S := R))
theorem ideal_prod_prime_aux {I : Ideal R} {J : Ideal S} :
(Ideal.prod I J).IsPrime → I = ⊤ ∨ J = ⊤ := by
contrapose!
simp only [ne_top_iff_one, isPrime_iff, not_and, not_forall, not_or]
exact fun ⟨hI, hJ⟩ _ => ⟨⟨0, 1⟩, ⟨1, 0⟩, by simp, by simp [hJ], by simp [hI]⟩
/-- Classification of prime ideals in product rings: the prime ideals of `R × S` are precisely the
ideals of the form `p × S` or `R × p`, where `p` is a prime ideal of `R` or `S`. -/
theorem ideal_prod_prime (I : Ideal (R × S)) :
I.IsPrime ↔
(∃ p : Ideal R, p.IsPrime ∧ I = Ideal.prod p ⊤) ∨
∃ p : Ideal S, p.IsPrime ∧ I = Ideal.prod ⊤ p := by
constructor
· rw [ideal_prod_eq I]
intro hI
rcases ideal_prod_prime_aux hI with (h | h)
· right
rw [h] at hI ⊢
exact ⟨_, ⟨isPrime_of_isPrime_prod_top' hI, rfl⟩⟩
· left
rw [h] at hI ⊢
exact ⟨_, ⟨isPrime_of_isPrime_prod_top hI, rfl⟩⟩
· rintro (⟨p, ⟨h, rfl⟩⟩ | ⟨p, ⟨h, rfl⟩⟩)
· exact isPrime_ideal_prod_top
· exact isPrime_ideal_prod_top'
end Ideal
|
RingTheory\Ideal\Quotient.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro, Anne Baanen
-/
import Mathlib.LinearAlgebra.Quotient
import Mathlib.RingTheory.Congruence.Basic
import Mathlib.RingTheory.Ideal.Basic
import Mathlib.Tactic.FinCases
/-!
# Ideal quotients
This file defines ideal quotients as a special case of submodule quotients and proves some basic
results about these quotients.
See `Algebra.RingQuot` for quotients of non-commutative rings.
## Main definitions
- `Ideal.Quotient`: the quotient of a commutative ring `R` by an ideal `I : Ideal R`
-/
universe u v w
namespace Ideal
open Set
variable {R : Type u} [CommRing R] (I : Ideal R) {a b : R}
variable {S : Type v}
-- Note that at present `Ideal` means a left-ideal,
-- so this quotient is only useful in a commutative ring.
-- We should develop quotients by two-sided ideals as well.
/-- The quotient `R/I` of a ring `R` by an ideal `I`.
The ideal quotient of `I` is defined to equal the quotient of `I` as an `R`-submodule of `R`.
This definition uses `abbrev` so that typeclass instances can be shared between
`Ideal.Quotient I` and `Submodule.Quotient I`.
-/
@[instance] abbrev instHasQuotient : HasQuotient R (Ideal R) :=
Submodule.hasQuotient
namespace Quotient
variable {I} {x y : R}
instance one (I : Ideal R) : One (R ⧸ I) :=
⟨Submodule.Quotient.mk 1⟩
/-- On `Ideal`s, `Submodule.quotientRel` is a ring congruence. -/
protected def ringCon (I : Ideal R) : RingCon R :=
{ QuotientAddGroup.con I.toAddSubgroup with
mul' := fun {a₁ b₁ a₂ b₂} h₁ h₂ => by
rw [Submodule.quotientRel_r_def] at h₁ h₂ ⊢
have F := I.add_mem (I.mul_mem_left a₂ h₁) (I.mul_mem_right b₁ h₂)
have : a₁ * a₂ - b₁ * b₂ = a₂ * (a₁ - b₁) + (a₂ - b₂) * b₁ := by
rw [mul_sub, sub_mul, sub_add_sub_cancel, mul_comm, mul_comm b₁]
rwa [← this] at F }
instance commRing (I : Ideal R) : CommRing (R ⧸ I) :=
inferInstanceAs (CommRing (Quotient.ringCon I).Quotient)
-- Sanity test to make sure no diamonds have emerged in `commRing`
example : (commRing I).toAddCommGroup = Submodule.Quotient.addCommGroup I := rfl
-- this instance is harder to find than the one via `Algebra α (R ⧸ I)`, so use a lower priority
instance (priority := 100) isScalarTower_right {α} [SMul α R] [IsScalarTower α R R] :
IsScalarTower α (R ⧸ I) (R ⧸ I) :=
(Quotient.ringCon I).isScalarTower_right
instance smulCommClass {α} [SMul α R] [IsScalarTower α R R] [SMulCommClass α R R] :
SMulCommClass α (R ⧸ I) (R ⧸ I) :=
(Quotient.ringCon I).smulCommClass
instance smulCommClass' {α} [SMul α R] [IsScalarTower α R R] [SMulCommClass R α R] :
SMulCommClass (R ⧸ I) α (R ⧸ I) :=
(Quotient.ringCon I).smulCommClass'
/-- The ring homomorphism from a ring `R` to a quotient ring `R/I`. -/
def mk (I : Ideal R) : R →+* R ⧸ I where
toFun a := Submodule.Quotient.mk a
map_zero' := rfl
map_one' := rfl
map_mul' _ _ := rfl
map_add' _ _ := rfl
instance {I : Ideal R} : Coe R (R ⧸ I) :=
⟨Ideal.Quotient.mk I⟩
/-- Two `RingHom`s from the quotient by an ideal are equal if their
compositions with `Ideal.Quotient.mk'` are equal.
See note [partially-applied ext lemmas]. -/
@[ext 1100]
theorem ringHom_ext [NonAssocSemiring S] ⦃f g : R ⧸ I →+* S⦄ (h : f.comp (mk I) = g.comp (mk I)) :
f = g :=
RingHom.ext fun x => Quotient.inductionOn' x <| (RingHom.congr_fun h : _)
instance inhabited : Inhabited (R ⧸ I) :=
⟨mk I 37⟩
protected theorem eq : mk I x = mk I y ↔ x - y ∈ I :=
Submodule.Quotient.eq I
@[simp]
theorem mk_eq_mk (x : R) : (Submodule.Quotient.mk x : R ⧸ I) = mk I x := rfl
theorem eq_zero_iff_mem {I : Ideal R} : mk I a = 0 ↔ a ∈ I :=
Submodule.Quotient.mk_eq_zero _
theorem eq_zero_iff_dvd (x y : R) : Ideal.Quotient.mk (Ideal.span ({x} : Set R)) y = 0 ↔ x ∣ y := by
rw [Ideal.Quotient.eq_zero_iff_mem, Ideal.mem_span_singleton]
@[simp]
lemma mk_singleton_self (x : R) : mk (Ideal.span {x}) x = 0 := by
rw [eq_zero_iff_dvd]
theorem mk_eq_mk_iff_sub_mem (x y : R) : mk I x = mk I y ↔ x - y ∈ I := by
rw [← eq_zero_iff_mem, map_sub, sub_eq_zero]
theorem zero_eq_one_iff {I : Ideal R} : (0 : R ⧸ I) = 1 ↔ I = ⊤ :=
eq_comm.trans <| eq_zero_iff_mem.trans (eq_top_iff_one _).symm
theorem zero_ne_one_iff {I : Ideal R} : (0 : R ⧸ I) ≠ 1 ↔ I ≠ ⊤ :=
not_congr zero_eq_one_iff
protected theorem nontrivial {I : Ideal R} (hI : I ≠ ⊤) : Nontrivial (R ⧸ I) :=
⟨⟨0, 1, zero_ne_one_iff.2 hI⟩⟩
theorem subsingleton_iff {I : Ideal R} : Subsingleton (R ⧸ I) ↔ I = ⊤ := by
rw [eq_top_iff_one, ← subsingleton_iff_zero_eq_one, eq_comm, ← (mk I).map_one,
Quotient.eq_zero_iff_mem]
instance : Unique (R ⧸ (⊤ : Ideal R)) :=
⟨⟨0⟩, by rintro ⟨x⟩; exact Quotient.eq_zero_iff_mem.mpr Submodule.mem_top⟩
theorem mk_surjective : Function.Surjective (mk I) := fun y =>
Quotient.inductionOn' y fun x => Exists.intro x rfl
instance : RingHomSurjective (mk I) :=
⟨mk_surjective⟩
/-- If `I` is an ideal of a commutative ring `R`, if `q : R → R/I` is the quotient map, and if
`s ⊆ R` is a subset, then `q⁻¹(q(s)) = ⋃ᵢ(i + s)`, the union running over all `i ∈ I`. -/
theorem quotient_ring_saturate (I : Ideal R) (s : Set R) :
mk I ⁻¹' (mk I '' s) = ⋃ x : I, (fun y => x.1 + y) '' s := by
ext x
simp only [mem_preimage, mem_image, mem_iUnion, Ideal.Quotient.eq]
exact
⟨fun ⟨a, a_in, h⟩ => ⟨⟨_, I.neg_mem h⟩, a, a_in, by simp⟩, fun ⟨⟨i, hi⟩, a, ha, Eq⟩ =>
⟨a, ha, by rw [← Eq, sub_add_eq_sub_sub_swap, sub_self, zero_sub]; exact I.neg_mem hi⟩⟩
instance noZeroDivisors (I : Ideal R) [hI : I.IsPrime] : NoZeroDivisors (R ⧸ I) where
eq_zero_or_eq_zero_of_mul_eq_zero {a b} := Quotient.inductionOn₂' a b fun {_ _} hab =>
(hI.mem_or_mem (eq_zero_iff_mem.1 hab)).elim (Or.inl ∘ eq_zero_iff_mem.2)
(Or.inr ∘ eq_zero_iff_mem.2)
instance isDomain (I : Ideal R) [hI : I.IsPrime] : IsDomain (R ⧸ I) :=
let _ := Quotient.nontrivial hI.1
NoZeroDivisors.to_isDomain _
theorem isDomain_iff_prime (I : Ideal R) : IsDomain (R ⧸ I) ↔ I.IsPrime := by
refine ⟨fun H => ⟨zero_ne_one_iff.1 ?_, fun {x y} h => ?_⟩, fun h => inferInstance⟩
· haveI : Nontrivial (R ⧸ I) := ⟨H.2.1⟩
exact zero_ne_one
· simp only [← eq_zero_iff_mem, (mk I).map_mul] at h ⊢
haveI := @IsDomain.to_noZeroDivisors (R ⧸ I) _ H
exact eq_zero_or_eq_zero_of_mul_eq_zero h
theorem exists_inv {I : Ideal R} [hI : I.IsMaximal] :
∀ {a : R ⧸ I}, a ≠ 0 → ∃ b : R ⧸ I, a * b = 1 := by
rintro ⟨a⟩ h
rcases hI.exists_inv (mt eq_zero_iff_mem.2 h) with ⟨b, c, hc, abc⟩
rw [mul_comm] at abc
refine ⟨mk _ b, Quot.sound ?_⟩
simp only [Submodule.quotientRel_r_def]
rw [← eq_sub_iff_add_eq'] at abc
rwa [abc, ← neg_mem_iff (G := R) (H := I), neg_sub] at hc
open Classical in
/-- The quotient by a maximal ideal is a group with zero. This is a `def` rather than `instance`,
since users will have computable inverses in some applications.
See note [reducible non-instances]. -/
protected noncomputable abbrev groupWithZero (I : Ideal R) [hI : I.IsMaximal] :
GroupWithZero (R ⧸ I) :=
{ inv := fun a => if ha : a = 0 then 0 else Classical.choose (exists_inv ha)
mul_inv_cancel := fun a (ha : a ≠ 0) =>
show a * dite _ _ _ = _ by rw [dif_neg ha]; exact Classical.choose_spec (exists_inv ha)
inv_zero := dif_pos rfl }
/-- The quotient by a maximal ideal is a field. This is a `def` rather than `instance`, since users
will have computable inverses (and `qsmul`, `ratCast`) in some applications.
See note [reducible non-instances]. -/
protected noncomputable abbrev field (I : Ideal R) [hI : I.IsMaximal] : Field (R ⧸ I) where
__ := commRing _
__ := Quotient.groupWithZero _
nnqsmul := _
nnqsmul_def := fun q a => rfl
qsmul := _
qsmul_def := fun q x => rfl
/-- If the quotient by an ideal is a field, then the ideal is maximal. -/
theorem maximal_of_isField (I : Ideal R) (hqf : IsField (R ⧸ I)) : I.IsMaximal := by
apply Ideal.isMaximal_iff.2
constructor
· intro h
rcases hqf.exists_pair_ne with ⟨⟨x⟩, ⟨y⟩, hxy⟩
exact hxy (Ideal.Quotient.eq.2 (mul_one (x - y) ▸ I.mul_mem_left _ h))
· intro J x hIJ hxnI hxJ
rcases hqf.mul_inv_cancel (mt Ideal.Quotient.eq_zero_iff_mem.1 hxnI) with ⟨⟨y⟩, hy⟩
rw [← zero_add (1 : R), ← sub_self (x * y), sub_add]
exact J.sub_mem (J.mul_mem_right _ hxJ) (hIJ (Ideal.Quotient.eq.1 hy))
/-- The quotient of a ring by an ideal is a field iff the ideal is maximal. -/
theorem maximal_ideal_iff_isField_quotient (I : Ideal R) : I.IsMaximal ↔ IsField (R ⧸ I) :=
⟨fun h =>
let _i := @Quotient.field _ _ I h
Field.toIsField _,
maximal_of_isField _⟩
variable [Semiring S]
/-- Given a ring homomorphism `f : R →+* S` sending all elements of an ideal to zero,
lift it to the quotient by this ideal. -/
def lift (I : Ideal R) (f : R →+* S) (H : ∀ a : R, a ∈ I → f a = 0) : R ⧸ I →+* S :=
{ QuotientAddGroup.lift I.toAddSubgroup f.toAddMonoidHom H with
map_one' := f.map_one
map_mul' := fun a₁ a₂ => Quotient.inductionOn₂' a₁ a₂ f.map_mul }
@[simp]
theorem lift_mk (I : Ideal R) (f : R →+* S) (H : ∀ a : R, a ∈ I → f a = 0) :
lift I f H (mk I a) = f a :=
rfl
theorem lift_surjective_of_surjective (I : Ideal R) {f : R →+* S} (H : ∀ a : R, a ∈ I → f a = 0)
(hf : Function.Surjective f) : Function.Surjective (Ideal.Quotient.lift I f H) := by
intro y
obtain ⟨x, rfl⟩ := hf y
use Ideal.Quotient.mk I x
simp only [Ideal.Quotient.lift_mk]
/-- The ring homomorphism from the quotient by a smaller ideal to the quotient by a larger ideal.
This is the `Ideal.Quotient` version of `Quot.Factor` -/
def factor (S T : Ideal R) (H : S ≤ T) : R ⧸ S →+* R ⧸ T :=
Ideal.Quotient.lift S (mk T) fun _ hx => eq_zero_iff_mem.2 (H hx)
@[simp]
theorem factor_mk (S T : Ideal R) (H : S ≤ T) (x : R) : factor S T H (mk S x) = mk T x :=
rfl
@[simp]
theorem factor_comp_mk (S T : Ideal R) (H : S ≤ T) : (factor S T H).comp (mk S) = mk T := by
ext x
rw [RingHom.comp_apply, factor_mk]
end Quotient
/-- Quotienting by equal ideals gives equivalent rings.
See also `Submodule.quotEquivOfEq` and `Ideal.quotientEquivAlgOfEq`.
-/
def quotEquivOfEq {R : Type*} [CommRing R] {I J : Ideal R} (h : I = J) : R ⧸ I ≃+* R ⧸ J :=
{ Submodule.quotEquivOfEq I J h with
map_mul' := by
rintro ⟨x⟩ ⟨y⟩
rfl }
@[simp]
theorem quotEquivOfEq_mk {R : Type*} [CommRing R] {I J : Ideal R} (h : I = J) (x : R) :
quotEquivOfEq h (Ideal.Quotient.mk I x) = Ideal.Quotient.mk J x :=
rfl
@[simp]
theorem quotEquivOfEq_symm {R : Type*} [CommRing R] {I J : Ideal R} (h : I = J) :
(Ideal.quotEquivOfEq h).symm = Ideal.quotEquivOfEq h.symm := by ext; rfl
section Pi
variable (ι : Type v)
/-- `R^n/I^n` is a `R/I`-module. -/
instance modulePi : Module (R ⧸ I) ((ι → R) ⧸ I.pi ι) where
smul c m :=
Quotient.liftOn₂' c m (fun r m => Submodule.Quotient.mk <| r • m) <| by
intro c₁ m₁ c₂ m₂ hc hm
apply Ideal.Quotient.eq.2
rw [Submodule.quotientRel_r_def] at hc hm
intro i
exact I.mul_sub_mul_mem hc (hm i)
one_smul := by
rintro ⟨a⟩
convert_to Ideal.Quotient.mk (I.pi ι) _ = Ideal.Quotient.mk (I.pi ι) _
congr with i; exact one_mul (a i)
mul_smul := by
rintro ⟨a⟩ ⟨b⟩ ⟨c⟩
convert_to Ideal.Quotient.mk (I.pi ι) _ = Ideal.Quotient.mk (I.pi ι) _
congr 1; funext i; exact mul_assoc a b (c i)
smul_add := by
rintro ⟨a⟩ ⟨b⟩ ⟨c⟩
convert_to Ideal.Quotient.mk (I.pi ι) _ = Ideal.Quotient.mk (I.pi ι) _
congr with i; exact mul_add a (b i) (c i)
smul_zero := by
rintro ⟨a⟩
convert_to Ideal.Quotient.mk (I.pi ι) _ = Ideal.Quotient.mk (I.pi ι) _
congr with _; exact mul_zero a
add_smul := by
rintro ⟨a⟩ ⟨b⟩ ⟨c⟩
convert_to Ideal.Quotient.mk (I.pi ι) _ = Ideal.Quotient.mk (I.pi ι) _
congr with i; exact add_mul a b (c i)
zero_smul := by
rintro ⟨a⟩
convert_to Ideal.Quotient.mk (I.pi ι) _ = Ideal.Quotient.mk (I.pi ι) _
congr with i; exact zero_mul (a i)
/-- `R^n/I^n` is isomorphic to `(R/I)^n` as an `R/I`-module. -/
noncomputable def piQuotEquiv : ((ι → R) ⧸ I.pi ι) ≃ₗ[R ⧸ I] ι → (R ⧸ I) where
toFun := fun x ↦
Quotient.liftOn' x (fun f i => Ideal.Quotient.mk I (f i)) fun a b hab =>
funext fun i => (Submodule.Quotient.eq' _).2 (QuotientAddGroup.leftRel_apply.mp hab i)
map_add' := by rintro ⟨_⟩ ⟨_⟩; rfl
map_smul' := by rintro ⟨_⟩ ⟨_⟩; rfl
invFun := fun x ↦ Ideal.Quotient.mk (I.pi ι) fun i ↦ Quotient.out' (x i)
left_inv := by
rintro ⟨x⟩
exact Ideal.Quotient.eq.2 fun i => Ideal.Quotient.eq.1 (Quotient.out_eq' _)
right_inv := by
intro x
ext i
obtain ⟨_, _⟩ := @Quot.exists_rep _ _ (x i)
convert Quotient.out_eq' (x i)
/-- If `f : R^n → R^m` is an `R`-linear map and `I ⊆ R` is an ideal, then the image of `I^n` is
contained in `I^m`. -/
theorem map_pi {ι : Type*} [Finite ι] {ι' : Type w} (x : ι → R) (hi : ∀ i, x i ∈ I)
(f : (ι → R) →ₗ[R] ι' → R) (i : ι') : f x i ∈ I := by
classical
cases nonempty_fintype ι
rw [pi_eq_sum_univ x]
simp only [Finset.sum_apply, smul_eq_mul, map_sum, Pi.smul_apply, map_smul]
exact I.sum_mem fun j _ => I.mul_mem_right _ (hi j)
end Pi
end Ideal
|
RingTheory\Ideal\QuotientOperations.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Patrick Massot
-/
import Mathlib.Algebra.Algebra.Subalgebra.Operations
import Mathlib.Algebra.Ring.Fin
import Mathlib.RingTheory.Ideal.Quotient
/-!
# More operations on modules and ideals related to quotients
## Main results:
- `RingHom.quotientKerEquivRange` : the **first isomorphism theorem** for commutative rings.
- `RingHom.quotientKerEquivRangeS` : the **first isomorphism theorem**
for a morphism from a commutative ring to a semiring.
- `AlgHom.quotientKerEquivRange` : the **first isomorphism theorem**
for a morphism of algebras (over a commutative semiring)
- `RingHom.quotientKerEquivRangeS` : the **first isomorphism theorem**
for a morphism from a commutative ring to a semiring.
- `Ideal.quotientInfRingEquivPiQuotient`: the **Chinese Remainder Theorem**, version for coprime
ideals (see also `ZMod.prodEquivPi` in `Data.ZMod.Quotient` for elementary versions about
`ZMod`).
-/
universe u v w
namespace RingHom
variable {R : Type u} {S : Type v} [CommRing R] [Semiring S] (f : R →+* S)
/-- The induced map from the quotient by the kernel to the codomain.
This is an isomorphism if `f` has a right inverse (`quotientKerEquivOfRightInverse`) /
is surjective (`quotientKerEquivOfSurjective`).
-/
def kerLift : R ⧸ ker f →+* S :=
Ideal.Quotient.lift _ f fun _ => f.mem_ker.mp
@[simp]
theorem kerLift_mk (r : R) : kerLift f (Ideal.Quotient.mk (ker f) r) = f r :=
Ideal.Quotient.lift_mk _ _ _
theorem lift_injective_of_ker_le_ideal (I : Ideal R) {f : R →+* S} (H : ∀ a : R, a ∈ I → f a = 0)
(hI : ker f ≤ I) : Function.Injective (Ideal.Quotient.lift I f H) := by
rw [RingHom.injective_iff_ker_eq_bot, RingHom.ker_eq_bot_iff_eq_zero]
intro u hu
obtain ⟨v, rfl⟩ := Ideal.Quotient.mk_surjective u
rw [Ideal.Quotient.lift_mk] at hu
rw [Ideal.Quotient.eq_zero_iff_mem]
exact hI ((RingHom.mem_ker f).mpr hu)
/-- The induced map from the quotient by the kernel is injective. -/
theorem kerLift_injective : Function.Injective (kerLift f) :=
lift_injective_of_ker_le_ideal (ker f) (fun a => by simp only [mem_ker, imp_self]) le_rfl
variable {f}
/-- The **first isomorphism theorem for commutative rings**, computable version. -/
def quotientKerEquivOfRightInverse {g : S → R} (hf : Function.RightInverse g f) :
R ⧸ ker f ≃+* S :=
{ kerLift f with
toFun := kerLift f
invFun := Ideal.Quotient.mk (ker f) ∘ g
left_inv := by
rintro ⟨x⟩
apply kerLift_injective
simp only [Submodule.Quotient.quot_mk_eq_mk, Ideal.Quotient.mk_eq_mk, kerLift_mk,
Function.comp_apply, hf (f x)]
right_inv := hf }
@[simp]
theorem quotientKerEquivOfRightInverse.apply {g : S → R} (hf : Function.RightInverse g f)
(x : R ⧸ ker f) : quotientKerEquivOfRightInverse hf x = kerLift f x :=
rfl
@[simp]
theorem quotientKerEquivOfRightInverse.Symm.apply {g : S → R} (hf : Function.RightInverse g f)
(x : S) : (quotientKerEquivOfRightInverse hf).symm x = Ideal.Quotient.mk (ker f) (g x) :=
rfl
variable (R) in
/-- The quotient of a ring by he zero ideal is isomorphic to the ring itself. -/
def _root_.RingEquiv.quotientBot : R ⧸ (⊥ : Ideal R) ≃+* R :=
(Ideal.quotEquivOfEq (RingHom.ker_coe_equiv <| .refl _).symm).trans <|
quotientKerEquivOfRightInverse (f := .id R) (g := _root_.id) fun _ ↦ rfl
/-- The **first isomorphism theorem** for commutative rings, surjective case. -/
noncomputable def quotientKerEquivOfSurjective (hf : Function.Surjective f) : R ⧸ (ker f) ≃+* S :=
quotientKerEquivOfRightInverse (Classical.choose_spec hf.hasRightInverse)
/-- The **first isomorphism theorem** for commutative rings (`RingHom.rangeS` version). -/
noncomputable def quotientKerEquivRangeS (f : R →+* S) : R ⧸ ker f ≃+* f.rangeS :=
(Ideal.quotEquivOfEq f.ker_rangeSRestrict.symm).trans <|
quotientKerEquivOfSurjective f.rangeSRestrict_surjective
variable {S : Type v} [Ring S] (f : R →+* S)
/-- The **first isomorphism theorem** for commutative rings (`RingHom.range` version). -/
noncomputable def quotientKerEquivRange (f : R →+* S) : R ⧸ ker f ≃+* f.range :=
(Ideal.quotEquivOfEq f.ker_rangeRestrict.symm).trans <|
quotientKerEquivOfSurjective f.rangeRestrict_surjective
end RingHom
namespace Ideal
open Function RingHom
variable {R : Type u} {S : Type v} {F : Type w} [CommRing R] [Semiring S]
@[simp]
theorem map_quotient_self (I : Ideal R) : map (Quotient.mk I) I = ⊥ :=
eq_bot_iff.2 <|
Ideal.map_le_iff_le_comap.2 fun _ hx =>
(Submodule.mem_bot (R ⧸ I)).2 <| Ideal.Quotient.eq_zero_iff_mem.2 hx
@[simp]
theorem mk_ker {I : Ideal R} : ker (Quotient.mk I) = I := by
ext
rw [ker, mem_comap, Submodule.mem_bot, Quotient.eq_zero_iff_mem]
theorem map_mk_eq_bot_of_le {I J : Ideal R} (h : I ≤ J) : I.map (Quotient.mk J) = ⊥ := by
rw [map_eq_bot_iff_le_ker, mk_ker]
exact h
theorem ker_quotient_lift {I : Ideal R} (f : R →+* S)
(H : I ≤ ker f) :
ker (Ideal.Quotient.lift I f H) = f.ker.map (Quotient.mk I) := by
apply Ideal.ext
intro x
constructor
· intro hx
obtain ⟨y, hy⟩ := Quotient.mk_surjective x
rw [mem_ker, ← hy, Ideal.Quotient.lift_mk, ← mem_ker] at hx
rw [← hy, mem_map_iff_of_surjective (Quotient.mk I) Quotient.mk_surjective]
exact ⟨y, hx, rfl⟩
· intro hx
rw [mem_map_iff_of_surjective (Quotient.mk I) Quotient.mk_surjective] at hx
obtain ⟨y, hy⟩ := hx
rw [mem_ker, ← hy.right, Ideal.Quotient.lift_mk]
exact hy.left
lemma injective_lift_iff {I : Ideal R} {f : R →+* S} (H : ∀ (a : R), a ∈ I → f a = 0) :
Injective (Quotient.lift I f H) ↔ ker f = I := by
rw [injective_iff_ker_eq_bot, ker_quotient_lift, map_eq_bot_iff_le_ker, mk_ker]
constructor
· exact fun h ↦ le_antisymm h H
· rintro rfl; rfl
lemma ker_Pi_Quotient_mk {ι : Type*} (I : ι → Ideal R) :
ker (Pi.ringHom fun i : ι ↦ Quotient.mk (I i)) = ⨅ i, I i := by
simp [Pi.ker_ringHom, mk_ker]
@[simp]
theorem bot_quotient_isMaximal_iff (I : Ideal R) : (⊥ : Ideal (R ⧸ I)).IsMaximal ↔ I.IsMaximal :=
⟨fun hI =>
mk_ker (I := I) ▸
comap_isMaximal_of_surjective (Quotient.mk I) Quotient.mk_surjective (K := ⊥) (H := hI),
fun hI => by
letI := Quotient.field I
exact bot_isMaximal⟩
/-- See also `Ideal.mem_quotient_iff_mem` in case `I ≤ J`. -/
@[simp]
theorem mem_quotient_iff_mem_sup {I J : Ideal R} {x : R} :
Quotient.mk I x ∈ J.map (Quotient.mk I) ↔ x ∈ J ⊔ I := by
rw [← mem_comap, comap_map_of_surjective (Quotient.mk I) Quotient.mk_surjective, ←
ker_eq_comap_bot, mk_ker]
/-- See also `Ideal.mem_quotient_iff_mem_sup` if the assumption `I ≤ J` is not available. -/
theorem mem_quotient_iff_mem {I J : Ideal R} (hIJ : I ≤ J) {x : R} :
Quotient.mk I x ∈ J.map (Quotient.mk I) ↔ x ∈ J := by
rw [mem_quotient_iff_mem_sup, sup_eq_left.mpr hIJ]
section ChineseRemainder
open Function Quotient Finset
variable {ι : Type*}
/-- The homomorphism from `R/(⋂ i, f i)` to `∏ i, (R / f i)` featured in the Chinese
Remainder Theorem. It is bijective if the ideals `f i` are coprime. -/
def quotientInfToPiQuotient (I : ι → Ideal R) : (R ⧸ ⨅ i, I i) →+* ∀ i, R ⧸ I i :=
Quotient.lift (⨅ i, I i) (Pi.ringHom fun i : ι ↦ Quotient.mk (I i))
(by simp [← RingHom.mem_ker, ker_Pi_Quotient_mk])
lemma quotientInfToPiQuotient_mk (I : ι → Ideal R) (x : R) :
quotientInfToPiQuotient I (Quotient.mk _ x) = fun i : ι ↦ Quotient.mk (I i) x :=
rfl
lemma quotientInfToPiQuotient_mk' (I : ι → Ideal R) (x : R) (i : ι) :
quotientInfToPiQuotient I (Quotient.mk _ x) i = Quotient.mk (I i) x :=
rfl
lemma quotientInfToPiQuotient_inj (I : ι → Ideal R) : Injective (quotientInfToPiQuotient I) := by
rw [quotientInfToPiQuotient, injective_lift_iff, ker_Pi_Quotient_mk]
lemma quotientInfToPiQuotient_surj [Finite ι] {I : ι → Ideal R}
(hI : Pairwise fun i j => IsCoprime (I i) (I j)) : Surjective (quotientInfToPiQuotient I) := by
classical
cases nonempty_fintype ι
intro g
choose f hf using fun i ↦ mk_surjective (g i)
have key : ∀ i, ∃ e : R, mk (I i) e = 1 ∧ ∀ j, j ≠ i → mk (I j) e = 0 := by
intro i
have hI' : ∀ j ∈ ({i} : Finset ι)ᶜ, IsCoprime (I i) (I j) := by
intros j hj
exact hI (by simpa [ne_comm, isCoprime_iff_add] using hj)
rcases isCoprime_iff_exists.mp (isCoprime_biInf hI') with ⟨u, hu, e, he, hue⟩
replace he : ∀ j, j ≠ i → e ∈ I j := by simpa using he
refine ⟨e, ?_, ?_⟩
· simp [eq_sub_of_add_eq' hue, map_sub, eq_zero_iff_mem.mpr hu]
· exact fun j hj ↦ eq_zero_iff_mem.mpr (he j hj)
choose e he using key
use mk _ (∑ i, f i*e i)
ext i
rw [quotientInfToPiQuotient_mk', map_sum, Fintype.sum_eq_single i]
· simp [(he i).1, hf]
· intros j hj
simp [(he j).2 i hj.symm]
/-- **Chinese Remainder Theorem**. Eisenbud Ex.2.6.
Similar to Atiyah-Macdonald 1.10 and Stacks 00DT -/
noncomputable def quotientInfRingEquivPiQuotient [Finite ι] (f : ι → Ideal R)
(hf : Pairwise fun i j => IsCoprime (f i) (f j)) : (R ⧸ ⨅ i, f i) ≃+* ∀ i, R ⧸ f i :=
{ Equiv.ofBijective _ ⟨quotientInfToPiQuotient_inj f, quotientInfToPiQuotient_surj hf⟩,
quotientInfToPiQuotient f with }
/-- Corollary of Chinese Remainder Theorem: if `Iᵢ` are pairwise coprime ideals in a
commutative ring then the canonical map `R → ∏ (R ⧸ Iᵢ)` is surjective. -/
lemma pi_quotient_surjective {R : Type*} [CommRing R] {ι : Type*} [Finite ι] {I : ι → Ideal R}
(hf : Pairwise fun i j ↦ IsCoprime (I i) (I j)) (x : (i : ι) → R ⧸ I i) :
∃ r : R, ∀ i, r = x i := by
obtain ⟨y, rfl⟩ := Ideal.quotientInfToPiQuotient_surj hf x
obtain ⟨r, rfl⟩ := Ideal.Quotient.mk_surjective y
exact ⟨r, fun i ↦ rfl⟩
-- variant of `IsDedekindDomain.exists_forall_sub_mem_ideal` which doesn't assume Dedekind domain!
/-- Corollary of Chinese Remainder Theorem: if `Iᵢ` are pairwise coprime ideals in a
commutative ring then given elements `xᵢ` you can find `r` with `r - xᵢ ∈ Iᵢ` for all `i`. -/
lemma exists_forall_sub_mem_ideal {R : Type*} [CommRing R] {ι : Type*} [Finite ι]
{I : ι → Ideal R} (hI : Pairwise fun i j ↦ IsCoprime (I i) (I j)) (x : ι → R) :
∃ r : R, ∀ i, r - x i ∈ I i := by
obtain ⟨y, hy⟩ := Ideal.pi_quotient_surjective hI (fun i ↦ x i)
exact ⟨y, fun i ↦ (Submodule.Quotient.eq (I i)).mp <| hy i⟩
/-- **Chinese remainder theorem**, specialized to two ideals. -/
noncomputable def quotientInfEquivQuotientProd (I J : Ideal R) (coprime : IsCoprime I J) :
R ⧸ I ⊓ J ≃+* (R ⧸ I) × R ⧸ J :=
let f : Fin 2 → Ideal R := ![I, J]
have hf : Pairwise fun i j => IsCoprime (f i) (f j) := by
intro i j h
fin_cases i <;> fin_cases j <;> try contradiction
· assumption
· exact coprime.symm
(Ideal.quotEquivOfEq (by simp [f, iInf, inf_comm])).trans <|
(Ideal.quotientInfRingEquivPiQuotient f hf).trans <| RingEquiv.piFinTwo fun i => R ⧸ f i
@[simp]
theorem quotientInfEquivQuotientProd_fst (I J : Ideal R) (coprime : IsCoprime I J) (x : R ⧸ I ⊓ J) :
(quotientInfEquivQuotientProd I J coprime x).fst =
Ideal.Quotient.factor (I ⊓ J) I inf_le_left x :=
Quot.inductionOn x fun _ => rfl
@[simp]
theorem quotientInfEquivQuotientProd_snd (I J : Ideal R) (coprime : IsCoprime I J) (x : R ⧸ I ⊓ J) :
(quotientInfEquivQuotientProd I J coprime x).snd =
Ideal.Quotient.factor (I ⊓ J) J inf_le_right x :=
Quot.inductionOn x fun _ => rfl
@[simp]
theorem fst_comp_quotientInfEquivQuotientProd (I J : Ideal R) (coprime : IsCoprime I J) :
(RingHom.fst _ _).comp
(quotientInfEquivQuotientProd I J coprime : R ⧸ I ⊓ J →+* (R ⧸ I) × R ⧸ J) =
Ideal.Quotient.factor (I ⊓ J) I inf_le_left := by
apply Quotient.ringHom_ext; ext; rfl
@[simp]
theorem snd_comp_quotientInfEquivQuotientProd (I J : Ideal R) (coprime : IsCoprime I J) :
(RingHom.snd _ _).comp
(quotientInfEquivQuotientProd I J coprime : R ⧸ I ⊓ J →+* (R ⧸ I) × R ⧸ J) =
Ideal.Quotient.factor (I ⊓ J) J inf_le_right := by
apply Quotient.ringHom_ext; ext; rfl
/-- **Chinese remainder theorem**, specialized to two ideals. -/
noncomputable def quotientMulEquivQuotientProd (I J : Ideal R) (coprime : IsCoprime I J) :
R ⧸ I * J ≃+* (R ⧸ I) × R ⧸ J :=
Ideal.quotEquivOfEq (inf_eq_mul_of_isCoprime coprime).symm |>.trans <|
Ideal.quotientInfEquivQuotientProd I J coprime
@[simp]
theorem quotientMulEquivQuotientProd_fst (I J : Ideal R) (coprime : IsCoprime I J) (x : R ⧸ I * J) :
(quotientMulEquivQuotientProd I J coprime x).fst =
Ideal.Quotient.factor (I * J) I mul_le_right x :=
Quot.inductionOn x fun _ => rfl
@[simp]
theorem quotientMulEquivQuotientProd_snd (I J : Ideal R) (coprime : IsCoprime I J) (x : R ⧸ I * J) :
(quotientMulEquivQuotientProd I J coprime x).snd =
Ideal.Quotient.factor (I * J) J mul_le_left x :=
Quot.inductionOn x fun _ => rfl
@[simp]
theorem fst_comp_quotientMulEquivQuotientProd (I J : Ideal R) (coprime : IsCoprime I J) :
(RingHom.fst _ _).comp
(quotientMulEquivQuotientProd I J coprime : R ⧸ I * J →+* (R ⧸ I) × R ⧸ J) =
Ideal.Quotient.factor (I * J) I mul_le_right := by
apply Quotient.ringHom_ext; ext; rfl
@[simp]
theorem snd_comp_quotientMulEquivQuotientProd (I J : Ideal R) (coprime : IsCoprime I J) :
(RingHom.snd _ _).comp
(quotientMulEquivQuotientProd I J coprime : R ⧸ I * J →+* (R ⧸ I) × R ⧸ J) =
Ideal.Quotient.factor (I * J) J mul_le_left := by
apply Quotient.ringHom_ext; ext; rfl
end ChineseRemainder
section QuotientAlgebra
variable (R₁ R₂ : Type*) {A B : Type*}
variable [CommSemiring R₁] [CommSemiring R₂] [CommRing A]
variable [Algebra R₁ A] [Algebra R₂ A]
/-- The `R₁`-algebra structure on `A/I` for an `R₁`-algebra `A` -/
instance Quotient.algebra {I : Ideal A} : Algebra R₁ (A ⧸ I) :=
{ toRingHom := (Ideal.Quotient.mk I).comp (algebraMap R₁ A)
smul_def' := fun _ x =>
Quotient.inductionOn' x fun _ =>
((Quotient.mk I).congr_arg <| Algebra.smul_def _ _).trans (RingHom.map_mul _ _ _)
commutes' := fun _ _ => mul_comm _ _ }
-- Lean can struggle to find this instance later if we don't provide this shortcut
-- Porting note: this can probably now be deleted
-- update: maybe not - removal causes timeouts
instance Quotient.isScalarTower [SMul R₁ R₂] [IsScalarTower R₁ R₂ A] (I : Ideal A) :
IsScalarTower R₁ R₂ (A ⧸ I) := by infer_instance
/-- The canonical morphism `A →ₐ[R₁] A ⧸ I` as morphism of `R₁`-algebras, for `I` an ideal of
`A`, where `A` is an `R₁`-algebra. -/
def Quotient.mkₐ (I : Ideal A) : A →ₐ[R₁] A ⧸ I :=
⟨⟨⟨⟨fun a => Submodule.Quotient.mk a, rfl⟩, fun _ _ => rfl⟩, rfl, fun _ _ => rfl⟩, fun _ => rfl⟩
theorem Quotient.algHom_ext {I : Ideal A} {S} [Semiring S] [Algebra R₁ S] ⦃f g : A ⧸ I →ₐ[R₁] S⦄
(h : f.comp (Quotient.mkₐ R₁ I) = g.comp (Quotient.mkₐ R₁ I)) : f = g :=
AlgHom.ext fun x => Quotient.inductionOn' x <| AlgHom.congr_fun h
theorem Quotient.alg_map_eq (I : Ideal A) :
algebraMap R₁ (A ⧸ I) = (algebraMap A (A ⧸ I)).comp (algebraMap R₁ A) :=
rfl
theorem Quotient.mkₐ_toRingHom (I : Ideal A) :
(Quotient.mkₐ R₁ I).toRingHom = Ideal.Quotient.mk I :=
rfl
@[simp]
theorem Quotient.mkₐ_eq_mk (I : Ideal A) : ⇑(Quotient.mkₐ R₁ I) = Quotient.mk I :=
rfl
@[simp]
theorem Quotient.algebraMap_eq (I : Ideal R) : algebraMap R (R ⧸ I) = Quotient.mk I :=
rfl
@[simp]
theorem Quotient.mk_comp_algebraMap (I : Ideal A) :
(Quotient.mk I).comp (algebraMap R₁ A) = algebraMap R₁ (A ⧸ I) :=
rfl
@[simp]
theorem Quotient.mk_algebraMap (I : Ideal A) (x : R₁) :
Quotient.mk I (algebraMap R₁ A x) = algebraMap R₁ (A ⧸ I) x :=
rfl
/-- The canonical morphism `A →ₐ[R₁] I.quotient` is surjective. -/
theorem Quotient.mkₐ_surjective (I : Ideal A) : Function.Surjective (Quotient.mkₐ R₁ I) :=
surjective_quot_mk _
/-- The kernel of `A →ₐ[R₁] I.quotient` is `I`. -/
@[simp]
theorem Quotient.mkₐ_ker (I : Ideal A) : RingHom.ker (Quotient.mkₐ R₁ I : A →+* A ⧸ I) = I :=
Ideal.mk_ker
variable {R₁}
section
variable [Semiring B] [Algebra R₁ B]
/-- `Ideal.quotient.lift` as an `AlgHom`. -/
def Quotient.liftₐ (I : Ideal A) (f : A →ₐ[R₁] B) (hI : ∀ a : A, a ∈ I → f a = 0) :
A ⧸ I →ₐ[R₁] B :=
{-- this is IsScalarTower.algebraMap_apply R₁ A (A ⧸ I) but the file `Algebra.Algebra.Tower`
-- imports this file.
Ideal.Quotient.lift
I (f : A →+* B) hI with
commutes' := fun r => by
have : algebraMap R₁ (A ⧸ I) r = algebraMap A (A ⧸ I) (algebraMap R₁ A r) := by
simp_rw [Algebra.algebraMap_eq_smul_one, smul_assoc, one_smul]
rw [this, Ideal.Quotient.algebraMap_eq, RingHom.toFun_eq_coe, Ideal.Quotient.lift_mk,
AlgHom.coe_toRingHom, Algebra.algebraMap_eq_smul_one, Algebra.algebraMap_eq_smul_one,
map_smul, map_one] }
@[simp]
theorem Quotient.liftₐ_apply (I : Ideal A) (f : A →ₐ[R₁] B) (hI : ∀ a : A, a ∈ I → f a = 0) (x) :
Ideal.Quotient.liftₐ I f hI x = Ideal.Quotient.lift I (f : A →+* B) hI x :=
rfl
theorem Quotient.liftₐ_comp (I : Ideal A) (f : A →ₐ[R₁] B) (hI : ∀ a : A, a ∈ I → f a = 0) :
(Ideal.Quotient.liftₐ I f hI).comp (Ideal.Quotient.mkₐ R₁ I) = f :=
AlgHom.ext fun _ => (Ideal.Quotient.lift_mk I (f : A →+* B) hI : _)
theorem KerLift.map_smul (f : A →ₐ[R₁] B) (r : R₁) (x : A ⧸ (RingHom.ker f)) :
f.kerLift (r • x) = r • f.kerLift x := by
obtain ⟨a, rfl⟩ := Quotient.mkₐ_surjective R₁ _ x
exact _root_.map_smul f _ _
/-- The induced algebras morphism from the quotient by the kernel to the codomain.
This is an isomorphism if `f` has a right inverse (`quotientKerAlgEquivOfRightInverse`) /
is surjective (`quotientKerAlgEquivOfSurjective`).
-/
def kerLiftAlg (f : A →ₐ[R₁] B) : A ⧸ (RingHom.ker f) →ₐ[R₁] B :=
AlgHom.mk' (RingHom.kerLift (f : A →+* B)) fun _ _ => KerLift.map_smul f _ _
@[simp]
theorem kerLiftAlg_mk (f : A →ₐ[R₁] B) (a : A) :
kerLiftAlg f (Quotient.mk (RingHom.ker f) a) = f a := by
rfl
@[simp]
theorem kerLiftAlg_toRingHom (f : A →ₐ[R₁] B) :
(kerLiftAlg f : A ⧸ ker f →+* B) = RingHom.kerLift (f : A →+* B) :=
rfl
/-- The induced algebra morphism from the quotient by the kernel is injective. -/
theorem kerLiftAlg_injective (f : A →ₐ[R₁] B) : Function.Injective (kerLiftAlg f) :=
RingHom.kerLift_injective (R := A) (S := B) f
/-- The **first isomorphism** theorem for algebras, computable version. -/
@[simps!]
def quotientKerAlgEquivOfRightInverse {f : A →ₐ[R₁] B} {g : B → A}
(hf : Function.RightInverse g f) : (A ⧸ RingHom.ker f) ≃ₐ[R₁] B :=
{ RingHom.quotientKerEquivOfRightInverse hf,
kerLiftAlg f with }
@[deprecated (since := "2024-02-27")]
alias quotientKerAlgEquivOfRightInverse.apply := quotientKerAlgEquivOfRightInverse_apply
@[deprecated (since := "2024-02-27")]
alias QuotientKerAlgEquivOfRightInverseSymm.apply := quotientKerAlgEquivOfRightInverse_symm_apply
/-- The **first isomorphism theorem** for algebras. -/
@[simps!]
noncomputable def quotientKerAlgEquivOfSurjective {f : A →ₐ[R₁] B} (hf : Function.Surjective f) :
(A ⧸ (RingHom.ker f)) ≃ₐ[R₁] B :=
quotientKerAlgEquivOfRightInverse (Classical.choose_spec hf.hasRightInverse)
end
section CommRing_CommRing
variable {S : Type v} [CommRing S]
/-- The ring hom `R/I →+* S/J` induced by a ring hom `f : R →+* S` with `I ≤ f⁻¹(J)` -/
def quotientMap {I : Ideal R} (J : Ideal S) (f : R →+* S) (hIJ : I ≤ J.comap f) : R ⧸ I →+* S ⧸ J :=
Quotient.lift I ((Quotient.mk J).comp f) fun _ ha => by
simpa [Function.comp_apply, RingHom.coe_comp, Quotient.eq_zero_iff_mem] using hIJ ha
@[simp]
theorem quotientMap_mk {J : Ideal R} {I : Ideal S} {f : R →+* S} {H : J ≤ I.comap f} {x : R} :
quotientMap I f H (Quotient.mk J x) = Quotient.mk I (f x) :=
Quotient.lift_mk J _ _
@[simp]
theorem quotientMap_algebraMap {J : Ideal A} {I : Ideal S} {f : A →+* S} {H : J ≤ I.comap f}
{x : R₁} : quotientMap I f H (algebraMap R₁ (A ⧸ J) x) = Quotient.mk I (f (algebraMap _ _ x)) :=
Quotient.lift_mk J _ _
theorem quotientMap_comp_mk {J : Ideal R} {I : Ideal S} {f : R →+* S} (H : J ≤ I.comap f) :
(quotientMap I f H).comp (Quotient.mk J) = (Quotient.mk I).comp f :=
RingHom.ext fun x => by simp only [Function.comp_apply, RingHom.coe_comp, Ideal.quotientMap_mk]
/-- The ring equiv `R/I ≃+* S/J` induced by a ring equiv `f : R ≃+* S`, where `J = f(I)`. -/
@[simps]
def quotientEquiv (I : Ideal R) (J : Ideal S) (f : R ≃+* S) (hIJ : J = I.map (f : R →+* S)) :
R ⧸ I ≃+* S ⧸ J :=
{
quotientMap J (↑f) (by
rw [hIJ]
exact le_comap_map)
with
invFun :=
quotientMap I (↑f.symm)
(by
rw [hIJ]
exact le_of_eq (map_comap_of_equiv I f))
left_inv := by
rintro ⟨r⟩
simp only [Submodule.Quotient.quot_mk_eq_mk, Quotient.mk_eq_mk, RingHom.toFun_eq_coe,
quotientMap_mk, RingEquiv.coe_toRingHom, RingEquiv.symm_apply_apply]
right_inv := by
rintro ⟨s⟩
simp only [Submodule.Quotient.quot_mk_eq_mk, Quotient.mk_eq_mk, RingHom.toFun_eq_coe,
quotientMap_mk, RingEquiv.coe_toRingHom, RingEquiv.apply_symm_apply] }
/- Porting note: removed simp. LHS simplified. Slightly different version of the simplified
form closed this and was itself closed by simp -/
theorem quotientEquiv_mk (I : Ideal R) (J : Ideal S) (f : R ≃+* S) (hIJ : J = I.map (f : R →+* S))
(x : R) : quotientEquiv I J f hIJ (Ideal.Quotient.mk I x) = Ideal.Quotient.mk J (f x) :=
rfl
@[simp]
theorem quotientEquiv_symm_mk (I : Ideal R) (J : Ideal S) (f : R ≃+* S)
(hIJ : J = I.map (f : R →+* S)) (x : S) :
(quotientEquiv I J f hIJ).symm (Ideal.Quotient.mk J x) = Ideal.Quotient.mk I (f.symm x) :=
rfl
/-- `H` and `h` are kept as separate hypothesis since H is used in constructing the quotient map. -/
theorem quotientMap_injective' {J : Ideal R} {I : Ideal S} {f : R →+* S} {H : J ≤ I.comap f}
(h : I.comap f ≤ J) : Function.Injective (quotientMap I f H) := by
refine (injective_iff_map_eq_zero (quotientMap I f H)).2 fun a ha => ?_
obtain ⟨r, rfl⟩ := Quotient.mk_surjective a
rw [quotientMap_mk, Quotient.eq_zero_iff_mem] at ha
exact Quotient.eq_zero_iff_mem.mpr (h ha)
/-- If we take `J = I.comap f` then `QuotientMap` is injective automatically. -/
theorem quotientMap_injective {I : Ideal S} {f : R →+* S} :
Function.Injective (quotientMap I f le_rfl) :=
quotientMap_injective' le_rfl
theorem quotientMap_surjective {J : Ideal R} {I : Ideal S} {f : R →+* S} {H : J ≤ I.comap f}
(hf : Function.Surjective f) : Function.Surjective (quotientMap I f H) := fun x =>
let ⟨x, hx⟩ := Quotient.mk_surjective x
let ⟨y, hy⟩ := hf x
⟨(Quotient.mk J) y, by simp [hx, hy]⟩
/-- Commutativity of a square is preserved when taking quotients by an ideal. -/
theorem comp_quotientMap_eq_of_comp_eq {R' S' : Type*} [CommRing R'] [CommRing S'] {f : R →+* S}
{f' : R' →+* S'} {g : R →+* R'} {g' : S →+* S'} (hfg : f'.comp g = g'.comp f) (I : Ideal S') :
-- Porting note: was losing track of I
let leq := le_of_eq (_root_.trans (comap_comap (I := I) f g') (hfg ▸ comap_comap (I := I) g f'))
(quotientMap I g' le_rfl).comp (quotientMap (I.comap g') f le_rfl) =
(quotientMap I f' le_rfl).comp (quotientMap (I.comap f') g leq) := by
refine RingHom.ext fun a => ?_
obtain ⟨r, rfl⟩ := Quotient.mk_surjective a
simp only [RingHom.comp_apply, quotientMap_mk]
exact (Ideal.Quotient.mk I).congr_arg (_root_.trans (g'.comp_apply f r).symm
(hfg ▸ f'.comp_apply g r))
end CommRing_CommRing
section
variable [CommRing B] [Algebra R₁ B]
/-- The algebra hom `A/I →+* B/J` induced by an algebra hom `f : A →ₐ[R₁] B` with `I ≤ f⁻¹(J)`. -/
def quotientMapₐ {I : Ideal A} (J : Ideal B) (f : A →ₐ[R₁] B) (hIJ : I ≤ J.comap f) :
A ⧸ I →ₐ[R₁] B ⧸ J :=
{ quotientMap J (f : A →+* B) hIJ with commutes' := fun r => by simp only [RingHom.toFun_eq_coe,
quotientMap_algebraMap, AlgHom.coe_toRingHom, AlgHom.commutes, Quotient.mk_algebraMap] }
@[simp]
theorem quotient_map_mkₐ {I : Ideal A} (J : Ideal B) (f : A →ₐ[R₁] B) (H : I ≤ J.comap f) {x : A} :
quotientMapₐ J f H (Quotient.mk I x) = Quotient.mkₐ R₁ J (f x) :=
rfl
theorem quotient_map_comp_mkₐ {I : Ideal A} (J : Ideal B) (f : A →ₐ[R₁] B) (H : I ≤ J.comap f) :
(quotientMapₐ J f H).comp (Quotient.mkₐ R₁ I) = (Quotient.mkₐ R₁ J).comp f :=
AlgHom.ext fun x => by simp only [quotient_map_mkₐ, Quotient.mkₐ_eq_mk, AlgHom.comp_apply]
/-- The algebra equiv `A/I ≃ₐ[R] B/J` induced by an algebra equiv `f : A ≃ₐ[R] B`,
where`J = f(I)`. -/
def quotientEquivAlg (I : Ideal A) (J : Ideal B) (f : A ≃ₐ[R₁] B) (hIJ : J = I.map (f : A →+* B)) :
(A ⧸ I) ≃ₐ[R₁] B ⧸ J :=
{ quotientEquiv I J (f : A ≃+* B) hIJ with
commutes' := fun r => by
-- Porting note: Needed to add the below lemma because Equivs coerce weird
have : ∀ (e : RingEquiv (A ⧸ I) (B ⧸ J)), Equiv.toFun e.toEquiv = DFunLike.coe e :=
fun _ ↦ rfl
rw [this]
simp only [quotientEquiv_apply, RingHom.toFun_eq_coe, quotientMap_algebraMap,
RingEquiv.coe_toRingHom, AlgEquiv.coe_ringEquiv, AlgEquiv.commutes, Quotient.mk_algebraMap]}
end
instance (priority := 100) quotientAlgebra {I : Ideal A} [Algebra R A] :
Algebra (R ⧸ I.comap (algebraMap R A)) (A ⧸ I) :=
(quotientMap I (algebraMap R A) (le_of_eq rfl)).toAlgebra
theorem algebraMap_quotient_injective {I : Ideal A} [Algebra R A] :
Function.Injective (algebraMap (R ⧸ I.comap (algebraMap R A)) (A ⧸ I)) := by
rintro ⟨a⟩ ⟨b⟩ hab
replace hab := Quotient.eq.mp hab
rw [← RingHom.map_sub] at hab
exact Quotient.eq.mpr hab
variable (R₁)
/-- Quotienting by equal ideals gives equivalent algebras. -/
def quotientEquivAlgOfEq {I J : Ideal A} (h : I = J) : (A ⧸ I) ≃ₐ[R₁] A ⧸ J :=
quotientEquivAlg I J AlgEquiv.refl <| h ▸ (map_id I).symm
@[simp]
theorem quotientEquivAlgOfEq_mk {I J : Ideal A} (h : I = J) (x : A) :
quotientEquivAlgOfEq R₁ h (Ideal.Quotient.mk I x) = Ideal.Quotient.mk J x :=
rfl
@[simp]
theorem quotientEquivAlgOfEq_symm {I J : Ideal A} (h : I = J) :
(quotientEquivAlgOfEq R₁ h).symm = quotientEquivAlgOfEq R₁ h.symm := by
ext
rfl
lemma comap_map_mk {I J : Ideal R} (h : I ≤ J) :
Ideal.comap (Ideal.Quotient.mk I) (Ideal.map (Ideal.Quotient.mk I) J) = J := by
ext; rw [← Ideal.mem_quotient_iff_mem h, Ideal.mem_comap]
/-- The **first isomorphism theorem** for commutative algebras (`AlgHom.range` version). -/
noncomputable def quotientKerEquivRange
{A B : Type*} [CommRing A] [Algebra R A] [Semiring B] [Algebra R B]
(f : A →ₐ[R] B) :
(A ⧸ RingHom.ker f) ≃ₐ[R] f.range :=
(Ideal.quotientEquivAlgOfEq R (AlgHom.ker_rangeRestrict f).symm).trans <|
Ideal.quotientKerAlgEquivOfSurjective f.rangeRestrict_surjective
end QuotientAlgebra
end Ideal
namespace DoubleQuot
open Ideal
variable {R : Type u}
section
variable [CommRing R] (I J : Ideal R)
/-- The obvious ring hom `R/I → R/(I ⊔ J)` -/
def quotLeftToQuotSup : R ⧸ I →+* R ⧸ I ⊔ J :=
Ideal.Quotient.factor I (I ⊔ J) le_sup_left
/-- The kernel of `quotLeftToQuotSup` -/
theorem ker_quotLeftToQuotSup : RingHom.ker (quotLeftToQuotSup I J) =
J.map (Ideal.Quotient.mk I) := by
simp only [mk_ker, sup_idem, sup_comm, quotLeftToQuotSup, Quotient.factor, ker_quotient_lift,
map_eq_iff_sup_ker_eq_of_surjective (Ideal.Quotient.mk I) Quotient.mk_surjective, ← sup_assoc]
/-- The ring homomorphism `(R/I)/J' -> R/(I ⊔ J)` induced by `quotLeftToQuotSup` where `J'`
is the image of `J` in `R/I`-/
def quotQuotToQuotSup : (R ⧸ I) ⧸ J.map (Ideal.Quotient.mk I) →+* R ⧸ I ⊔ J :=
Ideal.Quotient.lift (J.map (Ideal.Quotient.mk I)) (quotLeftToQuotSup I J)
(ker_quotLeftToQuotSup I J).symm.le
/-- The composite of the maps `R → (R/I)` and `(R/I) → (R/I)/J'` -/
def quotQuotMk : R →+* (R ⧸ I) ⧸ J.map (Ideal.Quotient.mk I) :=
(Ideal.Quotient.mk (J.map (Ideal.Quotient.mk I))).comp (Ideal.Quotient.mk I)
-- Porting note: mismatched instances
/-- The kernel of `quotQuotMk` -/
theorem ker_quotQuotMk : RingHom.ker (quotQuotMk I J) = I ⊔ J := by
rw [RingHom.ker_eq_comap_bot, quotQuotMk, ← comap_comap, ← RingHom.ker, mk_ker,
comap_map_of_surjective (Ideal.Quotient.mk I) Quotient.mk_surjective, ← RingHom.ker, mk_ker,
sup_comm]
/-- The ring homomorphism `R/(I ⊔ J) → (R/I)/J' `induced by `quotQuotMk` -/
def liftSupQuotQuotMk (I J : Ideal R) : R ⧸ I ⊔ J →+* (R ⧸ I) ⧸ J.map (Ideal.Quotient.mk I) :=
Ideal.Quotient.lift (I ⊔ J) (quotQuotMk I J) (ker_quotQuotMk I J).symm.le
/-- `quotQuotToQuotSup` and `liftSupQuotQuotMk` are inverse isomorphisms. In the case where
`I ≤ J`, this is the Third Isomorphism Theorem (see `quotQuotEquivQuotOfLe`)-/
def quotQuotEquivQuotSup : (R ⧸ I) ⧸ J.map (Ideal.Quotient.mk I) ≃+* R ⧸ I ⊔ J :=
RingEquiv.ofHomInv (quotQuotToQuotSup I J) (liftSupQuotQuotMk I J)
(by
repeat apply Ideal.Quotient.ringHom_ext
rfl)
(by
repeat apply Ideal.Quotient.ringHom_ext
rfl)
@[simp]
theorem quotQuotEquivQuotSup_quotQuotMk (x : R) :
quotQuotEquivQuotSup I J (quotQuotMk I J x) = Ideal.Quotient.mk (I ⊔ J) x :=
rfl
@[simp]
theorem quotQuotEquivQuotSup_symm_quotQuotMk (x : R) :
(quotQuotEquivQuotSup I J).symm (Ideal.Quotient.mk (I ⊔ J) x) = quotQuotMk I J x :=
rfl
/-- The obvious isomorphism `(R/I)/J' → (R/J)/I'` -/
def quotQuotEquivComm : (R ⧸ I) ⧸ J.map (Ideal.Quotient.mk I) ≃+*
(R ⧸ J) ⧸ I.map (Ideal.Quotient.mk J) :=
((quotQuotEquivQuotSup I J).trans (quotEquivOfEq (sup_comm ..))).trans
(quotQuotEquivQuotSup J I).symm
-- Porting note: mismatched instances
@[simp]
theorem quotQuotEquivComm_quotQuotMk (x : R) :
quotQuotEquivComm I J (quotQuotMk I J x) = quotQuotMk J I x :=
rfl
-- Porting note: mismatched instances
@[simp]
theorem quotQuotEquivComm_comp_quotQuotMk :
RingHom.comp (↑(quotQuotEquivComm I J)) (quotQuotMk I J) = quotQuotMk J I :=
RingHom.ext <| quotQuotEquivComm_quotQuotMk I J
@[simp]
theorem quotQuotEquivComm_symm : (quotQuotEquivComm I J).symm = quotQuotEquivComm J I := by
/- Porting note: this proof used to just be rfl but currently rfl opens up a bottomless pit
of processor cycles. Synthesizing instances does not seem to be an issue.
-/
change (((quotQuotEquivQuotSup I J).trans (quotEquivOfEq (sup_comm ..))).trans
(quotQuotEquivQuotSup J I).symm).symm =
((quotQuotEquivQuotSup J I).trans (quotEquivOfEq (sup_comm ..))).trans
(quotQuotEquivQuotSup I J).symm
ext r
dsimp
rfl
variable {I J}
/-- **The Third Isomorphism theorem** for rings. See `quotQuotEquivQuotSup` for a version
that does not assume an inclusion of ideals. -/
def quotQuotEquivQuotOfLE (h : I ≤ J) : (R ⧸ I) ⧸ J.map (Ideal.Quotient.mk I) ≃+* R ⧸ J :=
(quotQuotEquivQuotSup I J).trans (Ideal.quotEquivOfEq <| sup_eq_right.mpr h)
@[simp]
theorem quotQuotEquivQuotOfLE_quotQuotMk (x : R) (h : I ≤ J) :
quotQuotEquivQuotOfLE h (quotQuotMk I J x) = (Ideal.Quotient.mk J) x :=
rfl
@[simp]
theorem quotQuotEquivQuotOfLE_symm_mk (x : R) (h : I ≤ J) :
(quotQuotEquivQuotOfLE h).symm ((Ideal.Quotient.mk J) x) = quotQuotMk I J x :=
rfl
theorem quotQuotEquivQuotOfLE_comp_quotQuotMk (h : I ≤ J) :
RingHom.comp (↑(quotQuotEquivQuotOfLE h)) (quotQuotMk I J) = (Ideal.Quotient.mk J) := by
ext
rfl
theorem quotQuotEquivQuotOfLE_symm_comp_mk (h : I ≤ J) :
RingHom.comp (↑(quotQuotEquivQuotOfLE h).symm) (Ideal.Quotient.mk J) = quotQuotMk I J := by
ext
rfl
end
section Algebra
@[simp]
theorem quotQuotEquivComm_mk_mk [CommRing R] (I J : Ideal R) (x : R) :
quotQuotEquivComm I J (Ideal.Quotient.mk _ (Ideal.Quotient.mk _ x)) = algebraMap R _ x :=
rfl
variable [CommSemiring R] {A : Type v} [CommRing A] [Algebra R A] (I J : Ideal A)
@[simp]
theorem quotQuotEquivQuotSup_quot_quot_algebraMap (x : R) :
DoubleQuot.quotQuotEquivQuotSup I J (algebraMap R _ x) = algebraMap _ _ x :=
rfl
@[simp]
theorem quotQuotEquivComm_algebraMap (x : R) :
quotQuotEquivComm I J (algebraMap R _ x) = algebraMap _ _ x :=
rfl
end Algebra
section AlgebraQuotient
variable (R) {A : Type*} [CommSemiring R] [CommRing A] [Algebra R A] (I J : Ideal A)
/-- The natural algebra homomorphism `A / I → A / (I ⊔ J)`. -/
def quotLeftToQuotSupₐ : A ⧸ I →ₐ[R] A ⧸ I ⊔ J :=
AlgHom.mk (quotLeftToQuotSup I J) fun _ => rfl
@[simp]
theorem quotLeftToQuotSupₐ_toRingHom :
(quotLeftToQuotSupₐ R I J : _ →+* _) = quotLeftToQuotSup I J :=
rfl
@[simp]
theorem coe_quotLeftToQuotSupₐ : ⇑(quotLeftToQuotSupₐ R I J) = quotLeftToQuotSup I J :=
rfl
/-- The algebra homomorphism `(A / I) / J' -> A / (I ⊔ J)` induced by `quotQuotToQuotSup`,
where `J'` is the projection of `J` in `A / I`. -/
def quotQuotToQuotSupₐ : (A ⧸ I) ⧸ J.map (Quotient.mkₐ R I) →ₐ[R] A ⧸ I ⊔ J :=
AlgHom.mk (quotQuotToQuotSup I J) fun _ => rfl
@[simp]
theorem quotQuotToQuotSupₐ_toRingHom :
((quotQuotToQuotSupₐ R I J) : _ ⧸ map (Ideal.Quotient.mkₐ R I) J →+* _) =
quotQuotToQuotSup I J :=
rfl
@[simp]
theorem coe_quotQuotToQuotSupₐ : ⇑(quotQuotToQuotSupₐ R I J) = quotQuotToQuotSup I J :=
rfl
/-- The composition of the algebra homomorphisms `A → (A / I)` and `(A / I) → (A / I) / J'`,
where `J'` is the projection `J` in `A / I`. -/
def quotQuotMkₐ : A →ₐ[R] (A ⧸ I) ⧸ J.map (Quotient.mkₐ R I) :=
AlgHom.mk (quotQuotMk I J) fun _ => rfl
@[simp]
theorem quotQuotMkₐ_toRingHom :
(quotQuotMkₐ R I J : _ →+* _ ⧸ J.map (Quotient.mkₐ R I)) = quotQuotMk I J :=
rfl
@[simp]
theorem coe_quotQuotMkₐ : ⇑(quotQuotMkₐ R I J) = quotQuotMk I J :=
rfl
/-- The injective algebra homomorphism `A / (I ⊔ J) → (A / I) / J'`induced by `quot_quot_mk`,
where `J'` is the projection `J` in `A / I`. -/
def liftSupQuotQuotMkₐ (I J : Ideal A) : A ⧸ I ⊔ J →ₐ[R] (A ⧸ I) ⧸ J.map (Quotient.mkₐ R I) :=
AlgHom.mk (liftSupQuotQuotMk I J) fun _ => rfl
@[simp]
theorem liftSupQuotQuotMkₐ_toRingHom :
(liftSupQuotQuotMkₐ R I J : _ →+* _ ⧸ J.map (Quotient.mkₐ R I)) = liftSupQuotQuotMk I J :=
rfl
@[simp]
theorem coe_liftSupQuotQuotMkₐ : ⇑(liftSupQuotQuotMkₐ R I J) = liftSupQuotQuotMk I J :=
rfl
/-- `quotQuotToQuotSup` and `liftSupQuotQuotMk` are inverse isomorphisms. In the case where
`I ≤ J`, this is the Third Isomorphism Theorem (see `DoubleQuot.quotQuotEquivQuotOfLE`). -/
def quotQuotEquivQuotSupₐ : ((A ⧸ I) ⧸ J.map (Quotient.mkₐ R I)) ≃ₐ[R] A ⧸ I ⊔ J :=
AlgEquiv.ofRingEquiv (f := quotQuotEquivQuotSup I J) fun _ => rfl
@[simp]
theorem quotQuotEquivQuotSupₐ_toRingEquiv :
(quotQuotEquivQuotSupₐ R I J : _ ⧸ J.map (Quotient.mkₐ R I) ≃+* _) = quotQuotEquivQuotSup I J :=
rfl
@[simp]
-- Porting note: had to add an extra coercion arrow on the right hand side.
theorem coe_quotQuotEquivQuotSupₐ : ⇑(quotQuotEquivQuotSupₐ R I J) = ⇑(quotQuotEquivQuotSup I J) :=
rfl
@[simp]
theorem quotQuotEquivQuotSupₐ_symm_toRingEquiv :
((quotQuotEquivQuotSupₐ R I J).symm : _ ≃+* _ ⧸ J.map (Quotient.mkₐ R I)) =
(quotQuotEquivQuotSup I J).symm :=
rfl
@[simp]
-- Porting note: had to add an extra coercion arrow on the right hand side.
theorem coe_quotQuotEquivQuotSupₐ_symm :
⇑(quotQuotEquivQuotSupₐ R I J).symm = ⇑(quotQuotEquivQuotSup I J).symm :=
rfl
/-- The natural algebra isomorphism `(A / I) / J' → (A / J) / I'`,
where `J'` (resp. `I'`) is the projection of `J` in `A / I` (resp. `I` in `A / J`). -/
def quotQuotEquivCommₐ :
((A ⧸ I) ⧸ J.map (Quotient.mkₐ R I)) ≃ₐ[R] (A ⧸ J) ⧸ I.map (Quotient.mkₐ R J) :=
AlgEquiv.ofRingEquiv (f := quotQuotEquivComm I J) fun _ => rfl
@[simp]
theorem quotQuotEquivCommₐ_toRingEquiv :
(quotQuotEquivCommₐ R I J : _ ⧸ J.map (Quotient.mkₐ R I) ≃+* _ ⧸ I.map (Quotient.mkₐ R J)) =
quotQuotEquivComm I J :=
-- Porting note: should just be `rfl` but `AlgEquiv.toRingEquiv` and `AlgEquiv.ofRingEquiv`
-- involve repacking everything in the structure, so Lean ends up unfolding `quotQuotEquivComm`
-- and timing out.
RingEquiv.ext fun _ => rfl
@[simp]
theorem coe_quotQuotEquivCommₐ : ⇑(quotQuotEquivCommₐ R I J) = ⇑(quotQuotEquivComm I J) :=
rfl
@[simp]
theorem quotQuotEquivComm_symmₐ : (quotQuotEquivCommₐ R I J).symm = quotQuotEquivCommₐ R J I := by
-- Porting note: should just be `rfl` but `AlgEquiv.toRingEquiv` and `AlgEquiv.ofRingEquiv`
-- involve repacking everything in the structure, so Lean ends up unfolding `quotQuotEquivComm`
-- and timing out.
ext
unfold quotQuotEquivCommₐ
congr
@[simp]
theorem quotQuotEquivComm_comp_quotQuotMkₐ :
AlgHom.comp (↑(quotQuotEquivCommₐ R I J)) (quotQuotMkₐ R I J) = quotQuotMkₐ R J I :=
AlgHom.ext <| quotQuotEquivComm_quotQuotMk I J
variable {I J}
/-- The **third isomorphism theorem** for algebras. See `quotQuotEquivQuotSupₐ` for version
that does not assume an inclusion of ideals. -/
def quotQuotEquivQuotOfLEₐ (h : I ≤ J) : ((A ⧸ I) ⧸ J.map (Quotient.mkₐ R I)) ≃ₐ[R] A ⧸ J :=
AlgEquiv.ofRingEquiv (f := quotQuotEquivQuotOfLE h) fun _ => rfl
@[simp]
theorem quotQuotEquivQuotOfLEₐ_toRingEquiv (h : I ≤ J) :
(quotQuotEquivQuotOfLEₐ R h : _ ⧸ J.map (Quotient.mkₐ R I) ≃+* _) = quotQuotEquivQuotOfLE h :=
rfl
@[simp]
-- Porting note: had to add an extra coercion arrow on the right hand side.
theorem coe_quotQuotEquivQuotOfLEₐ (h : I ≤ J) :
⇑(quotQuotEquivQuotOfLEₐ R h) = ⇑(quotQuotEquivQuotOfLE h) :=
rfl
@[simp]
theorem quotQuotEquivQuotOfLEₐ_symm_toRingEquiv (h : I ≤ J) :
((quotQuotEquivQuotOfLEₐ R h).symm : _ ≃+* _ ⧸ J.map (Quotient.mkₐ R I)) =
(quotQuotEquivQuotOfLE h).symm :=
rfl
@[simp]
-- Porting note: had to add an extra coercion arrow on the right hand side.
theorem coe_quotQuotEquivQuotOfLEₐ_symm (h : I ≤ J) :
⇑(quotQuotEquivQuotOfLEₐ R h).symm = ⇑(quotQuotEquivQuotOfLE h).symm :=
rfl
@[simp]
theorem quotQuotEquivQuotOfLE_comp_quotQuotMkₐ (h : I ≤ J) :
AlgHom.comp (↑(quotQuotEquivQuotOfLEₐ R h)) (quotQuotMkₐ R I J) = Quotient.mkₐ R J :=
rfl
@[simp]
theorem quotQuotEquivQuotOfLE_symm_comp_mkₐ (h : I ≤ J) :
AlgHom.comp (↑(quotQuotEquivQuotOfLEₐ R h).symm) (Quotient.mkₐ R J) = quotQuotMkₐ R I J :=
rfl
end AlgebraQuotient
end DoubleQuot
|
RingTheory\Int\Basic.lean | /-
Copyright (c) 2018 Johannes Hölzl. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johannes Hölzl, Jens Wagemaker, Aaron Anderson
-/
import Mathlib.Algebra.EuclideanDomain.Basic
import Mathlib.Algebra.EuclideanDomain.Int
import Mathlib.RingTheory.PrincipalIdealDomain
import Mathlib.Algebra.GCDMonoid.Nat
import Mathlib.Data.Nat.Prime.Basic
/-!
# Divisibility over ℕ and ℤ
This file collects results for the integers and natural numbers that use ring theory in
their proofs or cases of ℕ and ℤ being examples of structures in ring theory.
## Main statements
* `Nat.factors_eq`: the multiset of elements of `Nat.factors` is equal to the factors
given by the `UniqueFactorizationMonoid` instance
## Tags
prime, irreducible, natural numbers, integers, normalization monoid, gcd monoid,
greatest common divisor, prime factorization, prime factors, unique factorization,
unique factors
-/
namespace Int
theorem gcd_eq_one_iff_coprime {a b : ℤ} : Int.gcd a b = 1 ↔ IsCoprime a b := by
constructor
· intro hg
obtain ⟨ua, -, ha⟩ := exists_unit_of_abs a
obtain ⟨ub, -, hb⟩ := exists_unit_of_abs b
use Nat.gcdA (Int.natAbs a) (Int.natAbs b) * ua, Nat.gcdB (Int.natAbs a) (Int.natAbs b) * ub
rw [mul_assoc, ← ha, mul_assoc, ← hb, mul_comm, mul_comm _ (Int.natAbs b : ℤ), ←
Nat.gcd_eq_gcd_ab, ← gcd_eq_natAbs, hg, Int.ofNat_one]
· rintro ⟨r, s, h⟩
by_contra hg
obtain ⟨p, ⟨hp, ha, hb⟩⟩ := Nat.Prime.not_coprime_iff_dvd.mp hg
apply Nat.Prime.not_dvd_one hp
rw [← natCast_dvd_natCast, Int.ofNat_one, ← h]
exact dvd_add ((natCast_dvd.mpr ha).mul_left _) ((natCast_dvd.mpr hb).mul_left _)
theorem coprime_iff_nat_coprime {a b : ℤ} : IsCoprime a b ↔ Nat.Coprime a.natAbs b.natAbs := by
rw [← gcd_eq_one_iff_coprime, Nat.coprime_iff_gcd_eq_one, gcd_eq_natAbs]
/-- If `gcd a (m * n) ≠ 1`, then `gcd a m ≠ 1` or `gcd a n ≠ 1`. -/
theorem gcd_ne_one_iff_gcd_mul_right_ne_one {a : ℤ} {m n : ℕ} :
a.gcd (m * n) ≠ 1 ↔ a.gcd m ≠ 1 ∨ a.gcd n ≠ 1 := by
simp only [gcd_eq_one_iff_coprime, ← not_and_or, not_iff_not, IsCoprime.mul_right_iff]
theorem sq_of_gcd_eq_one {a b c : ℤ} (h : Int.gcd a b = 1) (heq : a * b = c ^ 2) :
∃ a0 : ℤ, a = a0 ^ 2 ∨ a = -a0 ^ 2 := by
have h' : IsUnit (GCDMonoid.gcd a b) := by
rw [← coe_gcd, h, Int.ofNat_one]
exact isUnit_one
obtain ⟨d, ⟨u, hu⟩⟩ := exists_associated_pow_of_mul_eq_pow h' heq
use d
rw [← hu]
cases' Int.units_eq_one_or u with hu' hu' <;>
· rw [hu']
simp
theorem sq_of_coprime {a b c : ℤ} (h : IsCoprime a b) (heq : a * b = c ^ 2) :
∃ a0 : ℤ, a = a0 ^ 2 ∨ a = -a0 ^ 2 :=
sq_of_gcd_eq_one (gcd_eq_one_iff_coprime.mpr h) heq
theorem natAbs_euclideanDomain_gcd (a b : ℤ) :
Int.natAbs (EuclideanDomain.gcd a b) = Int.gcd a b := by
apply Nat.dvd_antisymm <;> rw [← Int.natCast_dvd_natCast]
· rw [Int.natAbs_dvd]
exact Int.dvd_gcd (EuclideanDomain.gcd_dvd_left _ _) (EuclideanDomain.gcd_dvd_right _ _)
· rw [Int.dvd_natAbs]
exact EuclideanDomain.dvd_gcd Int.gcd_dvd_left Int.gcd_dvd_right
end Int
theorem Int.Prime.dvd_mul {m n : ℤ} {p : ℕ} (hp : Nat.Prime p) (h : (p : ℤ) ∣ m * n) :
p ∣ m.natAbs ∨ p ∣ n.natAbs := by
rwa [← hp.dvd_mul, ← Int.natAbs_mul, ← Int.natCast_dvd]
theorem Int.Prime.dvd_mul' {m n : ℤ} {p : ℕ} (hp : Nat.Prime p) (h : (p : ℤ) ∣ m * n) :
(p : ℤ) ∣ m ∨ (p : ℤ) ∣ n := by
rw [Int.natCast_dvd, Int.natCast_dvd]
exact Int.Prime.dvd_mul hp h
theorem Int.Prime.dvd_pow {n : ℤ} {k p : ℕ} (hp : Nat.Prime p) (h : (p : ℤ) ∣ n ^ k) :
p ∣ n.natAbs := by
rw [Int.natCast_dvd, Int.natAbs_pow] at h
exact hp.dvd_of_dvd_pow h
theorem Int.Prime.dvd_pow' {n : ℤ} {k p : ℕ} (hp : Nat.Prime p) (h : (p : ℤ) ∣ n ^ k) :
(p : ℤ) ∣ n := by
rw [Int.natCast_dvd]
exact Int.Prime.dvd_pow hp h
theorem prime_two_or_dvd_of_dvd_two_mul_pow_self_two {m : ℤ} {p : ℕ} (hp : Nat.Prime p)
(h : (p : ℤ) ∣ 2 * m ^ 2) : p = 2 ∨ p ∣ Int.natAbs m := by
cases' Int.Prime.dvd_mul hp h with hp2 hpp
· apply Or.intro_left
exact le_antisymm (Nat.le_of_dvd zero_lt_two hp2) (Nat.Prime.two_le hp)
· apply Or.intro_right
rw [sq, Int.natAbs_mul] at hpp
exact or_self_iff.mp ((Nat.Prime.dvd_mul hp).mp hpp)
theorem Int.exists_prime_and_dvd {n : ℤ} (hn : n.natAbs ≠ 1) : ∃ p, Prime p ∧ p ∣ n := by
obtain ⟨p, pp, pd⟩ := Nat.exists_prime_and_dvd hn
exact ⟨p, Nat.prime_iff_prime_int.mp pp, Int.natCast_dvd.mpr pd⟩
theorem Int.prime_iff_natAbs_prime {k : ℤ} : Prime k ↔ Nat.Prime k.natAbs :=
(Int.associated_natAbs k).prime_iff.trans Nat.prime_iff_prime_int.symm
namespace Int
theorem zmultiples_natAbs (a : ℤ) :
AddSubgroup.zmultiples (a.natAbs : ℤ) = AddSubgroup.zmultiples a :=
le_antisymm (AddSubgroup.zmultiples_le_of_mem (mem_zmultiples_iff.mpr (dvd_natAbs.mpr dvd_rfl)))
(AddSubgroup.zmultiples_le_of_mem (mem_zmultiples_iff.mpr (natAbs_dvd.mpr dvd_rfl)))
theorem span_natAbs (a : ℤ) : Ideal.span ({(a.natAbs : ℤ)} : Set ℤ) = Ideal.span {a} := by
rw [Ideal.span_singleton_eq_span_singleton]
exact (associated_natAbs _).symm
theorem eq_pow_of_mul_eq_pow_odd_left {a b c : ℤ} (hab : IsCoprime a b) {k : ℕ} (hk : Odd k)
(h : a * b = c ^ k) : ∃ d, a = d ^ k := by
obtain ⟨d, hd⟩ := exists_associated_pow_of_mul_eq_pow' hab h
replace hd := hd.symm
rw [associated_iff_natAbs, natAbs_eq_natAbs_iff, ← hk.neg_pow] at hd
obtain rfl | rfl := hd <;> exact ⟨_, rfl⟩
@[deprecated (since := "2024-07-12")]
alias eq_pow_of_mul_eq_pow_bit1_left := eq_pow_of_mul_eq_pow_odd_left
theorem eq_pow_of_mul_eq_pow_odd_right {a b c : ℤ} (hab : IsCoprime a b) {k : ℕ} (hk : Odd k)
(h : a * b = c ^ k) : ∃ d, b = d ^ k :=
eq_pow_of_mul_eq_pow_odd_left hab.symm hk (by rwa [mul_comm] at h)
@[deprecated (since := "2024-07-12")]
alias eq_pow_of_mul_eq_pow_bit1_right := eq_pow_of_mul_eq_pow_odd_right
theorem eq_pow_of_mul_eq_pow_odd {a b c : ℤ} (hab : IsCoprime a b) {k : ℕ} (hk : Odd k)
(h : a * b = c ^ k) : (∃ d, a = d ^ k) ∧ ∃ e, b = e ^ k :=
⟨eq_pow_of_mul_eq_pow_odd_left hab hk h, eq_pow_of_mul_eq_pow_odd_right hab hk h⟩
@[deprecated (since := "2024-07-12")] alias eq_pow_of_mul_eq_pow_bit1 := eq_pow_of_mul_eq_pow_odd
end Int
|
RingTheory\IntegralClosure\IntegrallyClosed.lean | /-
Copyright (c) 2021 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen
-/
import Mathlib.RingTheory.IntegralClosure.Algebra.Basic
import Mathlib.RingTheory.Localization.Integral
/-!
# Integrally closed rings
An integrally closed ring `R` contains all the elements of `Frac(R)` that are
integral over `R`. A special case of integrally closed rings are the Dedekind domains.
## Main definitions
* `IsIntegrallyClosedIn R A` states `R` contains all integral elements of `A`
* `IsIntegrallyClosed R` states `R` contains all integral elements of `Frac(R)`
## Main results
* `isIntegrallyClosed_iff K`, where `K` is a fraction field of `R`, states `R`
is integrally closed iff it is the integral closure of `R` in `K`
## TODO Related notions
The following definitions are closely related, especially in their applications in Mathlib.
A *normal domain* is a domain that is integrally closed in its field of fractions.
[Stacks: normal domain](https://stacks.math.columbia.edu/tag/037B#0309)
Normal domains are the major use case of `IsIntegrallyClosed` at the time of writing, and we have
quite a few results that can be moved wholesale to a new `NormalDomain` definition.
In fact, before PR #6126 `IsIntegrallyClosed` was exactly defined to be a normal domain.
(So you might want to copy some of its API when you define normal domains.)
A normal ring means that localizations at all prime ideals are normal domains.
[Stacks: normal ring](https://stacks.math.columbia.edu/tag/037B#00GV)
This implies `IsIntegrallyClosed`,
[Stacks: Tag 034M](https://stacks.math.columbia.edu/tag/037B#034M)
but is equivalent to it only under some conditions (reduced + finitely many minimal primes),
[Stacks: Tag 030C](https://stacks.math.columbia.edu/tag/037B#030C)
in which case it's also equivalent to being a finite product of normal domains.
We'd need to add these conditions if we want exactly the products of Dedekind domains.
In fact noetherianity is sufficient to guarantee finitely many minimal primes, so `IsDedekindRing`
could be defined as `IsReduced`, `IsNoetherian`, `Ring.DimensionLEOne`, and either
`IsIntegrallyClosed` or `NormalDomain`. If we use `NormalDomain` then `IsReduced` is automatic,
but we could also consider a version of `NormalDomain` that only requires the localizations are
`IsIntegrallyClosed` but may not be domains, and that may not equivalent to the ring itself being
`IsIntegallyClosed` (even for noetherian rings?).
-/
open scoped nonZeroDivisors Polynomial
open Polynomial
/-- `R` is integrally closed in `A` if all integral elements of `A` are also elements of `R`.
-/
abbrev IsIntegrallyClosedIn (R A : Type*) [CommRing R] [CommRing A] [Algebra R A] :=
IsIntegralClosure R R A
/-- `R` is integrally closed if all integral elements of `Frac(R)` are also elements of `R`.
This definition uses `FractionRing R` to denote `Frac(R)`. See `isIntegrallyClosed_iff`
if you want to choose another field of fractions for `R`.
-/
abbrev IsIntegrallyClosed (R : Type*) [CommRing R] := IsIntegrallyClosedIn R (FractionRing R)
section Iff
variable {R : Type*} [CommRing R]
variable {A B : Type*} [CommRing A] [CommRing B] [Algebra R A] [Algebra R B]
/-- Being integrally closed is preserved under injective algebra homomorphisms. -/
theorem AlgHom.isIntegrallyClosedIn (f : A →ₐ[R] B) (hf : Function.Injective f) :
IsIntegrallyClosedIn R B → IsIntegrallyClosedIn R A := by
rintro ⟨inj, cl⟩
refine ⟨Function.Injective.of_comp (f := f) ?_, fun hx => ?_, ?_⟩
· convert inj
aesop
· obtain ⟨y, fx_eq⟩ := cl.mp ((isIntegral_algHom_iff f hf).mpr hx)
aesop
· rintro ⟨y, rfl⟩
apply (isIntegral_algHom_iff f hf).mp
aesop
/-- Being integrally closed is preserved under algebra isomorphisms. -/
theorem AlgEquiv.isIntegrallyClosedIn (e : A ≃ₐ[R] B) :
IsIntegrallyClosedIn R A ↔ IsIntegrallyClosedIn R B :=
⟨AlgHom.isIntegrallyClosedIn e.symm e.symm.injective, AlgHom.isIntegrallyClosedIn e e.injective⟩
variable (K : Type*) [CommRing K] [Algebra R K] [IsFractionRing R K]
/-- `R` is integrally closed iff it is the integral closure of itself in its field of fractions. -/
theorem isIntegrallyClosed_iff_isIntegrallyClosedIn :
IsIntegrallyClosed R ↔ IsIntegrallyClosedIn R K :=
(IsLocalization.algEquiv R⁰ _ _).isIntegrallyClosedIn
/-- `R` is integrally closed iff it is the integral closure of itself in its field of fractions. -/
theorem isIntegrallyClosed_iff_isIntegralClosure : IsIntegrallyClosed R ↔ IsIntegralClosure R R K :=
isIntegrallyClosed_iff_isIntegrallyClosedIn K
/-- `R` is integrally closed in `A` iff all integral elements of `A` are also elements of `R`. -/
theorem isIntegrallyClosedIn_iff {R A : Type*} [CommRing R] [CommRing A] [Algebra R A] :
IsIntegrallyClosedIn R A ↔
Function.Injective (algebraMap R A) ∧
∀ {x : A}, IsIntegral R x → ∃ y, algebraMap R A y = x := by
constructor
· rintro ⟨_, cl⟩
aesop
· rintro ⟨inj, cl⟩
refine ⟨inj, by aesop, ?_⟩
rintro ⟨y, rfl⟩
apply isIntegral_algebraMap
/-- `R` is integrally closed iff all integral elements of its fraction field `K`
are also elements of `R`. -/
theorem isIntegrallyClosed_iff :
IsIntegrallyClosed R ↔ ∀ {x : K}, IsIntegral R x → ∃ y, algebraMap R K y = x := by
simp [isIntegrallyClosed_iff_isIntegrallyClosedIn K, isIntegrallyClosedIn_iff,
IsFractionRing.injective R K]
end Iff
namespace IsIntegrallyClosedIn
variable {R A : Type*} [CommRing R] [CommRing A] [Algebra R A] [iic : IsIntegrallyClosedIn R A]
theorem algebraMap_eq_of_integral {x : A} : IsIntegral R x → ∃ y : R, algebraMap R A y = x :=
IsIntegralClosure.isIntegral_iff.mp
theorem isIntegral_iff {x : A} : IsIntegral R x ↔ ∃ y : R, algebraMap R A y = x :=
IsIntegralClosure.isIntegral_iff
theorem exists_algebraMap_eq_of_isIntegral_pow {x : A} {n : ℕ} (hn : 0 < n)
(hx : IsIntegral R <| x ^ n) : ∃ y : R, algebraMap R A y = x :=
isIntegral_iff.mp <| hx.of_pow hn
theorem exists_algebraMap_eq_of_pow_mem_subalgebra {A : Type*} [CommRing A] [Algebra R A]
{S : Subalgebra R A} [IsIntegrallyClosedIn S A] {x : A} {n : ℕ} (hn : 0 < n)
(hx : x ^ n ∈ S) : ∃ y : S, algebraMap S A y = x :=
exists_algebraMap_eq_of_isIntegral_pow hn <| isIntegral_iff.mpr ⟨⟨x ^ n, hx⟩, rfl⟩
variable (A)
theorem integralClosure_eq_bot_iff (hRA : Function.Injective (algebraMap R A)) :
integralClosure R A = ⊥ ↔ IsIntegrallyClosedIn R A := by
refine eq_bot_iff.trans ?_
constructor
· intro h
refine ⟨ hRA, fun hx => Set.mem_range.mp (Algebra.mem_bot.mp (h hx)), ?_⟩
rintro ⟨y, rfl⟩
apply isIntegral_algebraMap
· intro h x hx
rw [Algebra.mem_bot, Set.mem_range]
exact isIntegral_iff.mp hx
variable (R)
@[simp]
theorem integralClosure_eq_bot [NoZeroSMulDivisors R A] [Nontrivial A] : integralClosure R A = ⊥ :=
(integralClosure_eq_bot_iff A (NoZeroSMulDivisors.algebraMap_injective _ _)).mpr ‹_›
variable {A} {B : Type*} [CommRing B]
/-- If `R` is the integral closure of `S` in `A`, then it is integrally closed in `A`. -/
lemma of_isIntegralClosure [Algebra R B] [Algebra A B] [IsScalarTower R A B]
[IsIntegralClosure A R B] :
IsIntegrallyClosedIn A B :=
have : Algebra.IsIntegral R A := IsIntegralClosure.isIntegral_algebra R B
IsIntegralClosure.tower_top (R := R)
variable {R}
lemma _root_.IsIntegralClosure.of_isIntegrallyClosedIn
[Algebra R B] [Algebra A B] [IsScalarTower R A B]
[IsIntegrallyClosedIn A B] [Algebra.IsIntegral R A] :
IsIntegralClosure A R B := by
refine ⟨IsIntegralClosure.algebraMap_injective _ A _, fun {x} ↦
⟨fun hx ↦ IsIntegralClosure.isIntegral_iff.mp (IsIntegral.tower_top (A := A) hx), ?_⟩⟩
rintro ⟨y, rfl⟩
exact IsIntegral.map (IsScalarTower.toAlgHom A A B) (Algebra.IsIntegral.isIntegral y)
end IsIntegrallyClosedIn
namespace IsIntegrallyClosed
variable {R S : Type*} [CommRing R] [CommRing S] [id : IsDomain R] [iic : IsIntegrallyClosed R]
variable {K : Type*} [CommRing K] [Algebra R K] [ifr : IsFractionRing R K]
/-- Note that this is not a duplicate instance, since `IsIntegrallyClosed R` is instead defined
as `IsIntegrallyClosed R R (FractionRing R)`. -/
instance : IsIntegralClosure R R K :=
(isIntegrallyClosed_iff_isIntegralClosure K).mp iic
theorem algebraMap_eq_of_integral {x : K} : IsIntegral R x → ∃ y : R, algebraMap R K y = x :=
IsIntegralClosure.isIntegral_iff.mp
theorem isIntegral_iff {x : K} : IsIntegral R x ↔ ∃ y : R, algebraMap R K y = x :=
IsIntegrallyClosedIn.isIntegral_iff
theorem exists_algebraMap_eq_of_isIntegral_pow {x : K} {n : ℕ} (hn : 0 < n)
(hx : IsIntegral R <| x ^ n) : ∃ y : R, algebraMap R K y = x :=
IsIntegrallyClosedIn.exists_algebraMap_eq_of_isIntegral_pow hn hx
theorem exists_algebraMap_eq_of_pow_mem_subalgebra {K : Type*} [CommRing K] [Algebra R K]
{S : Subalgebra R K} [IsIntegrallyClosed S] [IsFractionRing S K] {x : K} {n : ℕ} (hn : 0 < n)
(hx : x ^ n ∈ S) : ∃ y : S, algebraMap S K y = x :=
IsIntegrallyClosedIn.exists_algebraMap_eq_of_pow_mem_subalgebra hn hx
variable (R S K)
instance _root_.IsIntegralClosure.of_isIntegrallyClosed
[Algebra S R] [Algebra S K] [IsScalarTower S R K] [Algebra.IsIntegral S R] :
IsIntegralClosure R S K :=
IsIntegralClosure.of_isIntegrallyClosedIn
variable {R}
theorem integralClosure_eq_bot_iff : integralClosure R K = ⊥ ↔ IsIntegrallyClosed R :=
(IsIntegrallyClosedIn.integralClosure_eq_bot_iff _ (IsFractionRing.injective _ _)).trans
(isIntegrallyClosed_iff_isIntegrallyClosedIn _).symm
@[simp]
theorem pow_dvd_pow_iff {n : ℕ} (hn : n ≠ 0) {a b : R} : a ^ n ∣ b ^ n ↔ a ∣ b := by
refine ⟨fun ⟨x, hx⟩ ↦ ?_, fun h ↦ pow_dvd_pow_of_dvd h n⟩
by_cases ha : a = 0
· simpa [ha, hn] using hx
let K := FractionRing R
replace ha : algebraMap R K a ≠ 0 := fun h ↦
ha <| (injective_iff_map_eq_zero _).1 (IsFractionRing.injective R K) _ h
let y := (algebraMap R K b) / (algebraMap R K a)
have hy : IsIntegral R y := by
refine ⟨X ^ n - C x, monic_X_pow_sub_C _ hn, ?_⟩
simp only [y, map_pow, eval₂_sub, eval₂_X_pow, div_pow, eval₂_pow', eval₂_C]
replace hx := congr_arg (algebraMap R K) hx
rw [map_pow] at hx
field_simp [hx, ha]
obtain ⟨k, hk⟩ := algebraMap_eq_of_integral hy
refine ⟨k, IsFractionRing.injective R K ?_⟩
rw [map_mul, hk, mul_div_cancel₀ _ ha]
variable (R)
/-- This is almost a duplicate of `IsIntegrallyClosedIn.integralClosure_eq_bot`,
except the `NoZeroSMulDivisors` hypothesis isn't inferred automatically from `IsFractionRing`. -/
@[simp]
theorem integralClosure_eq_bot : integralClosure R K = ⊥ :=
(integralClosure_eq_bot_iff K).mpr ‹_›
end IsIntegrallyClosed
namespace integralClosure
open IsIntegrallyClosed
variable {R : Type*} [CommRing R]
variable (K : Type*) [Field K] [Algebra R K]
variable [IsFractionRing R K]
variable {L : Type*} [Field L] [Algebra K L] [Algebra R L] [IsScalarTower R K L]
-- Can't be an instance because you need to supply `K`.
theorem isIntegrallyClosedOfFiniteExtension [IsDomain R] [FiniteDimensional K L] :
IsIntegrallyClosed (integralClosure R L) :=
letI : IsFractionRing (integralClosure R L) L := isFractionRing_of_finite_extension K L
(integralClosure_eq_bot_iff L).mp integralClosure_idem
end integralClosure
|
RingTheory\IntegralClosure\IntegralRestrict.lean | /-
Copyright (c) 2023 Andrew Yang, Patrick Lutz. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Andrew Yang
-/
import Mathlib.RingTheory.IntegralClosure.IntegrallyClosed
import Mathlib.RingTheory.LocalProperties
import Mathlib.RingTheory.Localization.NormTrace
import Mathlib.RingTheory.Localization.LocalizationLocalization
import Mathlib.RingTheory.DedekindDomain.IntegralClosure
/-!
# Restriction of various maps between fields to integrally closed subrings.
In this file, we assume `A` is an integrally closed domain; `K` is the fraction ring of `A`;
`L` is a finite (separable) extension of `K`; `B` is the integral closure of `A` in `L`.
We call this the AKLB setup.
## Main definition
- `galRestrict`: The restriction `Aut(L/K) → Aut(B/A)` as an `MulEquiv` in an AKLB setup.
- `Algebra.intTrace`: The trace map of a finite extension of integrally closed domains `B/A` is
defined to be the restriction of the trace map of `Frac(B)/Frac(A)`.
- `Algebra.intNorm`: The norm map of a finite extension of integrally closed domains `B/A` is
defined to be the restriction of the norm map of `Frac(B)/Frac(A)`.
-/
open nonZeroDivisors
variable (A K L B : Type*) [CommRing A] [CommRing B] [Algebra A B] [Field K] [Field L]
[Algebra A K] [IsFractionRing A K] [Algebra B L]
[Algebra K L] [Algebra A L] [IsScalarTower A B L] [IsScalarTower A K L]
[IsIntegralClosure B A L] [FiniteDimensional K L]
section galois
/-- The lift `End(B/A) → End(L/K)` in an ALKB setup.
This is inverse to the restriction. See `galRestrictHom`. -/
noncomputable
def galLift (σ : B →ₐ[A] B) : L →ₐ[K] L :=
haveI := (IsFractionRing.injective A K).isDomain
haveI := NoZeroSMulDivisors.trans A K L
haveI := IsIntegralClosure.isLocalization A K L B
haveI H : ∀ (y : Algebra.algebraMapSubmonoid B A⁰),
IsUnit (((algebraMap B L).comp σ) (y : B)) := by
rintro ⟨_, x, hx, rfl⟩
simpa only [RingHom.coe_comp, RingHom.coe_coe, Function.comp_apply, AlgHom.commutes,
isUnit_iff_ne_zero, ne_eq, map_eq_zero_iff _ (NoZeroSMulDivisors.algebraMap_injective _ _),
← IsScalarTower.algebraMap_apply] using nonZeroDivisors.ne_zero hx
haveI H_eq : (IsLocalization.lift (S := L) H).comp (algebraMap K L) = (algebraMap K L) := by
apply IsLocalization.ringHom_ext A⁰
ext
simp only [RingHom.coe_comp, Function.comp_apply, ← IsScalarTower.algebraMap_apply A K L,
IsScalarTower.algebraMap_apply A B L, IsLocalization.lift_eq,
RingHom.coe_coe, AlgHom.commutes]
{ IsLocalization.lift (S := L) H with commutes' := DFunLike.congr_fun H_eq }
/-- The restriction `End(L/K) → End(B/A)` in an AKLB setup.
Also see `galRestrict` for the `AlgEquiv` version. -/
noncomputable
def galRestrictHom : (L →ₐ[K] L) ≃* (B →ₐ[A] B) where
toFun := fun f ↦ (IsIntegralClosure.equiv A (integralClosure A L) L B).toAlgHom.comp
(((f.restrictScalars A).comp (IsScalarTower.toAlgHom A B L)).codRestrict
(integralClosure A L) (fun x ↦ IsIntegral.map _ (IsIntegralClosure.isIntegral A L x)))
map_mul' := by
intros σ₁ σ₂
ext x
apply (IsIntegralClosure.equiv A (integralClosure A L) L B).symm.injective
ext
dsimp
simp only [AlgEquiv.symm_apply_apply, AlgHom.coe_codRestrict, AlgHom.coe_comp,
AlgHom.coe_restrictScalars', IsScalarTower.coe_toAlgHom', Function.comp_apply,
AlgHom.mul_apply, IsIntegralClosure.algebraMap_equiv, Subalgebra.algebraMap_eq]
rfl
invFun := galLift A K L B
left_inv σ :=
have := (IsFractionRing.injective A K).isDomain
have := IsIntegralClosure.isLocalization A K L B
AlgHom.coe_ringHom_injective <| IsLocalization.ringHom_ext (Algebra.algebraMapSubmonoid B A⁰)
<| RingHom.ext fun x ↦ by simp [Subalgebra.algebraMap_eq, galLift]
right_inv σ :=
have := (IsFractionRing.injective A K).isDomain
have := IsIntegralClosure.isLocalization A K L B
AlgHom.ext fun x ↦
IsIntegralClosure.algebraMap_injective B A L (by simp [Subalgebra.algebraMap_eq, galLift])
@[simp]
lemma algebraMap_galRestrictHom_apply (σ : L →ₐ[K] L) (x : B) :
algebraMap B L (galRestrictHom A K L B σ x) = σ (algebraMap B L x) := by
simp [galRestrictHom, Subalgebra.algebraMap_eq]
@[simp, nolint unusedHavesSuffices] -- false positive from unfolding galRestrictHom
lemma galRestrictHom_symm_algebraMap_apply (σ : B →ₐ[A] B) (x : B) :
(galRestrictHom A K L B).symm σ (algebraMap B L x) = algebraMap B L (σ x) := by
have := (IsFractionRing.injective A K).isDomain
have := IsIntegralClosure.isLocalization A K L B
simp [galRestrictHom, galLift, Subalgebra.algebraMap_eq]
/-- The restriction `Aut(L/K) → Aut(B/A)` in an AKLB setup. -/
noncomputable
def galRestrict : (L ≃ₐ[K] L) ≃* (B ≃ₐ[A] B) :=
(AlgEquiv.algHomUnitsEquiv K L).symm.trans
((Units.mapEquiv <| galRestrictHom A K L B).trans (AlgEquiv.algHomUnitsEquiv A B))
variable {K L}
lemma coe_galRestrict_apply (σ : L ≃ₐ[K] L) :
(galRestrict A K L B σ : B →ₐ[A] B) = galRestrictHom A K L B σ := rfl
variable {B}
lemma galRestrict_apply (σ : L ≃ₐ[K] L) (x : B) :
galRestrict A K L B σ x = galRestrictHom A K L B σ x := rfl
lemma algebraMap_galRestrict_apply (σ : L ≃ₐ[K] L) (x : B) :
algebraMap B L (galRestrict A K L B σ x) = σ (algebraMap B L x) :=
algebraMap_galRestrictHom_apply A K L B σ.toAlgHom x
variable (K L B)
lemma prod_galRestrict_eq_norm [IsGalois K L] [IsIntegrallyClosed A] (x : B) :
(∏ σ : L ≃ₐ[K] L, galRestrict A K L B σ x) =
algebraMap A B (IsIntegralClosure.mk' (R := A) A (Algebra.norm K <| algebraMap B L x)
(Algebra.isIntegral_norm K (IsIntegralClosure.isIntegral A L x).algebraMap)) := by
apply IsIntegralClosure.algebraMap_injective B A L
rw [← IsScalarTower.algebraMap_apply, IsScalarTower.algebraMap_eq A K L]
simp only [map_prod, algebraMap_galRestrict_apply, IsIntegralClosure.algebraMap_mk',
Algebra.norm_eq_prod_automorphisms, AlgHom.coe_coe, RingHom.coe_comp, Function.comp_apply]
end galois
attribute [local instance] FractionRing.liftAlgebra FractionRing.isScalarTower_liftAlgebra
noncomputable
instance (priority := 900) [IsDomain A] [IsDomain B] [IsIntegrallyClosed B]
[Module.Finite A B] [NoZeroSMulDivisors A B] : Fintype (B ≃ₐ[A] B) :=
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (Algebra.algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
Fintype.ofEquiv _ (galRestrict A (FractionRing A) (FractionRing B) B).toEquiv
variable {Aₘ Bₘ} [CommRing Aₘ] [CommRing Bₘ] [Algebra Aₘ Bₘ] [Algebra A Aₘ] [Algebra B Bₘ]
variable [Algebra A Bₘ] [IsScalarTower A Aₘ Bₘ] [IsScalarTower A B Bₘ]
variable (M : Submonoid A) [IsLocalization M Aₘ]
variable [IsLocalization (Algebra.algebraMapSubmonoid B M) Bₘ]
section trace
/-- The restriction of the trace on `L/K` restricted onto `B/A` in an AKLB setup.
See `Algebra.intTrace` instead. -/
noncomputable
def Algebra.intTraceAux [IsIntegrallyClosed A] :
B →ₗ[A] A :=
(IsIntegralClosure.equiv A (integralClosure A K) K A).toLinearMap.comp
((((Algebra.trace K L).restrictScalars A).comp
(IsScalarTower.toAlgHom A B L).toLinearMap).codRestrict
(Subalgebra.toSubmodule <| integralClosure A K) (fun x ↦ isIntegral_trace
(IsIntegral.algebraMap (IsIntegralClosure.isIntegral A L x))))
variable {A K L B}
lemma Algebra.map_intTraceAux [IsIntegrallyClosed A] (x : B) :
algebraMap A K (Algebra.intTraceAux A K L B x) = Algebra.trace K L (algebraMap B L x) :=
IsIntegralClosure.algebraMap_equiv A (integralClosure A K) K A _
variable (A B)
variable [IsDomain A] [IsIntegrallyClosed A] [IsDomain B] [IsIntegrallyClosed B]
variable [Module.Finite A B] [NoZeroSMulDivisors A B]
/-- The trace of a finite extension of integrally closed domains `B/A` is the restriction of
the trace on `Frac(B)/Frac(A)` onto `B/A`. See `Algebra.algebraMap_intTrace`. -/
noncomputable
def Algebra.intTrace : B →ₗ[A] A :=
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
Algebra.intTraceAux A (FractionRing A) (FractionRing B) B
variable {A B}
lemma Algebra.algebraMap_intTrace (x : B) :
algebraMap A K (Algebra.intTrace A B x) = Algebra.trace K L (algebraMap B L x) := by
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
haveI := IsIntegralClosure.isFractionRing_of_finite_extension A K L B
apply (FractionRing.algEquiv A K).symm.injective
rw [AlgEquiv.commutes, Algebra.intTrace, Algebra.map_intTraceAux,
← AlgEquiv.commutes (FractionRing.algEquiv B L)]
apply Algebra.trace_eq_of_equiv_equiv (FractionRing.algEquiv A K).toRingEquiv
(FractionRing.algEquiv B L).toRingEquiv
apply IsLocalization.ringHom_ext A⁰
simp only [AlgEquiv.toRingEquiv_eq_coe, ← AlgEquiv.coe_ringHom_commutes, RingHom.comp_assoc,
AlgHom.comp_algebraMap_of_tower, ← IsScalarTower.algebraMap_eq, RingHom.comp_assoc]
lemma Algebra.algebraMap_intTrace_fractionRing (x : B) :
algebraMap A (FractionRing A) (Algebra.intTrace A B x) =
Algebra.trace (FractionRing A) (FractionRing B) (algebraMap B _ x) := by
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
exact Algebra.map_intTraceAux x
variable (A B)
lemma Algebra.intTrace_eq_trace [Module.Free A B] : Algebra.intTrace A B = Algebra.trace A B := by
ext x
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
apply IsFractionRing.injective A (FractionRing A)
rw [Algebra.algebraMap_intTrace_fractionRing, Algebra.trace_localization A A⁰]
open nonZeroDivisors
variable [IsDomain Aₘ] [IsIntegrallyClosed Aₘ] [IsDomain Bₘ] [IsIntegrallyClosed Bₘ]
variable [NoZeroSMulDivisors Aₘ Bₘ] [Module.Finite Aₘ Bₘ]
lemma Algebra.intTrace_eq_of_isLocalization
(x : B) :
algebraMap A Aₘ (Algebra.intTrace A B x) = Algebra.intTrace Aₘ Bₘ (algebraMap B Bₘ x) := by
by_cases hM : 0 ∈ M
· subsingleton [IsLocalization.uniqueOfZeroMem (S := Aₘ) hM]
replace hM : M ≤ A⁰ := fun x hx ↦ mem_nonZeroDivisors_iff_ne_zero.mpr (fun e ↦ hM (e ▸ hx))
let K := FractionRing A
let L := FractionRing B
have : IsIntegralClosure B A L :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
have : IsLocalization (algebraMapSubmonoid B A⁰) L :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
let f : Aₘ →+* K := IsLocalization.map _ (T := A⁰) (RingHom.id A) hM
letI := f.toAlgebra
have : IsScalarTower A Aₘ K := IsScalarTower.of_algebraMap_eq'
(by rw [RingHom.algebraMap_toAlgebra, IsLocalization.map_comp, RingHomCompTriple.comp_eq])
letI := IsFractionRing.isFractionRing_of_isDomain_of_isLocalization M Aₘ K
let g : Bₘ →+* L := IsLocalization.map _
(M := algebraMapSubmonoid B M) (T := algebraMapSubmonoid B A⁰)
(RingHom.id B) (Submonoid.monotone_map hM)
letI := g.toAlgebra
have : IsScalarTower B Bₘ L := IsScalarTower.of_algebraMap_eq'
(by rw [RingHom.algebraMap_toAlgebra, IsLocalization.map_comp, RingHomCompTriple.comp_eq])
letI := ((algebraMap K L).comp f).toAlgebra
have : IsScalarTower Aₘ K L := IsScalarTower.of_algebraMap_eq' rfl
have : IsScalarTower Aₘ Bₘ L := by
apply IsScalarTower.of_algebraMap_eq'
apply IsLocalization.ringHom_ext M
rw [RingHom.algebraMap_toAlgebra, RingHom.algebraMap_toAlgebra (R := Bₘ), RingHom.comp_assoc,
RingHom.comp_assoc, ← IsScalarTower.algebraMap_eq, IsScalarTower.algebraMap_eq A B Bₘ,
IsLocalization.map_comp, RingHom.comp_id, ← RingHom.comp_assoc, IsLocalization.map_comp,
RingHom.comp_id, ← IsScalarTower.algebraMap_eq, ← IsScalarTower.algebraMap_eq]
letI := IsFractionRing.isFractionRing_of_isDomain_of_isLocalization
(algebraMapSubmonoid B M) Bₘ L
have : FiniteDimensional K L := Module.Finite_of_isLocalization A B _ _ A⁰
have : IsIntegralClosure Bₘ Aₘ L :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
apply IsFractionRing.injective Aₘ K
rw [← IsScalarTower.algebraMap_apply, Algebra.algebraMap_intTrace_fractionRing,
Algebra.algebraMap_intTrace (L := L), ← IsScalarTower.algebraMap_apply]
end trace
section norm
variable [IsIntegrallyClosed A]
/-- The restriction of the norm on `L/K` restricted onto `B/A` in an AKLB setup.
See `Algebra.intNorm` instead. -/
noncomputable
def Algebra.intNormAux [Algebra.IsSeparable K L] :
B →* A where
toFun := fun s ↦ IsIntegralClosure.mk' (R := A) A (Algebra.norm K (algebraMap B L s))
(isIntegral_norm K <| IsIntegral.map (IsScalarTower.toAlgHom A B L)
(IsIntegralClosure.isIntegral A L s))
map_one' := by simp
map_mul' := fun x y ↦ by simpa using IsIntegralClosure.mk'_mul _ _ _ _ _
variable {A K L B}
lemma Algebra.map_intNormAux [Algebra.IsSeparable K L] (x : B) :
algebraMap A K (Algebra.intNormAux A K L B x) = Algebra.norm K (algebraMap B L x) := by
dsimp [Algebra.intNormAux]
exact IsIntegralClosure.algebraMap_mk' _ _ _
variable (A B)
variable [IsDomain A] [IsDomain B] [IsIntegrallyClosed B]
variable [Module.Finite A B] [NoZeroSMulDivisors A B]
variable [Algebra.IsSeparable (FractionRing A) (FractionRing B)] -- TODO: remove this
/-- The norm of a finite extension of integrally closed domains `B/A` is the restriction of
the norm on `Frac(B)/Frac(A)` onto `B/A`. See `Algebra.algebraMap_intNorm`. -/
noncomputable
def Algebra.intNorm : B →* A :=
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
Algebra.intNormAux A (FractionRing A) (FractionRing B) B
variable {A B}
lemma Algebra.algebraMap_intNorm (x : B) :
algebraMap A K (Algebra.intNorm A B x) = Algebra.norm K (algebraMap B L x) := by
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
haveI := IsIntegralClosure.isFractionRing_of_finite_extension A K L B
apply (FractionRing.algEquiv A K).symm.injective
rw [AlgEquiv.commutes, Algebra.intNorm, Algebra.map_intNormAux,
← AlgEquiv.commutes (FractionRing.algEquiv B L)]
apply Algebra.norm_eq_of_equiv_equiv (FractionRing.algEquiv A K).toRingEquiv
(FractionRing.algEquiv B L).toRingEquiv
apply IsLocalization.ringHom_ext A⁰
simp only [AlgEquiv.toRingEquiv_eq_coe, ← AlgEquiv.coe_ringHom_commutes, RingHom.comp_assoc,
AlgHom.comp_algebraMap_of_tower, ← IsScalarTower.algebraMap_eq, RingHom.comp_assoc]
@[simp]
lemma Algebra.algebraMap_intNorm_fractionRing (x : B) :
algebraMap A (FractionRing A) (Algebra.intNorm A B x) =
Algebra.norm (FractionRing A) (algebraMap B (FractionRing B) x) := by
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
exact Algebra.map_intNormAux x
variable (A B)
lemma Algebra.intNorm_eq_norm [Module.Free A B] : Algebra.intNorm A B = Algebra.norm A := by
ext x
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
apply IsFractionRing.injective A (FractionRing A)
rw [Algebra.algebraMap_intNorm_fractionRing, Algebra.norm_localization A A⁰]
@[simp]
lemma Algebra.intNorm_zero : Algebra.intNorm A B 0 = 0 := by
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
apply IsFractionRing.injective A (FractionRing A)
simp only [algebraMap_intNorm_fractionRing, map_zero, norm_zero]
variable {A B}
@[simp]
lemma Algebra.intNorm_eq_zero {x : B} : Algebra.intNorm A B x = 0 ↔ x = 0 := by
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
rw [← (IsFractionRing.injective A (FractionRing A)).eq_iff,
← (IsFractionRing.injective B (FractionRing B)).eq_iff]
simp only [algebraMap_intNorm_fractionRing, map_zero, norm_eq_zero_iff]
lemma Algebra.intNorm_ne_zero {x : B} : Algebra.intNorm A B x ≠ 0 ↔ x ≠ 0 := by simp
variable [IsDomain Aₘ] [IsIntegrallyClosed Aₘ] [IsDomain Bₘ] [IsIntegrallyClosed Bₘ]
variable [NoZeroSMulDivisors Aₘ Bₘ] [Module.Finite Aₘ Bₘ]
variable [Algebra.IsSeparable (FractionRing Aₘ) (FractionRing Bₘ)]
lemma Algebra.intNorm_eq_of_isLocalization (x : B) :
algebraMap A Aₘ (Algebra.intNorm A B x) = Algebra.intNorm Aₘ Bₘ (algebraMap B Bₘ x) := by
by_cases hM : 0 ∈ M
· subsingleton [IsLocalization.uniqueOfZeroMem (S := Aₘ) hM]
replace hM : M ≤ A⁰ := fun x hx ↦ mem_nonZeroDivisors_iff_ne_zero.mpr (fun e ↦ hM (e ▸ hx))
let K := FractionRing A
let L := FractionRing B
have : IsIntegralClosure B A L :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
have : IsLocalization (algebraMapSubmonoid B A⁰) L :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
let f : Aₘ →+* K := IsLocalization.map _ (T := A⁰) (RingHom.id A) hM
letI := f.toAlgebra
have : IsScalarTower A Aₘ K := IsScalarTower.of_algebraMap_eq'
(by rw [RingHom.algebraMap_toAlgebra, IsLocalization.map_comp, RingHomCompTriple.comp_eq])
letI := IsFractionRing.isFractionRing_of_isDomain_of_isLocalization M Aₘ K
let g : Bₘ →+* L := IsLocalization.map _
(M := algebraMapSubmonoid B M) (T := algebraMapSubmonoid B A⁰)
(RingHom.id B) (Submonoid.monotone_map hM)
letI := g.toAlgebra
have : IsScalarTower B Bₘ L := IsScalarTower.of_algebraMap_eq'
(by rw [RingHom.algebraMap_toAlgebra, IsLocalization.map_comp, RingHomCompTriple.comp_eq])
letI := ((algebraMap K L).comp f).toAlgebra
have : IsScalarTower Aₘ K L := IsScalarTower.of_algebraMap_eq' rfl
have : IsScalarTower Aₘ Bₘ L := by
apply IsScalarTower.of_algebraMap_eq'
apply IsLocalization.ringHom_ext M
rw [RingHom.algebraMap_toAlgebra, RingHom.algebraMap_toAlgebra (R := Bₘ), RingHom.comp_assoc,
RingHom.comp_assoc, ← IsScalarTower.algebraMap_eq, IsScalarTower.algebraMap_eq A B Bₘ,
IsLocalization.map_comp, RingHom.comp_id, ← RingHom.comp_assoc, IsLocalization.map_comp,
RingHom.comp_id, ← IsScalarTower.algebraMap_eq, ← IsScalarTower.algebraMap_eq]
letI := IsFractionRing.isFractionRing_of_isDomain_of_isLocalization
(algebraMapSubmonoid B M) Bₘ L
have : FiniteDimensional K L := Module.Finite_of_isLocalization A B _ _ A⁰
have : IsIntegralClosure Bₘ Aₘ L :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
apply IsFractionRing.injective Aₘ K
rw [← IsScalarTower.algebraMap_apply, Algebra.algebraMap_intNorm_fractionRing,
Algebra.algebraMap_intNorm (L := L), ← IsScalarTower.algebraMap_apply]
end norm
lemma Algebra.algebraMap_intNorm_of_isGalois
[IsDomain A] [IsIntegrallyClosed A] [IsDomain B] [IsIntegrallyClosed B]
[Module.Finite A B] [NoZeroSMulDivisors A B] [IsGalois (FractionRing A) (FractionRing B)]
{x : B} :
algebraMap A B (Algebra.intNorm A B x) = ∏ σ : B ≃ₐ[A] B, σ x := by
haveI : IsIntegralClosure B A (FractionRing B) :=
IsIntegralClosure.of_isIntegrallyClosed _ _ _
haveI : IsLocalization (Algebra.algebraMapSubmonoid B A⁰) (FractionRing B) :=
IsIntegralClosure.isLocalization _ (FractionRing A) _ _
haveI : FiniteDimensional (FractionRing A) (FractionRing B) :=
Module.Finite_of_isLocalization A B _ _ A⁰
rw [← (galRestrict A (FractionRing A) (FractionRing B) B).toEquiv.prod_comp]
simp only [MulEquiv.toEquiv_eq_coe, EquivLike.coe_coe]
convert (prod_galRestrict_eq_norm A (FractionRing A) (FractionRing B) B x).symm
|
RingTheory\IntegralClosure\Algebra\Basic.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.RingTheory.IntegralClosure.Algebra.Defs
import Mathlib.RingTheory.IntegralClosure.IsIntegral.Basic
import Mathlib.LinearAlgebra.Matrix.Charpoly.LinearMap
/-!
# Integral closure of a subring.
Let `A` be an `R`-algebra. We prove that integral elements form a sub-`R`-algebra of `A`.
## Main definitions
Let `R` be a `CommRing` and let `A` be an R-algebra.
* `integralClosure R A` : the integral closure of `R` in an `R`-algebra `A`.
-/
open Polynomial Submodule
section
variable {R A B S : Type*}
variable [CommRing R] [CommRing A] [Ring B] [CommRing S]
variable [Algebra R A] [Algebra R B] (f : R →+* S)
section
variable {A B : Type*} [Ring A] [Ring B] [Algebra R A] [Algebra R B]
variable (f : A →ₐ[R] B) (hf : Function.Injective f)
end
instance Module.End.isIntegral {M : Type*} [AddCommGroup M] [Module R M] [Module.Finite R M] :
Algebra.IsIntegral R (Module.End R M) :=
⟨LinearMap.exists_monic_and_aeval_eq_zero R⟩
variable (R)
theorem IsIntegral.of_finite [Module.Finite R B] (x : B) : IsIntegral R x :=
(isIntegral_algHom_iff (Algebra.lmul R B) Algebra.lmul_injective).mp
(Algebra.IsIntegral.isIntegral _)
variable (B)
instance Algebra.IsIntegral.of_finite [Module.Finite R B] : Algebra.IsIntegral R B :=
⟨.of_finite R⟩
variable {R B}
/-- If `S` is a sub-`R`-algebra of `A` and `S` is finitely-generated as an `R`-module,
then all elements of `S` are integral over `R`. -/
theorem IsIntegral.of_mem_of_fg {A} [Ring A] [Algebra R A] (S : Subalgebra R A)
(HS : S.toSubmodule.FG) (x : A) (hx : x ∈ S) : IsIntegral R x :=
have : Module.Finite R S := ⟨(fg_top _).mpr HS⟩
(isIntegral_algHom_iff S.val Subtype.val_injective).mpr (.of_finite R (⟨x, hx⟩ : S))
theorem RingHom.IsIntegralElem.of_mem_closure {x y z : S} (hx : f.IsIntegralElem x)
(hy : f.IsIntegralElem y) (hz : z ∈ Subring.closure ({x, y} : Set S)) : f.IsIntegralElem z := by
letI : Algebra R S := f.toAlgebra
have := (IsIntegral.fg_adjoin_singleton hx).mul (IsIntegral.fg_adjoin_singleton hy)
rw [← Algebra.adjoin_union_coe_submodule, Set.singleton_union] at this
exact
IsIntegral.of_mem_of_fg (Algebra.adjoin R {x, y}) this z
(Algebra.mem_adjoin_iff.2 <| Subring.closure_mono Set.subset_union_right hz)
nonrec theorem IsIntegral.of_mem_closure {x y z : A} (hx : IsIntegral R x) (hy : IsIntegral R y)
(hz : z ∈ Subring.closure ({x, y} : Set A)) : IsIntegral R z :=
hx.of_mem_closure (algebraMap R A) hy hz
variable (f : R →+* B)
theorem RingHom.IsIntegralElem.add (f : R →+* S) {x y : S}
(hx : f.IsIntegralElem x) (hy : f.IsIntegralElem y) :
f.IsIntegralElem (x + y) :=
hx.of_mem_closure f hy <|
Subring.add_mem _ (Subring.subset_closure (Or.inl rfl)) (Subring.subset_closure (Or.inr rfl))
nonrec theorem IsIntegral.add {x y : A} (hx : IsIntegral R x) (hy : IsIntegral R y) :
IsIntegral R (x + y) :=
hx.add (algebraMap R A) hy
variable (f : R →+* S)
-- can be generalized to noncommutative S.
theorem RingHom.IsIntegralElem.neg {x : S} (hx : f.IsIntegralElem x) : f.IsIntegralElem (-x) :=
hx.of_mem_closure f hx (Subring.neg_mem _ (Subring.subset_closure (Or.inl rfl)))
theorem IsIntegral.neg {x : B} (hx : IsIntegral R x) : IsIntegral R (-x) :=
.of_mem_of_fg _ hx.fg_adjoin_singleton _ (Subalgebra.neg_mem _ <| Algebra.subset_adjoin rfl)
theorem RingHom.IsIntegralElem.sub {x y : S} (hx : f.IsIntegralElem x) (hy : f.IsIntegralElem y) :
f.IsIntegralElem (x - y) := by
simpa only [sub_eq_add_neg] using hx.add f (hy.neg f)
nonrec theorem IsIntegral.sub {x y : A} (hx : IsIntegral R x) (hy : IsIntegral R y) :
IsIntegral R (x - y) :=
hx.sub (algebraMap R A) hy
theorem RingHom.IsIntegralElem.mul {x y : S} (hx : f.IsIntegralElem x) (hy : f.IsIntegralElem y) :
f.IsIntegralElem (x * y) :=
hx.of_mem_closure f hy
(Subring.mul_mem _ (Subring.subset_closure (Or.inl rfl)) (Subring.subset_closure (Or.inr rfl)))
nonrec theorem IsIntegral.mul {x y : A} (hx : IsIntegral R x) (hy : IsIntegral R y) :
IsIntegral R (x * y) :=
hx.mul (algebraMap R A) hy
theorem IsIntegral.smul {R} [CommSemiring R] [CommRing S] [Algebra R B] [Algebra S B] [Algebra R S]
[IsScalarTower R S B] {x : B} (r : R)(hx : IsIntegral S x) : IsIntegral S (r • x) :=
.of_mem_of_fg _ hx.fg_adjoin_singleton _ <| by
rw [← algebraMap_smul S]; apply Subalgebra.smul_mem; exact Algebra.subset_adjoin rfl
variable (R A)
/-- The integral closure of `R` in an `R`-algebra `A`. -/
def integralClosure : Subalgebra R A where
carrier := { r | IsIntegral R r }
zero_mem' := isIntegral_zero
one_mem' := isIntegral_one
add_mem' := IsIntegral.add
mul_mem' := IsIntegral.mul
algebraMap_mem' _ := isIntegral_algebraMap
end
|
RingTheory\IntegralClosure\Algebra\Defs.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.RingTheory.IntegralClosure.IsIntegral.Defs
/-!
# Integral algebras
## Main definitions
Let `R` be a `CommRing` and let `A` be an R-algebra.
* `Algebra.IsIntegral R A` : An algebra is integral if every element of the extension is integral
over the base ring.
-/
open Polynomial Submodule
section Ring
variable {R S A : Type*}
variable [CommRing R] [Ring A] [Ring S] (f : R →+* S)
variable [Algebra R A] (R)
variable (A)
/-- An algebra is integral if every element of the extension is integral over the base ring. -/
protected class Algebra.IsIntegral : Prop :=
isIntegral : ∀ x : A, IsIntegral R x
variable {R A}
lemma Algebra.isIntegral_def : Algebra.IsIntegral R A ↔ ∀ x : A, IsIntegral R x :=
⟨fun ⟨h⟩ ↦ h, fun h ↦ ⟨h⟩⟩
end Ring
|
RingTheory\IntegralClosure\IsIntegral\Basic.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.RingTheory.IntegralClosure.IsIntegral.Defs
import Mathlib.Algebra.Polynomial.Expand
import Mathlib.RingTheory.Polynomial.Tower
/-!
# Properties of integral elements.
We prove basic properties of integral elements in a ring extension.
-/
open Polynomial Submodule
section Ring
variable {R S A : Type*}
variable [CommRing R] [Ring A] [Ring S] (f : R →+* S)
variable [Algebra R A]
theorem RingHom.isIntegralElem_map {x : R} : f.IsIntegralElem (f x) :=
⟨X - C x, monic_X_sub_C _, by simp⟩
theorem isIntegral_algebraMap {x : R} : IsIntegral R (algebraMap R A x) :=
(algebraMap R A).isIntegralElem_map
end Ring
section
variable {R A B S : Type*}
variable [CommRing R] [CommRing A] [Ring B] [CommRing S]
variable [Algebra R A] (f : R →+* S)
theorem IsIntegral.map {B C F : Type*} [Ring B] [Ring C] [Algebra R B] [Algebra A B] [Algebra R C]
[IsScalarTower R A B] [Algebra A C] [IsScalarTower R A C] {b : B}
[FunLike F B C] [AlgHomClass F A B C] (f : F)
(hb : IsIntegral R b) : IsIntegral R (f b) := by
obtain ⟨P, hP⟩ := hb
refine ⟨P, hP.1, ?_⟩
rw [← aeval_def, ← aeval_map_algebraMap A,
aeval_algHom_apply, aeval_map_algebraMap, aeval_def, hP.2, _root_.map_zero]
section
variable {A B : Type*} [Ring A] [Ring B] [Algebra R A] [Algebra R B]
theorem isIntegral_algHom_iff (f : A →ₐ[R] B) (hf : Function.Injective f) {x : A} :
IsIntegral R (f x) ↔ IsIntegral R x := by
refine ⟨fun ⟨p, hp, hx⟩ ↦ ⟨p, hp, ?_⟩, IsIntegral.map f⟩
rwa [← f.comp_algebraMap, ← AlgHom.coe_toRingHom, ← hom_eval₂, AlgHom.coe_toRingHom,
map_eq_zero_iff f hf] at hx
end
open Classical in
theorem Submodule.span_range_natDegree_eq_adjoin {R A} [CommRing R] [Semiring A] [Algebra R A]
{x : A} {f : R[X]} (hf : f.Monic) (hfx : aeval x f = 0) :
span R (Finset.image (x ^ ·) (Finset.range (natDegree f))) =
Subalgebra.toSubmodule (Algebra.adjoin R {x}) := by
nontriviality A
have hf1 : f ≠ 1 := by rintro rfl; simp [one_ne_zero' A] at hfx
refine (span_le.mpr fun s hs ↦ ?_).antisymm fun r hr ↦ ?_
· rcases Finset.mem_image.1 hs with ⟨k, -, rfl⟩
exact (Algebra.adjoin R {x}).pow_mem (Algebra.subset_adjoin rfl) k
rw [Subalgebra.mem_toSubmodule, Algebra.adjoin_singleton_eq_range_aeval] at hr
rcases (aeval x).mem_range.mp hr with ⟨p, rfl⟩
rw [← modByMonic_add_div p hf, map_add, map_mul, hfx,
zero_mul, add_zero, ← sum_C_mul_X_pow_eq (p %ₘ f), aeval_def, eval₂_sum, sum_def]
refine sum_mem fun k hkq ↦ ?_
rw [C_mul_X_pow_eq_monomial, eval₂_monomial, ← Algebra.smul_def]
exact smul_mem _ _ (subset_span <| Finset.mem_image_of_mem _ <| Finset.mem_range.mpr <|
(le_natDegree_of_mem_supp _ hkq).trans_lt <| natDegree_modByMonic_lt p hf hf1)
theorem IsIntegral.fg_adjoin_singleton [Algebra R B] {x : B} (hx : IsIntegral R x) :
(Algebra.adjoin R {x}).toSubmodule.FG := by
classical
rcases hx with ⟨f, hfm, hfx⟩
use (Finset.range <| f.natDegree).image (x ^ ·)
exact span_range_natDegree_eq_adjoin hfm (by rwa [aeval_def])
variable (f : R →+* B)
theorem RingHom.isIntegralElem_zero : f.IsIntegralElem 0 :=
f.map_zero ▸ f.isIntegralElem_map
theorem isIntegral_zero [Algebra R B] : IsIntegral R (0 : B) :=
(algebraMap R B).isIntegralElem_zero
theorem RingHom.isIntegralElem_one : f.IsIntegralElem 1 :=
f.map_one ▸ f.isIntegralElem_map
theorem isIntegral_one [Algebra R B] : IsIntegral R (1 : B) :=
(algebraMap R B).isIntegralElem_one
variable (f : R →+* S)
theorem IsIntegral.of_pow [Algebra R B] {x : B} {n : ℕ} (hn : 0 < n) (hx : IsIntegral R <| x ^ n) :
IsIntegral R x := by
rcases hx with ⟨p, hmonic, heval⟩
exact ⟨expand R n p, hmonic.expand hn, by rwa [← aeval_def, expand_aeval]⟩
end
|
RingTheory\IntegralClosure\IsIntegral\Defs.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.Algebra.Polynomial.Eval
/-!
# Integral closure of a subring.
If A is an R-algebra then `a : A` is integral over R if it is a root of a monic polynomial
with coefficients in R.
## Main definitions
Let `R` be a `CommRing` and let `A` be an R-algebra.
* `RingHom.IsIntegralElem (f : R →+* A) (x : A)` : `x` is integral with respect to the map `f`,
* `IsIntegral (x : A)` : `x` is integral over `R`, i.e., is a root of a monic polynomial with
coefficients in `R`.
-/
open Polynomial
section Ring
variable {R S A : Type*}
variable [CommRing R] [Ring A] [Ring S] (f : R →+* S)
/-- An element `x` of `A` is said to be integral over `R` with respect to `f`
if it is a root of a monic polynomial `p : R[X]` evaluated under `f` -/
def RingHom.IsIntegralElem (f : R →+* A) (x : A) :=
∃ p : R[X], Monic p ∧ eval₂ f x p = 0
/-- A ring homomorphism `f : R →+* A` is said to be integral
if every element `A` is integral with respect to the map `f` -/
def RingHom.IsIntegral (f : R →+* A) :=
∀ x : A, f.IsIntegralElem x
variable [Algebra R A] (R)
/-- An element `x` of an algebra `A` over a commutative ring `R` is said to be *integral*,
if it is a root of some monic polynomial `p : R[X]`.
Equivalently, the element is integral over `R` with respect to the induced `algebraMap` -/
def IsIntegral (x : A) : Prop :=
(algebraMap R A).IsIntegralElem x
end Ring
|
RingTheory\IntegralClosure\IsIntegralClosure\Basic.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.RingTheory.IntegralClosure.IsIntegralClosure.Defs
import Mathlib.RingTheory.IntegralClosure.Algebra.Basic
import Mathlib.RingTheory.FiniteType
import Mathlib.RingTheory.Polynomial.ScaleRoots
/-!
# # Integral closure as a characteristic predicate
We prove basic properties of `IsIntegralClosure`.
-/
open scoped Classical
open Polynomial Submodule
section
variable {R A B S : Type*}
variable [CommRing R] [CommRing A] [Ring B] [CommRing S]
variable [Algebra R A] [Algebra R B] (f : R →+* S)
theorem IsIntegral.map_of_comp_eq {R S T U : Type*} [CommRing R] [Ring S]
[CommRing T] [Ring U] [Algebra R S] [Algebra T U] (φ : R →+* T) (ψ : S →+* U)
(h : (algebraMap T U).comp φ = ψ.comp (algebraMap R S)) {a : S} (ha : IsIntegral R a) :
IsIntegral T (ψ a) :=
let ⟨p, hp⟩ := ha
⟨p.map φ, hp.1.map _, by
rw [← eval_map, map_map, h, ← map_map, eval_map, eval₂_at_apply, eval_map, hp.2, ψ.map_zero]⟩
section
variable {A B : Type*} [Ring A] [Ring B] [Algebra R A] [Algebra R B]
variable (f : A →ₐ[R] B)
theorem Algebra.IsIntegral.of_injective (hf : Function.Injective f) [Algebra.IsIntegral R B] :
Algebra.IsIntegral R A :=
⟨fun _ ↦ (isIntegral_algHom_iff f hf).mp (isIntegral _)⟩
end
@[simp]
theorem isIntegral_algEquiv {A B : Type*} [Ring A] [Ring B] [Algebra R A] [Algebra R B]
(f : A ≃ₐ[R] B) {x : A} : IsIntegral R (f x) ↔ IsIntegral R x :=
⟨fun h ↦ by simpa using h.map f.symm, IsIntegral.map f⟩
/-- If `R → A → B` is an algebra tower,
then if the entire tower is an integral extension so is `A → B`. -/
theorem IsIntegral.tower_top [Algebra A B] [IsScalarTower R A B] {x : B}
(hx : IsIntegral R x) : IsIntegral A x :=
let ⟨p, hp, hpx⟩ := hx
⟨p.map <| algebraMap R A, hp.map _, by rw [← aeval_def, aeval_map_algebraMap, aeval_def, hpx]⟩
theorem map_isIntegral_int {B C F : Type*} [Ring B] [Ring C] {b : B}
[FunLike F B C] [RingHomClass F B C] (f : F)
(hb : IsIntegral ℤ b) : IsIntegral ℤ (f b) :=
hb.map (f : B →+* C).toIntAlgHom
theorem IsIntegral.of_subring {x : B} (T : Subring R) (hx : IsIntegral T x) : IsIntegral R x :=
hx.tower_top
protected theorem IsIntegral.algebraMap [Algebra A B] [IsScalarTower R A B] {x : A}
(h : IsIntegral R x) : IsIntegral R (algebraMap A B x) := by
rcases h with ⟨f, hf, hx⟩
use f, hf
rw [IsScalarTower.algebraMap_eq R A B, ← hom_eval₂, hx, RingHom.map_zero]
theorem isIntegral_algebraMap_iff [Algebra A B] [IsScalarTower R A B] {x : A}
(hAB : Function.Injective (algebraMap A B)) :
IsIntegral R (algebraMap A B x) ↔ IsIntegral R x :=
isIntegral_algHom_iff (IsScalarTower.toAlgHom R A B) hAB
theorem isIntegral_iff_isIntegral_closure_finite {r : B} :
IsIntegral R r ↔ ∃ s : Set R, s.Finite ∧ IsIntegral (Subring.closure s) r := by
constructor <;> intro hr
· rcases hr with ⟨p, hmp, hpr⟩
refine ⟨_, Finset.finite_toSet _, p.restriction, monic_restriction.2 hmp, ?_⟩
rw [← aeval_def, ← aeval_map_algebraMap R r p.restriction, map_restriction, aeval_def, hpr]
rcases hr with ⟨s, _, hsr⟩
exact hsr.of_subring _
theorem fg_adjoin_of_finite {s : Set A} (hfs : s.Finite) (his : ∀ x ∈ s, IsIntegral R x) :
(Algebra.adjoin R s).toSubmodule.FG :=
Set.Finite.induction_on hfs
(fun _ =>
⟨{1},
Submodule.ext fun x => by
rw [Algebra.adjoin_empty, Finset.coe_singleton, ← one_eq_span, Algebra.toSubmodule_bot]⟩)
(fun {a s} _ _ ih his => by
rw [← Set.union_singleton, Algebra.adjoin_union_coe_submodule]
exact
FG.mul (ih fun i hi => his i <| Set.mem_insert_of_mem a hi)
(his a <| Set.mem_insert a s).fg_adjoin_singleton)
his
theorem isNoetherian_adjoin_finset [IsNoetherianRing R] (s : Finset A)
(hs : ∀ x ∈ s, IsIntegral R x) : IsNoetherian R (Algebra.adjoin R (s : Set A)) :=
isNoetherian_of_fg_of_noetherian _ (fg_adjoin_of_finite s.finite_toSet hs)
theorem isIntegral_of_noetherian (_ : IsNoetherian R B) (x : B) : IsIntegral R x :=
.of_finite R x
theorem isIntegral_of_submodule_noetherian (S : Subalgebra R B)
(H : IsNoetherian R (Subalgebra.toSubmodule S)) (x : B) (hx : x ∈ S) : IsIntegral R x :=
.of_mem_of_fg _ ((fg_top _).mp <| H.noetherian _) _ hx
/-- Suppose `A` is an `R`-algebra, `M` is an `A`-module such that `a • m ≠ 0` for all non-zero `a`
and `m`. If `x : A` fixes a nontrivial f.g. `R`-submodule `N` of `M`, then `x` is `R`-integral. -/
theorem isIntegral_of_smul_mem_submodule {M : Type*} [AddCommGroup M] [Module R M] [Module A M]
[IsScalarTower R A M] [NoZeroSMulDivisors A M] (N : Submodule R M) (hN : N ≠ ⊥) (hN' : N.FG)
(x : A) (hx : ∀ n ∈ N, x • n ∈ N) : IsIntegral R x := by
let A' : Subalgebra R A :=
{ carrier := { x | ∀ n ∈ N, x • n ∈ N }
mul_mem' := fun {a b} ha hb n hn => smul_smul a b n ▸ ha _ (hb _ hn)
one_mem' := fun n hn => (one_smul A n).symm ▸ hn
add_mem' := fun {a b} ha hb n hn => (add_smul a b n).symm ▸ N.add_mem (ha _ hn) (hb _ hn)
zero_mem' := fun n _hn => (zero_smul A n).symm ▸ N.zero_mem
algebraMap_mem' := fun r n hn => (algebraMap_smul A r n).symm ▸ N.smul_mem r hn }
let f : A' →ₐ[R] Module.End R N :=
AlgHom.ofLinearMap
{ toFun := fun x => (DistribMulAction.toLinearMap R M x).restrict x.prop
-- Porting note: was
-- `fun x y => LinearMap.ext fun n => Subtype.ext <| add_smul x y n`
map_add' := by intros x y; ext; exact add_smul _ _ _
-- Porting note: was
-- `fun r s => LinearMap.ext fun n => Subtype.ext <| smul_assoc r s n`
map_smul' := by intros r s; ext; apply smul_assoc }
-- Porting note: the next two lines were
--`(LinearMap.ext fun n => Subtype.ext <| one_smul _ _) fun x y =>`
--`LinearMap.ext fun n => Subtype.ext <| mul_smul x y n`
(by ext; apply one_smul)
(by intros x y; ext; apply mul_smul)
obtain ⟨a, ha₁, ha₂⟩ : ∃ a ∈ N, a ≠ (0 : M) := by
by_contra! h'
apply hN
rwa [eq_bot_iff]
have : Function.Injective f := by
show Function.Injective f.toLinearMap
rw [← LinearMap.ker_eq_bot, eq_bot_iff]
intro s hs
have : s.1 • a = 0 := congr_arg Subtype.val (LinearMap.congr_fun hs ⟨a, ha₁⟩)
exact Subtype.ext ((eq_zero_or_eq_zero_of_smul_eq_zero this).resolve_right ha₂)
show IsIntegral R (A'.val ⟨x, hx⟩)
rw [isIntegral_algHom_iff A'.val Subtype.val_injective, ← isIntegral_algHom_iff f this]
haveI : Module.Finite R N := by rwa [Module.finite_def, Submodule.fg_top]
apply Algebra.IsIntegral.isIntegral
variable {f}
theorem RingHom.Finite.to_isIntegral (h : f.Finite) : f.IsIntegral :=
letI := f.toAlgebra
fun _ ↦ IsIntegral.of_mem_of_fg ⊤ h.1 _ trivial
alias RingHom.IsIntegral.of_finite := RingHom.Finite.to_isIntegral
/-- The [Kurosh problem](https://en.wikipedia.org/wiki/Kurosh_problem) asks to show that
this is still true when `A` is not necessarily commutative and `R` is a field, but it has
been solved in the negative. See https://arxiv.org/pdf/1706.02383.pdf for criteria for a
finitely generated algebraic (= integral) algebra over a field to be finite dimensional.
This could be an `instance`, but we tend to go from `Module.Finite` to `IsIntegral`/`IsAlgebraic`,
and making it an instance will cause the search to be complicated a lot.
-/
theorem Algebra.IsIntegral.finite [Algebra.IsIntegral R A] [h' : Algebra.FiniteType R A] :
Module.Finite R A :=
have ⟨s, hs⟩ := h'
⟨by apply hs ▸ fg_adjoin_of_finite s.finite_toSet fun x _ ↦ Algebra.IsIntegral.isIntegral x⟩
/-- finite = integral + finite type -/
theorem Algebra.finite_iff_isIntegral_and_finiteType :
Module.Finite R A ↔ Algebra.IsIntegral R A ∧ Algebra.FiniteType R A :=
⟨fun _ ↦ ⟨⟨.of_finite R⟩, inferInstance⟩, fun ⟨h, _⟩ ↦ h.finite⟩
theorem RingHom.IsIntegral.to_finite (h : f.IsIntegral) (h' : f.FiniteType) : f.Finite :=
let _ := f.toAlgebra
let _ : Algebra.IsIntegral R S := ⟨h⟩
Algebra.IsIntegral.finite (h' := h')
alias RingHom.Finite.of_isIntegral_of_finiteType := RingHom.IsIntegral.to_finite
/-- finite = integral + finite type -/
theorem RingHom.finite_iff_isIntegral_and_finiteType : f.Finite ↔ f.IsIntegral ∧ f.FiniteType :=
⟨fun h ↦ ⟨h.to_isIntegral, h.to_finiteType⟩, fun ⟨h, h'⟩ ↦ h.to_finite h'⟩
variable (f : R →+* S) (R A)
theorem mem_integralClosure_iff_mem_fg {r : A} :
r ∈ integralClosure R A ↔ ∃ M : Subalgebra R A, M.toSubmodule.FG ∧ r ∈ M :=
⟨fun hr =>
⟨Algebra.adjoin R {r}, hr.fg_adjoin_singleton, Algebra.subset_adjoin rfl⟩,
fun ⟨M, Hf, hrM⟩ => .of_mem_of_fg M Hf _ hrM⟩
variable {R A}
theorem adjoin_le_integralClosure {x : A} (hx : IsIntegral R x) :
Algebra.adjoin R {x} ≤ integralClosure R A := by
rw [Algebra.adjoin_le_iff]
simp only [SetLike.mem_coe, Set.singleton_subset_iff]
exact hx
theorem le_integralClosure_iff_isIntegral {S : Subalgebra R A} :
S ≤ integralClosure R A ↔ Algebra.IsIntegral R S :=
SetLike.forall.symm.trans <|
(forall_congr' fun x =>
show IsIntegral R (algebraMap S A x) ↔ IsIntegral R x from
isIntegral_algebraMap_iff Subtype.coe_injective).trans
Algebra.isIntegral_def.symm
theorem Algebra.isIntegral_sup {S T : Subalgebra R A} :
Algebra.IsIntegral R (S ⊔ T : Subalgebra R A) ↔
Algebra.IsIntegral R S ∧ Algebra.IsIntegral R T := by
simp only [← le_integralClosure_iff_isIntegral, sup_le_iff]
/-- Mapping an integral closure along an `AlgEquiv` gives the integral closure. -/
theorem integralClosure_map_algEquiv [Algebra R S] (f : A ≃ₐ[R] S) :
(integralClosure R A).map (f : A →ₐ[R] S) = integralClosure R S := by
ext y
rw [Subalgebra.mem_map]
constructor
· rintro ⟨x, hx, rfl⟩
exact hx.map f
· intro hy
use f.symm y, hy.map (f.symm : S →ₐ[R] A)
simp
/-- An `AlgHom` between two rings restrict to an `AlgHom` between the integral closures inside
them. -/
def AlgHom.mapIntegralClosure [Algebra R S] (f : A →ₐ[R] S) :
integralClosure R A →ₐ[R] integralClosure R S :=
(f.restrictDomain (integralClosure R A)).codRestrict (integralClosure R S) (fun ⟨_, h⟩ => h.map f)
@[simp]
theorem AlgHom.coe_mapIntegralClosure [Algebra R S] (f : A →ₐ[R] S)
(x : integralClosure R A) : (f.mapIntegralClosure x : S) = f (x : A) := rfl
/-- An `AlgEquiv` between two rings restrict to an `AlgEquiv` between the integral closures inside
them. -/
def AlgEquiv.mapIntegralClosure [Algebra R S] (f : A ≃ₐ[R] S) :
integralClosure R A ≃ₐ[R] integralClosure R S :=
AlgEquiv.ofAlgHom (f : A →ₐ[R] S).mapIntegralClosure (f.symm : S →ₐ[R] A).mapIntegralClosure
(AlgHom.ext fun _ ↦ Subtype.ext (f.right_inv _))
(AlgHom.ext fun _ ↦ Subtype.ext (f.left_inv _))
@[simp]
theorem AlgEquiv.coe_mapIntegralClosure [Algebra R S] (f : A ≃ₐ[R] S)
(x : integralClosure R A) : (f.mapIntegralClosure x : S) = f (x : A) := rfl
theorem integralClosure.isIntegral (x : integralClosure R A) : IsIntegral R x :=
let ⟨p, hpm, hpx⟩ := x.2
⟨p, hpm,
Subtype.eq <| by
rwa [← aeval_def, ← Subalgebra.val_apply, aeval_algHom_apply] at hpx⟩
instance integralClosure.AlgebraIsIntegral : Algebra.IsIntegral R (integralClosure R A) :=
⟨integralClosure.isIntegral⟩
theorem IsIntegral.of_mul_unit {x y : B} {r : R} (hr : algebraMap R B r * y = 1)
(hx : IsIntegral R (x * y)) : IsIntegral R x := by
obtain ⟨p, p_monic, hp⟩ := hx
refine ⟨scaleRoots p r, (monic_scaleRoots_iff r).2 p_monic, ?_⟩
convert scaleRoots_aeval_eq_zero hp
rw [Algebra.commutes] at hr ⊢
rw [mul_assoc, hr, mul_one]; rfl
theorem RingHom.IsIntegralElem.of_mul_unit (x y : S) (r : R) (hr : f r * y = 1)
(hx : f.IsIntegralElem (x * y)) : f.IsIntegralElem x :=
letI : Algebra R S := f.toAlgebra
IsIntegral.of_mul_unit hr hx
/-- Generalization of `IsIntegral.of_mem_closure` bootstrapped up from that lemma -/
theorem IsIntegral.of_mem_closure' (G : Set A) (hG : ∀ x ∈ G, IsIntegral R x) :
∀ x ∈ Subring.closure G, IsIntegral R x := fun _ hx ↦
Subring.closure_induction hx hG isIntegral_zero isIntegral_one (fun _ _ ↦ IsIntegral.add)
(fun _ ↦ IsIntegral.neg) fun _ _ ↦ IsIntegral.mul
theorem IsIntegral.of_mem_closure'' {S : Type*} [CommRing S] {f : R →+* S} (G : Set S)
(hG : ∀ x ∈ G, f.IsIntegralElem x) : ∀ x ∈ Subring.closure G, f.IsIntegralElem x := fun x hx =>
@IsIntegral.of_mem_closure' R S _ _ f.toAlgebra G hG x hx
theorem IsIntegral.pow {x : B} (h : IsIntegral R x) (n : ℕ) : IsIntegral R (x ^ n) :=
.of_mem_of_fg _ h.fg_adjoin_singleton _ <|
Subalgebra.pow_mem _ (by exact Algebra.subset_adjoin rfl) _
theorem IsIntegral.nsmul {x : B} (h : IsIntegral R x) (n : ℕ) : IsIntegral R (n • x) :=
h.smul n
theorem IsIntegral.zsmul {x : B} (h : IsIntegral R x) (n : ℤ) : IsIntegral R (n • x) :=
h.smul n
theorem IsIntegral.multiset_prod {s : Multiset A} (h : ∀ x ∈ s, IsIntegral R x) :
IsIntegral R s.prod :=
(integralClosure R A).multiset_prod_mem h
theorem IsIntegral.multiset_sum {s : Multiset A} (h : ∀ x ∈ s, IsIntegral R x) :
IsIntegral R s.sum :=
(integralClosure R A).multiset_sum_mem h
theorem IsIntegral.prod {α : Type*} {s : Finset α} (f : α → A) (h : ∀ x ∈ s, IsIntegral R (f x)) :
IsIntegral R (∏ x ∈ s, f x) :=
(integralClosure R A).prod_mem h
theorem IsIntegral.sum {α : Type*} {s : Finset α} (f : α → A) (h : ∀ x ∈ s, IsIntegral R (f x)) :
IsIntegral R (∑ x ∈ s, f x) :=
(integralClosure R A).sum_mem h
theorem IsIntegral.det {n : Type*} [Fintype n] [DecidableEq n] {M : Matrix n n A}
(h : ∀ i j, IsIntegral R (M i j)) : IsIntegral R M.det := by
rw [Matrix.det_apply]
exact IsIntegral.sum _ fun σ _hσ ↦ (IsIntegral.prod _ fun i _hi => h _ _).zsmul _
@[simp]
theorem IsIntegral.pow_iff {x : A} {n : ℕ} (hn : 0 < n) : IsIntegral R (x ^ n) ↔ IsIntegral R x :=
⟨IsIntegral.of_pow hn, fun hx ↦ hx.pow n⟩
open TensorProduct
theorem IsIntegral.tmul (x : A) {y : B} (h : IsIntegral R y) : IsIntegral A (x ⊗ₜ[R] y) := by
rw [← mul_one x, ← smul_eq_mul, ← smul_tmul']
exact smul _ (h.map_of_comp_eq (algebraMap R A)
(Algebra.TensorProduct.includeRight (R := R) (A := A) (B := B)).toRingHom
Algebra.TensorProduct.includeLeftRingHom_comp_algebraMap)
section
variable (p : R[X]) (x : S)
/-- The monic polynomial whose roots are `p.leadingCoeff * x` for roots `x` of `p`. -/
noncomputable def normalizeScaleRoots (p : R[X]) : R[X] :=
∑ i ∈ p.support,
monomial i (if i = p.natDegree then 1 else p.coeff i * p.leadingCoeff ^ (p.natDegree - 1 - i))
theorem normalizeScaleRoots_coeff_mul_leadingCoeff_pow (i : ℕ) (hp : 1 ≤ natDegree p) :
(normalizeScaleRoots p).coeff i * p.leadingCoeff ^ i =
p.coeff i * p.leadingCoeff ^ (p.natDegree - 1) := by
simp only [normalizeScaleRoots, finset_sum_coeff, coeff_monomial, Finset.sum_ite_eq', one_mul,
zero_mul, mem_support_iff, ite_mul, Ne, ite_not]
split_ifs with h₁ h₂
· simp [h₁]
· rw [h₂, leadingCoeff, ← pow_succ', tsub_add_cancel_of_le hp]
· rw [mul_assoc, ← pow_add, tsub_add_cancel_of_le]
apply Nat.le_sub_one_of_lt
rw [lt_iff_le_and_ne]
exact ⟨le_natDegree_of_ne_zero h₁, h₂⟩
theorem leadingCoeff_smul_normalizeScaleRoots (p : R[X]) :
p.leadingCoeff • normalizeScaleRoots p = scaleRoots p p.leadingCoeff := by
ext
simp only [coeff_scaleRoots, normalizeScaleRoots, coeff_monomial, coeff_smul, Finset.smul_sum,
Ne, Finset.sum_ite_eq', finset_sum_coeff, smul_ite, smul_zero, mem_support_iff]
-- Porting note: added the following `simp only`
simp only [tsub_le_iff_right, smul_eq_mul, mul_ite, mul_one, mul_zero,
Finset.sum_ite_eq', mem_support_iff, ne_eq, ite_not]
split_ifs with h₁ h₂
· simp [*]
· simp [*]
· rw [mul_comm, mul_assoc, ← pow_succ, tsub_right_comm,
tsub_add_cancel_of_le]
rw [Nat.succ_le_iff]
exact tsub_pos_of_lt (lt_of_le_of_ne (le_natDegree_of_ne_zero h₁) h₂)
theorem normalizeScaleRoots_support : (normalizeScaleRoots p).support ≤ p.support := by
intro x
contrapose
simp only [not_mem_support_iff, normalizeScaleRoots, finset_sum_coeff, coeff_monomial,
Finset.sum_ite_eq', mem_support_iff, Ne, Classical.not_not, ite_eq_right_iff]
intro h₁ h₂
exact (h₂ h₁).elim
theorem normalizeScaleRoots_degree : (normalizeScaleRoots p).degree = p.degree := by
apply le_antisymm
· exact Finset.sup_mono (normalizeScaleRoots_support p)
· rw [← degree_scaleRoots, ← leadingCoeff_smul_normalizeScaleRoots]
exact degree_smul_le _ _
theorem normalizeScaleRoots_eval₂_leadingCoeff_mul (h : 1 ≤ p.natDegree) (f : R →+* S) (x : S) :
(normalizeScaleRoots p).eval₂ f (f p.leadingCoeff * x) =
f p.leadingCoeff ^ (p.natDegree - 1) * p.eval₂ f x := by
rw [eval₂_eq_sum_range, eval₂_eq_sum_range, Finset.mul_sum]
apply Finset.sum_congr
· rw [natDegree_eq_of_degree_eq (normalizeScaleRoots_degree p)]
intro n _hn
rw [mul_pow, ← mul_assoc, ← f.map_pow, ← f.map_mul,
normalizeScaleRoots_coeff_mul_leadingCoeff_pow _ _ h, f.map_mul, f.map_pow]
ring
theorem normalizeScaleRoots_monic (h : p ≠ 0) : (normalizeScaleRoots p).Monic := by
delta Monic leadingCoeff
rw [natDegree_eq_of_degree_eq (normalizeScaleRoots_degree p)]
suffices p = 0 → (0 : R) = 1 by simpa [normalizeScaleRoots, coeff_monomial]
exact fun h' => (h h').elim
/-- Given a `p : R[X]` and a `x : S` such that `p.eval₂ f x = 0`,
`f p.leadingCoeff * x` is integral. -/
theorem RingHom.isIntegralElem_leadingCoeff_mul (h : p.eval₂ f x = 0) :
f.IsIntegralElem (f p.leadingCoeff * x) := by
by_cases h' : 1 ≤ p.natDegree
· use normalizeScaleRoots p
have : p ≠ 0 := fun h'' => by
rw [h'', natDegree_zero] at h'
exact Nat.not_succ_le_zero 0 h'
use normalizeScaleRoots_monic p this
rw [normalizeScaleRoots_eval₂_leadingCoeff_mul p h' f x, h, mul_zero]
· by_cases hp : p.map f = 0
· apply_fun fun q => coeff q p.natDegree at hp
rw [coeff_map, coeff_zero, coeff_natDegree] at hp
rw [hp, zero_mul]
exact f.isIntegralElem_zero
· rw [Nat.one_le_iff_ne_zero, Classical.not_not] at h'
rw [eq_C_of_natDegree_eq_zero h', eval₂_C] at h
suffices p.map f = 0 by exact (hp this).elim
rw [eq_C_of_natDegree_eq_zero h', map_C, h, C_eq_zero]
/-- Given a `p : R[X]` and a root `x : S`,
then `p.leadingCoeff • x : S` is integral over `R`. -/
theorem isIntegral_leadingCoeff_smul [Algebra R S] (h : aeval x p = 0) :
IsIntegral R (p.leadingCoeff • x) := by
rw [aeval_def] at h
rw [Algebra.smul_def]
exact (algebraMap R S).isIntegralElem_leadingCoeff_mul p x h
end
end
section IsIntegralClosure
instance integralClosure.isIntegralClosure (R A : Type*) [CommRing R] [CommRing A] [Algebra R A] :
IsIntegralClosure (integralClosure R A) R A where
algebraMap_injective' := Subtype.coe_injective
isIntegral_iff {x} := ⟨fun h => ⟨⟨x, h⟩, rfl⟩, by rintro ⟨⟨_, h⟩, rfl⟩; exact h⟩
namespace IsIntegralClosure
-- Porting note: added to work around missing infer kind support
theorem algebraMap_injective (A R B : Type*) [CommRing R] [CommSemiring A] [CommRing B]
[Algebra R B] [Algebra A B] [IsIntegralClosure A R B] : Function.Injective (algebraMap A B) :=
algebraMap_injective' R
variable {R A B : Type*} [CommRing R] [CommRing A] [CommRing B]
variable [Algebra R B] [Algebra A B] [IsIntegralClosure A R B]
variable (R B)
protected theorem isIntegral [Algebra R A] [IsScalarTower R A B] (x : A) : IsIntegral R x :=
(isIntegral_algebraMap_iff (algebraMap_injective A R B)).mp <|
show IsIntegral R (algebraMap A B x) from isIntegral_iff.mpr ⟨x, rfl⟩
theorem isIntegral_algebra [Algebra R A] [IsScalarTower R A B] : Algebra.IsIntegral R A :=
⟨fun x => IsIntegralClosure.isIntegral R B x⟩
theorem noZeroSMulDivisors [Algebra R A] [IsScalarTower R A B] [NoZeroSMulDivisors R B] :
NoZeroSMulDivisors R A := by
refine
Function.Injective.noZeroSMulDivisors _ (IsIntegralClosure.algebraMap_injective A R B)
(map_zero _) fun _ _ => ?_
simp only [Algebra.algebraMap_eq_smul_one, IsScalarTower.smul_assoc]
variable {R} (A) {B}
/-- If `x : B` is integral over `R`, then it is an element of the integral closure of `R` in `B`. -/
noncomputable def mk' (x : B) (hx : IsIntegral R x) : A :=
Classical.choose (isIntegral_iff.mp hx)
@[simp]
theorem algebraMap_mk' (x : B) (hx : IsIntegral R x) : algebraMap A B (mk' A x hx) = x :=
Classical.choose_spec (isIntegral_iff.mp hx)
@[simp]
theorem mk'_one (h : IsIntegral R (1 : B) := isIntegral_one) : mk' A 1 h = 1 :=
algebraMap_injective A R B <| by rw [algebraMap_mk', RingHom.map_one]
@[simp]
theorem mk'_zero (h : IsIntegral R (0 : B) := isIntegral_zero) : mk' A 0 h = 0 :=
algebraMap_injective A R B <| by rw [algebraMap_mk', RingHom.map_zero]
-- Porting note: Left-hand side does not simplify @[simp]
theorem mk'_add (x y : B) (hx : IsIntegral R x) (hy : IsIntegral R y) :
mk' A (x + y) (hx.add hy) = mk' A x hx + mk' A y hy :=
algebraMap_injective A R B <| by simp only [algebraMap_mk', RingHom.map_add]
-- Porting note: Left-hand side does not simplify @[simp]
theorem mk'_mul (x y : B) (hx : IsIntegral R x) (hy : IsIntegral R y) :
mk' A (x * y) (hx.mul hy) = mk' A x hx * mk' A y hy :=
algebraMap_injective A R B <| by simp only [algebraMap_mk', RingHom.map_mul]
@[simp]
theorem mk'_algebraMap [Algebra R A] [IsScalarTower R A B] (x : R)
(h : IsIntegral R (algebraMap R B x) := isIntegral_algebraMap) :
IsIntegralClosure.mk' A (algebraMap R B x) h = algebraMap R A x :=
algebraMap_injective A R B <| by rw [algebraMap_mk', ← IsScalarTower.algebraMap_apply]
section lift
variable (B) {S : Type*} [CommRing S] [Algebra R S]
-- split from above, since otherwise it does not synthesize `Semiring S`
variable [Algebra S B] [IsScalarTower R S B]
variable [Algebra R A] [IsScalarTower R A B] [isIntegral : Algebra.IsIntegral R S]
variable (R)
/-- If `B / S / R` is a tower of ring extensions where `S` is integral over `R`,
then `S` maps (uniquely) into an integral closure `B / A / R`. -/
noncomputable def lift : S →ₐ[R] A where
toFun x := mk' A (algebraMap S B x) (IsIntegral.algebraMap
(Algebra.IsIntegral.isIntegral (R := R) x))
map_one' := by simp only [RingHom.map_one, mk'_one]
map_zero' := by simp only [RingHom.map_zero, mk'_zero]
map_add' x y := by simp_rw [← mk'_add, map_add]
map_mul' x y := by simp_rw [← mk'_mul, RingHom.map_mul]
commutes' x := by simp_rw [← IsScalarTower.algebraMap_apply, mk'_algebraMap]
@[simp]
theorem algebraMap_lift (x : S) : algebraMap A B (lift R A B x) = algebraMap S B x :=
algebraMap_mk' A (algebraMap S B x) (IsIntegral.algebraMap
(Algebra.IsIntegral.isIntegral (R := R) x))
end lift
section Equiv
variable (R B) (A' : Type*) [CommRing A']
variable [Algebra A' B] [IsIntegralClosure A' R B]
variable [Algebra R A] [Algebra R A'] [IsScalarTower R A B] [IsScalarTower R A' B]
/-- Integral closures are all isomorphic to each other. -/
noncomputable def equiv : A ≃ₐ[R] A' :=
AlgEquiv.ofAlgHom
(lift _ B (isIntegral := isIntegral_algebra R B))
(lift _ B (isIntegral := isIntegral_algebra R B))
(by ext x; apply algebraMap_injective A' R B; simp)
(by ext x; apply algebraMap_injective A R B; simp)
@[simp]
theorem algebraMap_equiv (x : A) : algebraMap A' B (equiv R A B A' x) = algebraMap A B x :=
algebraMap_lift A' B (isIntegral := isIntegral_algebra R B) x
end Equiv
end IsIntegralClosure
end IsIntegralClosure
section Algebra
open Algebra
variable {R A B S T : Type*}
variable [CommRing R] [CommRing A] [Ring B] [CommRing S] [CommRing T]
variable [Algebra A B] [Algebra R B] (f : R →+* S) (g : S →+* T)
variable [Algebra R A] [IsScalarTower R A B]
/-- If A is an R-algebra all of whose elements are integral over R,
and x is an element of an A-algebra that is integral over A, then x is integral over R. -/
theorem isIntegral_trans [Algebra.IsIntegral R A] (x : B) (hx : IsIntegral A x) :
IsIntegral R x := by
rcases hx with ⟨p, pmonic, hp⟩
let S := adjoin R (p.coeffs : Set A)
have : Module.Finite R S := ⟨(Subalgebra.toSubmodule S).fg_top.mpr <|
fg_adjoin_of_finite p.coeffs.finite_toSet fun a _ ↦ Algebra.IsIntegral.isIntegral a⟩
let p' : S[X] := p.toSubring S.toSubring subset_adjoin
have hSx : IsIntegral S x := ⟨p', (p.monic_toSubring _ _).mpr pmonic, by
rw [IsScalarTower.algebraMap_eq S A B, ← eval₂_map]
convert hp; apply p.map_toSubring S.toSubring⟩
let Sx := Subalgebra.toSubmodule (adjoin S {x})
let MSx : Module S Sx := SMulMemClass.toModule _ -- the next line times out without this
have : Module.Finite S Sx := ⟨(Submodule.fg_top _).mpr hSx.fg_adjoin_singleton⟩
refine .of_mem_of_fg ((adjoin S {x}).restrictScalars R) ?_ _
((Subalgebra.mem_restrictScalars R).mpr <| subset_adjoin rfl)
rw [← Submodule.fg_top, ← Module.finite_def]
letI : SMul S Sx := { MSx with } -- need this even though MSx is there
have : IsScalarTower R S Sx :=
Submodule.isScalarTower Sx -- Lean looks for `Module A Sx` without this
exact Module.Finite.trans S Sx
variable (A) in
/-- If A is an R-algebra all of whose elements are integral over R,
and B is an A-algebra all of whose elements are integral over A,
then all elements of B are integral over R. -/
protected theorem Algebra.IsIntegral.trans
[Algebra.IsIntegral R A] [Algebra.IsIntegral A B] : Algebra.IsIntegral R B :=
⟨fun x ↦ isIntegral_trans x (Algebra.IsIntegral.isIntegral (R := A) x)⟩
protected theorem RingHom.IsIntegral.trans
(hf : f.IsIntegral) (hg : g.IsIntegral) : (g.comp f).IsIntegral :=
let _ := f.toAlgebra; let _ := g.toAlgebra; let _ := (g.comp f).toAlgebra
have : IsScalarTower R S T := IsScalarTower.of_algebraMap_eq fun _ ↦ rfl
have : Algebra.IsIntegral R S := ⟨hf⟩
have : Algebra.IsIntegral S T := ⟨hg⟩
have : Algebra.IsIntegral R T := Algebra.IsIntegral.trans S
Algebra.IsIntegral.isIntegral
/-- If `R → A → B` is an algebra tower, `C` is the integral closure of `R` in `B`
and `A` is integral over `R`, then `C` is the integral closure of `A` in `B`. -/
lemma IsIntegralClosure.tower_top {B C : Type*} [CommRing C] [CommRing B]
[Algebra R B] [Algebra A B] [Algebra C B] [IsScalarTower R A B]
[IsIntegralClosure C R B] [Algebra.IsIntegral R A] :
IsIntegralClosure C A B :=
⟨IsIntegralClosure.algebraMap_injective _ R _,
fun hx => (IsIntegralClosure.isIntegral_iff).mp (isIntegral_trans (R := R) _ hx),
fun hx => ((IsIntegralClosure.isIntegral_iff (R := R)).mpr hx).tower_top⟩
theorem RingHom.isIntegral_of_surjective (hf : Function.Surjective f) : f.IsIntegral :=
fun x ↦ (hf x).recOn fun _y hy ↦ hy ▸ f.isIntegralElem_map
theorem Algebra.isIntegral_of_surjective (h : Function.Surjective (algebraMap R A)) :
Algebra.IsIntegral R A :=
⟨(algebraMap R A).isIntegral_of_surjective h⟩
/-- If `R → A → B` is an algebra tower with `A → B` injective,
then if the entire tower is an integral extension so is `R → A` -/
theorem IsIntegral.tower_bot (H : Function.Injective (algebraMap A B)) {x : A}
(h : IsIntegral R (algebraMap A B x)) : IsIntegral R x :=
(isIntegral_algHom_iff (IsScalarTower.toAlgHom R A B) H).mp h
nonrec theorem RingHom.IsIntegral.tower_bot (hg : Function.Injective g)
(hfg : (g.comp f).IsIntegral) : f.IsIntegral :=
letI := f.toAlgebra; letI := g.toAlgebra; letI := (g.comp f).toAlgebra
haveI : IsScalarTower R S T := IsScalarTower.of_algebraMap_eq fun _ ↦ rfl
fun x ↦ IsIntegral.tower_bot hg (hfg (g x))
theorem IsIntegral.tower_bot_of_field {R A B : Type*} [CommRing R] [Field A]
[CommRing B] [Nontrivial B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B]
{x : A} (h : IsIntegral R (algebraMap A B x)) : IsIntegral R x :=
h.tower_bot (algebraMap A B).injective
theorem RingHom.isIntegralElem.of_comp {x : T} (h : (g.comp f).IsIntegralElem x) :
g.IsIntegralElem x :=
let ⟨p, hp, hp'⟩ := h
⟨p.map f, hp.map f, by rwa [← eval₂_map] at hp'⟩
theorem RingHom.IsIntegral.tower_top (h : (g.comp f).IsIntegral) : g.IsIntegral :=
fun x ↦ RingHom.isIntegralElem.of_comp f g (h x)
theorem RingHom.IsIntegral.quotient {I : Ideal S} (hf : f.IsIntegral) :
(Ideal.quotientMap I f le_rfl).IsIntegral := by
rintro ⟨x⟩
obtain ⟨p, p_monic, hpx⟩ := hf x
refine ⟨p.map (Ideal.Quotient.mk _), p_monic.map _, ?_⟩
simpa only [hom_eval₂, eval₂_map] using congr_arg (Ideal.Quotient.mk I) hpx
instance Algebra.IsIntegral.quotient {I : Ideal A} [Algebra.IsIntegral R A] :
Algebra.IsIntegral (R ⧸ I.comap (algebraMap R A)) (A ⧸ I) :=
⟨RingHom.IsIntegral.quotient (algebraMap R A) Algebra.IsIntegral.isIntegral⟩
theorem isIntegral_quotientMap_iff {I : Ideal S} :
(Ideal.quotientMap I f le_rfl).IsIntegral ↔
((Ideal.Quotient.mk I).comp f : R →+* S ⧸ I).IsIntegral := by
let g := Ideal.Quotient.mk (I.comap f)
-- Porting note: added type ascription
have : (Ideal.quotientMap I f le_rfl).comp g = (Ideal.Quotient.mk I).comp f :=
Ideal.quotientMap_comp_mk le_rfl
refine ⟨fun h => ?_, fun h => RingHom.IsIntegral.tower_top g _ (this ▸ h)⟩
refine this ▸ RingHom.IsIntegral.trans g (Ideal.quotientMap I f le_rfl) ?_ h
exact g.isIntegral_of_surjective Ideal.Quotient.mk_surjective
/-- If the integral extension `R → S` is injective, and `S` is a field, then `R` is also a field. -/
theorem isField_of_isIntegral_of_isField {R S : Type*} [CommRing R] [CommRing S]
[Algebra R S] [Algebra.IsIntegral R S]
(hRS : Function.Injective (algebraMap R S)) (hS : IsField S) : IsField R := by
have := hS.nontrivial; have := Module.nontrivial R S
refine ⟨⟨0, 1, zero_ne_one⟩, mul_comm, fun {a} ha ↦ ?_⟩
-- Let `a_inv` be the inverse of `algebraMap R S a`,
-- then we need to show that `a_inv` is of the form `algebraMap R S b`.
obtain ⟨a_inv, ha_inv⟩ := hS.mul_inv_cancel fun h ↦ ha (hRS (h.trans (RingHom.map_zero _).symm))
letI : Invertible a_inv := (Units.mkOfMulEqOne a_inv _ <| mul_comm _ a_inv ▸ ha_inv).invertible
-- Let `p : R[X]` be monic with root `a_inv`,
obtain ⟨p, p_monic, hp⟩ := Algebra.IsIntegral.isIntegral (R := R) a_inv
-- and `q` be `p` with coefficients reversed (so `q(a) = q'(a) * a + 1`).
-- We have `q(a) = 0`, so `-q'(a)` is the inverse of `a`.
use -p.reverse.divX.eval a -- -q'(a)
nth_rewrite 1 [mul_neg, ← eval_X (x := a), ← eval_mul, ← p_monic, ← coeff_zero_reverse,
← add_eq_zero_iff_neg_eq, ← eval_C (a := p.reverse.coeff 0), ← eval_add, X_mul_divX_add,
← (injective_iff_map_eq_zero' _).mp hRS, ← aeval_algebraMap_apply_eq_algebraMap_eval]
rwa [← eval₂_reverse_eq_zero_iff] at hp
theorem isField_of_isIntegral_of_isField' {R S : Type*} [CommRing R] [CommRing S] [IsDomain S]
[Algebra R S] [Algebra.IsIntegral R S] (hR : IsField R) : IsField S := by
refine ⟨⟨0, 1, zero_ne_one⟩, mul_comm, fun {x} hx ↦ ?_⟩
have : Module.Finite R (adjoin R {x}) := ⟨(Submodule.fg_top _).mpr
(Algebra.IsIntegral.isIntegral x).fg_adjoin_singleton⟩
letI := hR.toField
obtain ⟨y, hy⟩ := FiniteDimensional.exists_mul_eq_one R
(K := adjoin R {x}) (x := ⟨x, subset_adjoin rfl⟩) (mt Subtype.ext_iff.mp hx)
exact ⟨y, Subtype.ext_iff.mp hy⟩
theorem Algebra.IsIntegral.isField_iff_isField {R S : Type*} [CommRing R]
[CommRing S] [IsDomain S] [Algebra R S] [Algebra.IsIntegral R S]
(hRS : Function.Injective (algebraMap R S)) : IsField R ↔ IsField S :=
⟨isField_of_isIntegral_of_isField', isField_of_isIntegral_of_isField hRS⟩
end Algebra
theorem integralClosure_idem {R A : Type*} [CommRing R] [CommRing A] [Algebra R A] :
integralClosure (integralClosure R A : Set A) A = ⊥ :=
letI := (integralClosure R A).algebra
eq_bot_iff.2 fun x hx ↦ Algebra.mem_bot.2
⟨⟨x, isIntegral_trans (A := integralClosure R A) x hx⟩, rfl⟩
section IsDomain
variable {R S : Type*} [CommRing R] [CommRing S] [IsDomain S] [Algebra R S]
instance : IsDomain (integralClosure R S) :=
inferInstance
theorem roots_mem_integralClosure {f : R[X]} (hf : f.Monic) {a : S}
(ha : a ∈ f.aroots S) : a ∈ integralClosure R S :=
⟨f, hf, (eval₂_eq_eval_map _).trans <| (mem_roots <| (hf.map _).ne_zero).1 ha⟩
end IsDomain
|
RingTheory\IntegralClosure\IsIntegralClosure\Defs.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.RingTheory.IntegralClosure.IsIntegral.Defs
/-!
# Integral closure as a characteristic predicate
## Main definitions
Let `R` be a `CommRing` and let `A` be an R-algebra.
* `IsIntegralClosure R A` : the characteristic predicate stating `A` is the integral closure of `R`
in `B`, i.e. that an element of `B` is integral over `R` iff it is an element of
(the image of) `A`.
-/
open Polynomial
section IsIntegralClosure
/-- `IsIntegralClosure A R B` is the characteristic predicate stating `A` is
the integral closure of `R` in `B`,
i.e. that an element of `B` is integral over `R` iff it is an element of (the image of) `A`.
-/
class IsIntegralClosure (A R B : Type*) [CommRing R] [CommSemiring A] [CommRing B] [Algebra R B]
[Algebra A B] : Prop where
algebraMap_injective' : Function.Injective (algebraMap A B)
isIntegral_iff : ∀ {x : B}, IsIntegral R x ↔ ∃ y, algebraMap A B y = x
end IsIntegralClosure
|
RingTheory\Kaehler\Basic.lean | /-
Copyright (c) 2020 Nicolò Cavalleri. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Nicolò Cavalleri, Andrew Yang
-/
import Mathlib.RingTheory.Derivation.ToSquareZero
import Mathlib.RingTheory.Ideal.Cotangent
import Mathlib.RingTheory.IsTensorProduct
import Mathlib.RingTheory.EssentialFiniteness
import Mathlib.Algebra.Exact
import Mathlib.LinearAlgebra.TensorProduct.RightExactness
/-!
# The module of kaehler differentials
## Main results
- `KaehlerDifferential`: The module of kaehler differentials. For an `R`-algebra `S`, we provide
the notation `Ω[S⁄R]` for `KaehlerDifferential R S`.
Note that the slash is `\textfractionsolidus`.
- `KaehlerDifferential.D`: The derivation into the module of kaehler differentials.
- `KaehlerDifferential.span_range_derivation`: The image of `D` spans `Ω[S⁄R]` as an `S`-module.
- `KaehlerDifferential.linearMapEquivDerivation`:
The isomorphism `Hom_R(Ω[S⁄R], M) ≃ₗ[S] Der_R(S, M)`.
- `KaehlerDifferential.quotKerTotalEquiv`: An alternative description of `Ω[S⁄R]` as `S` copies
of `S` with kernel (`KaehlerDifferential.kerTotal`) generated by the relations:
1. `dx + dy = d(x + y)`
2. `x dy + y dx = d(x * y)`
3. `dr = 0` for `r ∈ R`
- `KaehlerDifferential.map`: Given a map between the arrows `R →+* A` and `S →+* B`, we have an
`A`-linear map `Ω[A⁄R] → Ω[B⁄S]`.
- `KaehlerDifferential.map_surjective`:
The sequence `Ω[B⁄R] → Ω[B⁄A] → 0` is exact.
- `KaehlerDifferential.exact_mapBaseChange_map`:
The sequence `B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → Ω[B⁄A]` is exact.
- `KaehlerDifferential.exact_kerCotangentToTensor_mapBaseChange`:
If `A → B` is surjective with kernel `I`, then
the sequence `I/I² → B ⊗[A] Ω[A⁄R] → Ω[B⁄R]` is exact.
- `KaehlerDifferential.mapBaseChange_surjective`:
If `A → B` is surjective, then the sequence `B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → 0` is exact.
## Future project
- Define the `IsKaehlerDifferential` predicate.
-/
suppress_compilation
section KaehlerDifferential
open scoped TensorProduct
open Algebra
universe u v
variable (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S]
/-- The kernel of the multiplication map `S ⊗[R] S →ₐ[R] S`. -/
abbrev KaehlerDifferential.ideal : Ideal (S ⊗[R] S) :=
RingHom.ker (TensorProduct.lmul' R : S ⊗[R] S →ₐ[R] S)
variable {S}
theorem KaehlerDifferential.one_smul_sub_smul_one_mem_ideal (a : S) :
(1 : S) ⊗ₜ[R] a - a ⊗ₜ[R] (1 : S) ∈ KaehlerDifferential.ideal R S := by simp [RingHom.mem_ker]
variable {R}
variable {M : Type*} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M]
/-- For a `R`-derivation `S → M`, this is the map `S ⊗[R] S →ₗ[S] M` sending `s ⊗ₜ t ↦ s • D t`. -/
def Derivation.tensorProductTo (D : Derivation R S M) : S ⊗[R] S →ₗ[S] M :=
TensorProduct.AlgebraTensorModule.lift ((LinearMap.lsmul S (S →ₗ[R] M)).flip D.toLinearMap)
theorem Derivation.tensorProductTo_tmul (D : Derivation R S M) (s t : S) :
D.tensorProductTo (s ⊗ₜ t) = s • D t := rfl
theorem Derivation.tensorProductTo_mul (D : Derivation R S M) (x y : S ⊗[R] S) :
D.tensorProductTo (x * y) =
TensorProduct.lmul' (S := S) R x • D.tensorProductTo y +
TensorProduct.lmul' (S := S) R y • D.tensorProductTo x := by
refine TensorProduct.induction_on x ?_ ?_ ?_
· rw [zero_mul, map_zero, map_zero, zero_smul, smul_zero, add_zero]
swap
· intro x₁ y₁ h₁ h₂
rw [add_mul, map_add, map_add, map_add, add_smul, smul_add, h₁, h₂, add_add_add_comm]
intro x₁ x₂
refine TensorProduct.induction_on y ?_ ?_ ?_
· rw [mul_zero, map_zero, map_zero, zero_smul, smul_zero, add_zero]
swap
· intro x₁ y₁ h₁ h₂
rw [mul_add, map_add, map_add, map_add, add_smul, smul_add, h₁, h₂, add_add_add_comm]
intro x y
simp only [TensorProduct.tmul_mul_tmul, Derivation.tensorProductTo,
TensorProduct.AlgebraTensorModule.lift_apply, TensorProduct.lift.tmul',
TensorProduct.lmul'_apply_tmul]
dsimp
rw [D.leibniz]
simp only [smul_smul, smul_add, mul_comm (x * y) x₁, mul_right_comm x₁ x₂, ← mul_assoc]
variable (R S)
/-- The kernel of `S ⊗[R] S →ₐ[R] S` is generated by `1 ⊗ s - s ⊗ 1` as a `S`-module. -/
theorem KaehlerDifferential.submodule_span_range_eq_ideal :
Submodule.span S (Set.range fun s : S => (1 : S) ⊗ₜ[R] s - s ⊗ₜ[R] (1 : S)) =
(KaehlerDifferential.ideal R S).restrictScalars S := by
apply le_antisymm
· rw [Submodule.span_le]
rintro _ ⟨s, rfl⟩
exact KaehlerDifferential.one_smul_sub_smul_one_mem_ideal _ _
· rintro x (hx : _ = _)
have : x - TensorProduct.lmul' (S := S) R x ⊗ₜ[R] (1 : S) = x := by
rw [hx, TensorProduct.zero_tmul, sub_zero]
rw [← this]
clear this hx
refine TensorProduct.induction_on x ?_ ?_ ?_
· rw [map_zero, TensorProduct.zero_tmul, sub_zero]; exact zero_mem _
· intro x y
have : x ⊗ₜ[R] y - (x * y) ⊗ₜ[R] (1 : S) = x • ((1 : S) ⊗ₜ y - y ⊗ₜ (1 : S)) := by
simp_rw [smul_sub, TensorProduct.smul_tmul', smul_eq_mul, mul_one]
rw [TensorProduct.lmul'_apply_tmul, this]
refine Submodule.smul_mem _ x ?_
apply Submodule.subset_span
exact Set.mem_range_self y
· intro x y hx hy
rw [map_add, TensorProduct.add_tmul, ← sub_add_sub_comm]
exact add_mem hx hy
theorem KaehlerDifferential.span_range_eq_ideal :
Ideal.span (Set.range fun s : S => (1 : S) ⊗ₜ[R] s - s ⊗ₜ[R] (1 : S)) =
KaehlerDifferential.ideal R S := by
apply le_antisymm
· rw [Ideal.span_le]
rintro _ ⟨s, rfl⟩
exact KaehlerDifferential.one_smul_sub_smul_one_mem_ideal _ _
· change (KaehlerDifferential.ideal R S).restrictScalars S ≤ (Ideal.span _).restrictScalars S
rw [← KaehlerDifferential.submodule_span_range_eq_ideal, Ideal.span]
conv_rhs => rw [← Submodule.span_span_of_tower S]
exact Submodule.subset_span
/-- The module of Kähler differentials (Kahler differentials, Kaehler differentials).
This is implemented as `I / I ^ 2` with `I` the kernel of the multiplication map `S ⊗[R] S →ₐ[R] S`.
To view elements as a linear combination of the form `s • D s'`, use
`KaehlerDifferential.tensorProductTo_surjective` and `Derivation.tensorProductTo_tmul`.
We also provide the notation `Ω[S⁄R]` for `KaehlerDifferential R S`.
Note that the slash is `\textfractionsolidus`.
-/
def KaehlerDifferential : Type v :=
(KaehlerDifferential.ideal R S).Cotangent
instance : AddCommGroup (KaehlerDifferential R S) := inferInstanceAs <|
AddCommGroup (KaehlerDifferential.ideal R S).Cotangent
instance KaehlerDifferential.module : Module (S ⊗[R] S) (KaehlerDifferential R S) :=
Ideal.Cotangent.moduleOfTower _
@[inherit_doc KaehlerDifferential]
notation:100 "Ω[" S "⁄" R "]" => KaehlerDifferential R S
instance : Nonempty (Ω[S⁄R]) := ⟨0⟩
instance KaehlerDifferential.module' {R' : Type*} [CommRing R'] [Algebra R' S]
[SMulCommClass R R' S] :
Module R' (Ω[S⁄R]) :=
Submodule.Quotient.module' _
instance : IsScalarTower S (S ⊗[R] S) (Ω[S⁄R]) :=
Ideal.Cotangent.isScalarTower _
instance KaehlerDifferential.isScalarTower_of_tower {R₁ R₂ : Type*} [CommRing R₁] [CommRing R₂]
[Algebra R₁ S] [Algebra R₂ S] [SMul R₁ R₂]
[SMulCommClass R R₁ S] [SMulCommClass R R₂ S] [IsScalarTower R₁ R₂ S] :
IsScalarTower R₁ R₂ (Ω[S⁄R]) :=
Submodule.Quotient.isScalarTower _ _
instance KaehlerDifferential.isScalarTower' : IsScalarTower R (S ⊗[R] S) (Ω[S⁄R]) :=
Submodule.Quotient.isScalarTower _ _
/-- The quotient map `I → Ω[S⁄R]` with `I` being the kernel of `S ⊗[R] S → S`. -/
def KaehlerDifferential.fromIdeal : KaehlerDifferential.ideal R S →ₗ[S ⊗[R] S] Ω[S⁄R] :=
(KaehlerDifferential.ideal R S).toCotangent
/-- (Implementation) The underlying linear map of the derivation into `Ω[S⁄R]`. -/
def KaehlerDifferential.DLinearMap : S →ₗ[R] Ω[S⁄R] :=
((KaehlerDifferential.fromIdeal R S).restrictScalars R).comp
((TensorProduct.includeRight.toLinearMap - TensorProduct.includeLeft.toLinearMap :
S →ₗ[R] S ⊗[R] S).codRestrict
((KaehlerDifferential.ideal R S).restrictScalars R)
(KaehlerDifferential.one_smul_sub_smul_one_mem_ideal R) :
_ →ₗ[R] _)
theorem KaehlerDifferential.DLinearMap_apply (s : S) :
KaehlerDifferential.DLinearMap R S s =
(KaehlerDifferential.ideal R S).toCotangent
⟨1 ⊗ₜ s - s ⊗ₜ 1, KaehlerDifferential.one_smul_sub_smul_one_mem_ideal R s⟩ := rfl
/-- The universal derivation into `Ω[S⁄R]`. -/
def KaehlerDifferential.D : Derivation R S (Ω[S⁄R]) :=
{ toLinearMap := KaehlerDifferential.DLinearMap R S
map_one_eq_zero' := by
dsimp [KaehlerDifferential.DLinearMap_apply, Ideal.toCotangent_apply]
congr
rw [sub_self]
leibniz' := fun a b => by
have : LinearMap.CompatibleSMul { x // x ∈ ideal R S } (Ω[S⁄R]) S (S ⊗[R] S) := inferInstance
dsimp [KaehlerDifferential.DLinearMap_apply]
-- This used to be `rw`, but we need `erw` after leanprover/lean4#2644
erw [← LinearMap.map_smul_of_tower (M₂ := Ω[S⁄R]),
← LinearMap.map_smul_of_tower (M₂ := Ω[S⁄R]), ← map_add, Ideal.toCotangent_eq, pow_two]
convert Submodule.mul_mem_mul (KaehlerDifferential.one_smul_sub_smul_one_mem_ideal R a : _)
(KaehlerDifferential.one_smul_sub_smul_one_mem_ideal R b : _) using 1
simp only [AddSubgroupClass.coe_sub, Submodule.coe_add, Submodule.coe_mk,
TensorProduct.tmul_mul_tmul, mul_sub, sub_mul, mul_comm b, Submodule.coe_smul_of_tower,
smul_sub, TensorProduct.smul_tmul', smul_eq_mul, mul_one]
ring_nf }
theorem KaehlerDifferential.D_apply (s : S) :
KaehlerDifferential.D R S s =
(KaehlerDifferential.ideal R S).toCotangent
⟨1 ⊗ₜ s - s ⊗ₜ 1, KaehlerDifferential.one_smul_sub_smul_one_mem_ideal R s⟩ := rfl
theorem KaehlerDifferential.span_range_derivation :
Submodule.span S (Set.range <| KaehlerDifferential.D R S) = ⊤ := by
rw [_root_.eq_top_iff]
rintro x -
obtain ⟨⟨x, hx⟩, rfl⟩ := Ideal.toCotangent_surjective _ x
have : x ∈ (KaehlerDifferential.ideal R S).restrictScalars S := hx
rw [← KaehlerDifferential.submodule_span_range_eq_ideal] at this
suffices ∃ hx, (KaehlerDifferential.ideal R S).toCotangent ⟨x, hx⟩ ∈
Submodule.span S (Set.range <| KaehlerDifferential.D R S) by
exact this.choose_spec
refine Submodule.span_induction this ?_ ?_ ?_ ?_
· rintro _ ⟨x, rfl⟩
refine ⟨KaehlerDifferential.one_smul_sub_smul_one_mem_ideal R x, ?_⟩
apply Submodule.subset_span
exact ⟨x, KaehlerDifferential.DLinearMap_apply R S x⟩
· exact ⟨zero_mem _, Submodule.zero_mem _⟩
· rintro x y ⟨hx₁, hx₂⟩ ⟨hy₁, hy₂⟩; exact ⟨add_mem hx₁ hy₁, Submodule.add_mem _ hx₂ hy₂⟩
· rintro r x ⟨hx₁, hx₂⟩
exact ⟨((KaehlerDifferential.ideal R S).restrictScalars S).smul_mem r hx₁,
Submodule.smul_mem _ r hx₂⟩
/-- `Ω[S⁄R]` is trivial if `R → S` is surjective.
Also see `Algebra.FormallyUnramified.iff_subsingleton_kaehlerDifferential`. -/
lemma KaehlerDifferential.subsingleton_of_surjective (h : Function.Surjective (algebraMap R S)) :
Subsingleton (Ω[S⁄R]) := by
suffices (⊤ : Submodule S (Ω[S⁄R])) ≤ ⊥ from
(subsingleton_iff_forall_eq 0).mpr fun y ↦ this trivial
rw [← KaehlerDifferential.span_range_derivation, Submodule.span_le]
rintro _ ⟨x, rfl⟩; obtain ⟨x, rfl⟩ := h x; simp
variable {R S}
/-- The linear map from `Ω[S⁄R]`, associated with a derivation. -/
def Derivation.liftKaehlerDifferential (D : Derivation R S M) : Ω[S⁄R] →ₗ[S] M := by
refine LinearMap.comp ((((KaehlerDifferential.ideal R S) •
(⊤ : Submodule (S ⊗[R] S) (KaehlerDifferential.ideal R S))).restrictScalars S).liftQ ?_ ?_)
(Submodule.Quotient.restrictScalarsEquiv S _).symm.toLinearMap
· exact D.tensorProductTo.comp ((KaehlerDifferential.ideal R S).subtype.restrictScalars S)
· intro x hx
rw [LinearMap.mem_ker]
refine Submodule.smul_induction_on hx ?_ ?_
· rintro x hx y -
rw [RingHom.mem_ker] at hx
dsimp
rw [Derivation.tensorProductTo_mul, hx, y.prop, zero_smul, zero_smul, zero_add]
· intro x y ex ey; rw [map_add, ex, ey, zero_add]
theorem Derivation.liftKaehlerDifferential_apply (D : Derivation R S M) (x) :
D.liftKaehlerDifferential ((KaehlerDifferential.ideal R S).toCotangent x) =
D.tensorProductTo x := rfl
theorem Derivation.liftKaehlerDifferential_comp (D : Derivation R S M) :
D.liftKaehlerDifferential.compDer (KaehlerDifferential.D R S) = D := by
ext a
dsimp [KaehlerDifferential.D_apply]
refine (D.liftKaehlerDifferential_apply _).trans ?_
rw [Subtype.coe_mk, map_sub, Derivation.tensorProductTo_tmul, Derivation.tensorProductTo_tmul,
one_smul, D.map_one_eq_zero, smul_zero, sub_zero]
@[simp]
theorem Derivation.liftKaehlerDifferential_comp_D (D' : Derivation R S M) (x : S) :
D'.liftKaehlerDifferential (KaehlerDifferential.D R S x) = D' x :=
Derivation.congr_fun D'.liftKaehlerDifferential_comp x
@[ext]
theorem Derivation.liftKaehlerDifferential_unique (f f' : Ω[S⁄R] →ₗ[S] M)
(hf : f.compDer (KaehlerDifferential.D R S) = f'.compDer (KaehlerDifferential.D R S)) :
f = f' := by
apply LinearMap.ext
intro x
have : x ∈ Submodule.span S (Set.range <| KaehlerDifferential.D R S) := by
rw [KaehlerDifferential.span_range_derivation]; trivial
refine Submodule.span_induction this ?_ ?_ ?_ ?_
· rintro _ ⟨x, rfl⟩; exact congr_arg (fun D : Derivation R S M => D x) hf
· rw [map_zero, map_zero]
· intro x y hx hy; rw [map_add, map_add, hx, hy]
· intro a x e; simp [e]
variable (R S)
theorem Derivation.liftKaehlerDifferential_D :
(KaehlerDifferential.D R S).liftKaehlerDifferential = LinearMap.id :=
Derivation.liftKaehlerDifferential_unique _ _
(KaehlerDifferential.D R S).liftKaehlerDifferential_comp
variable {R S}
theorem KaehlerDifferential.D_tensorProductTo (x : KaehlerDifferential.ideal R S) :
(KaehlerDifferential.D R S).tensorProductTo x =
(KaehlerDifferential.ideal R S).toCotangent x := by
rw [← Derivation.liftKaehlerDifferential_apply, Derivation.liftKaehlerDifferential_D]
rfl
variable (R S)
theorem KaehlerDifferential.tensorProductTo_surjective :
Function.Surjective (KaehlerDifferential.D R S).tensorProductTo := by
intro x; obtain ⟨x, rfl⟩ := (KaehlerDifferential.ideal R S).toCotangent_surjective x
exact ⟨x, KaehlerDifferential.D_tensorProductTo x⟩
/-- The `S`-linear maps from `Ω[S⁄R]` to `M` are (`S`-linearly) equivalent to `R`-derivations
from `S` to `M`. -/
@[simps! symm_apply apply_apply]
def KaehlerDifferential.linearMapEquivDerivation : (Ω[S⁄R] →ₗ[S] M) ≃ₗ[S] Derivation R S M :=
{ Derivation.llcomp.flip <| KaehlerDifferential.D R S with
invFun := Derivation.liftKaehlerDifferential
left_inv := fun _ =>
Derivation.liftKaehlerDifferential_unique _ _ (Derivation.liftKaehlerDifferential_comp _)
right_inv := Derivation.liftKaehlerDifferential_comp }
/-- The quotient ring of `S ⊗ S ⧸ J ^ 2` by `Ω[S⁄R]` is isomorphic to `S`. -/
def KaehlerDifferential.quotientCotangentIdealRingEquiv :
(S ⊗ S ⧸ KaehlerDifferential.ideal R S ^ 2) ⧸ (KaehlerDifferential.ideal R S).cotangentIdeal ≃+*
S := by
have : Function.RightInverse (TensorProduct.includeLeft (R := R) (S := R) (A := S) (B := S))
(↑(TensorProduct.lmul' R : S ⊗[R] S →ₐ[R] S) : S ⊗[R] S →+* S) := by
intro x; rw [AlgHom.coe_toRingHom, ← AlgHom.comp_apply, TensorProduct.lmul'_comp_includeLeft]
rfl
refine (Ideal.quotCotangent _).trans ?_
refine (Ideal.quotEquivOfEq ?_).trans (RingHom.quotientKerEquivOfRightInverse this)
ext; rfl
/-- The quotient ring of `S ⊗ S ⧸ J ^ 2` by `Ω[S⁄R]` is isomorphic to `S` as an `S`-algebra. -/
def KaehlerDifferential.quotientCotangentIdeal :
((S ⊗ S ⧸ KaehlerDifferential.ideal R S ^ 2) ⧸
(KaehlerDifferential.ideal R S).cotangentIdeal) ≃ₐ[S] S :=
{ KaehlerDifferential.quotientCotangentIdealRingEquiv R S with
commutes' := (KaehlerDifferential.quotientCotangentIdealRingEquiv R S).apply_symm_apply }
theorem KaehlerDifferential.End_equiv_aux (f : S →ₐ[R] S ⊗ S ⧸ KaehlerDifferential.ideal R S ^ 2) :
(Ideal.Quotient.mkₐ R (KaehlerDifferential.ideal R S).cotangentIdeal).comp f =
IsScalarTower.toAlgHom R S _ ↔
(TensorProduct.lmul' R : S ⊗[R] S →ₐ[R] S).kerSquareLift.comp f = AlgHom.id R S := by
rw [AlgHom.ext_iff, AlgHom.ext_iff]
apply forall_congr'
intro x
have e₁ : (TensorProduct.lmul' R : S ⊗[R] S →ₐ[R] S).kerSquareLift (f x) =
KaehlerDifferential.quotientCotangentIdealRingEquiv R S
(Ideal.Quotient.mk (KaehlerDifferential.ideal R S).cotangentIdeal <| f x) := by
generalize f x = y; obtain ⟨y, rfl⟩ := Ideal.Quotient.mk_surjective y; rfl
have e₂ :
x = KaehlerDifferential.quotientCotangentIdealRingEquiv R S (IsScalarTower.toAlgHom R S _ x) :=
(mul_one x).symm
constructor
· intro e
exact (e₁.trans (@RingEquiv.congr_arg _ _ _ _ _ _
(KaehlerDifferential.quotientCotangentIdealRingEquiv R S) _ _ e)).trans e₂.symm
· intro e; apply (KaehlerDifferential.quotientCotangentIdealRingEquiv R S).injective
exact e₁.symm.trans (e.trans e₂)
/- Note: Lean is slow to synthesize theses instances (times out).
Without them the endEquivDerivation' and endEquivAuxEquiv both have significant timeouts.
In Mathlib 3, it was slow but not this slow. -/
/-- A shortcut instance to prevent timing out. Hopefully to be removed in the future. -/
local instance smul_SSmod_SSmod : SMul (S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2)
(S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2) := Mul.toSMul _
/-- A shortcut instance to prevent timing out. Hopefully to be removed in the future. -/
@[nolint defLemma]
local instance isScalarTower_S_right :
IsScalarTower S (S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2)
(S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2) := Ideal.Quotient.isScalarTower_right
/-- A shortcut instance to prevent timing out. Hopefully to be removed in the future. -/
@[nolint defLemma]
local instance isScalarTower_R_right :
IsScalarTower R (S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2)
(S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2) := Ideal.Quotient.isScalarTower_right
/-- A shortcut instance to prevent timing out. Hopefully to be removed in the future. -/
@[nolint defLemma]
local instance isScalarTower_SS_right : IsScalarTower (S ⊗[R] S)
(S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2) (S ⊗[R] S ⧸ KaehlerDifferential.ideal R S ^ 2) :=
Ideal.Quotient.isScalarTower_right
/-- A shortcut instance to prevent timing out. Hopefully to be removed in the future. -/
local instance instS : Module S (KaehlerDifferential.ideal R S).cotangentIdeal :=
Submodule.module' _
/-- A shortcut instance to prevent timing out. Hopefully to be removed in the future. -/
local instance instR : Module R (KaehlerDifferential.ideal R S).cotangentIdeal :=
Submodule.module' _
/-- A shortcut instance to prevent timing out. Hopefully to be removed in the future. -/
local instance instSS : Module (S ⊗[R] S) (KaehlerDifferential.ideal R S).cotangentIdeal :=
Submodule.module' _
/-- Derivations into `Ω[S⁄R]` is equivalent to derivations
into `(KaehlerDifferential.ideal R S).cotangentIdeal`. -/
noncomputable def KaehlerDifferential.endEquivDerivation' :
Derivation R S (Ω[S⁄R]) ≃ₗ[R] Derivation R S (ideal R S).cotangentIdeal :=
LinearEquiv.compDer ((KaehlerDifferential.ideal R S).cotangentEquivIdeal.restrictScalars S)
/-- (Implementation) An `Equiv` version of `KaehlerDifferential.End_equiv_aux`.
Used in `KaehlerDifferential.endEquiv`. -/
def KaehlerDifferential.endEquivAuxEquiv :
{ f //
(Ideal.Quotient.mkₐ R (KaehlerDifferential.ideal R S).cotangentIdeal).comp f =
IsScalarTower.toAlgHom R S _ } ≃
{ f // (TensorProduct.lmul' R : S ⊗[R] S →ₐ[R] S).kerSquareLift.comp f = AlgHom.id R S } :=
(Equiv.refl _).subtypeEquiv (KaehlerDifferential.End_equiv_aux R S)
/--
The endomorphisms of `Ω[S⁄R]` corresponds to sections of the surjection `S ⊗[R] S ⧸ J ^ 2 →ₐ[R] S`,
with `J` being the kernel of the multiplication map `S ⊗[R] S →ₐ[R] S`.
-/
noncomputable def KaehlerDifferential.endEquiv :
Module.End S (Ω[S⁄R]) ≃
{ f // (TensorProduct.lmul' R : S ⊗[R] S →ₐ[R] S).kerSquareLift.comp f = AlgHom.id R S } :=
(KaehlerDifferential.linearMapEquivDerivation R S).toEquiv.trans <|
(KaehlerDifferential.endEquivDerivation' R S).toEquiv.trans <|
(derivationToSquareZeroEquivLift (KaehlerDifferential.ideal R S).cotangentIdeal
(KaehlerDifferential.ideal R S).cotangentIdeal_square).trans <|
KaehlerDifferential.endEquivAuxEquiv R S
section Finiteness
theorem KaehlerDifferential.ideal_fg [EssFiniteType R S] :
(KaehlerDifferential.ideal R S).FG := by
classical
use (EssFiniteType.finset R S).image (fun s ↦ (1 : S) ⊗ₜ[R] s - s ⊗ₜ[R] (1 : S))
apply le_antisymm
· rw [Finset.coe_image, Ideal.span_le]
rintro _ ⟨x, _, rfl⟩
exact KaehlerDifferential.one_smul_sub_smul_one_mem_ideal R x
· rw [← KaehlerDifferential.span_range_eq_ideal, Ideal.span_le]
rintro _ ⟨x, rfl⟩
let I : Ideal (S ⊗[R] S) := Ideal.span
((EssFiniteType.finset R S).image (fun s ↦ (1 : S) ⊗ₜ[R] s - s ⊗ₜ[R] (1 : S)))
show _ - _ ∈ I
have : (IsScalarTower.toAlgHom R (S ⊗[R] S) (S ⊗[R] S ⧸ I)).comp TensorProduct.includeRight =
(IsScalarTower.toAlgHom R (S ⊗[R] S) (S ⊗[R] S ⧸ I)).comp TensorProduct.includeLeft := by
apply EssFiniteType.algHom_ext
intro a ha
simp only [AlgHom.coe_comp, IsScalarTower.coe_toAlgHom', Ideal.Quotient.algebraMap_eq,
Function.comp_apply, TensorProduct.includeLeft_apply, TensorProduct.includeRight_apply,
Ideal.Quotient.mk_eq_mk_iff_sub_mem]
refine Ideal.subset_span ?_
simp only [Finset.coe_image, Set.mem_image, Finset.mem_coe]
exact ⟨a, ha, rfl⟩
simpa [Ideal.Quotient.mk_eq_mk_iff_sub_mem] using AlgHom.congr_fun this x
instance KaehlerDifferential.finite [EssFiniteType R S] :
Module.Finite S (Ω[S⁄R]) := by
classical
let s := (EssFiniteType.finset R S).image (fun s ↦ D R S s)
refine ⟨⟨s, top_le_iff.mp ?_⟩⟩
rw [← span_range_derivation, Submodule.span_le]
rintro _ ⟨x, rfl⟩
have : ∀ x ∈ adjoin R (EssFiniteType.finset R S).toSet,
.D _ _ x ∈ Submodule.span S s.toSet := by
intro x hx
refine adjoin_induction hx ?_ ?_ ?_ ?_
· exact fun x hx ↦ Submodule.subset_span (Finset.mem_image_of_mem _ hx)
· simp
· exact fun x y hx hy ↦ (D R S).map_add x y ▸ add_mem hx hy
· intro x y hx hy
simp only [Derivation.leibniz]
exact add_mem (Submodule.smul_mem _ _ hy) (Submodule.smul_mem _ _ hx)
obtain ⟨t, ht, ht', hxt⟩ := (essFiniteType_cond_iff R S (EssFiniteType.finset R S)).mp
EssFiniteType.cond.choose_spec x
rw [show D R S x =
ht'.unit⁻¹ • (D R S (x * t) - x • D R S t) by simp [smul_smul, Units.smul_def]]
exact Submodule.smul_mem _ _ (sub_mem (this _ hxt) (Submodule.smul_mem _ _ (this _ ht)))
end Finiteness
section Presentation
open KaehlerDifferential (D)
open Finsupp (single)
/-- The `S`-submodule of `S →₀ S` (the direct sum of copies of `S` indexed by `S`) generated by
the relations:
1. `dx + dy = d(x + y)`
2. `x dy + y dx = d(x * y)`
3. `dr = 0` for `r ∈ R`
where `db` is the unit in the copy of `S` with index `b`.
This is the kernel of the surjection `Finsupp.total S Ω[S⁄R] S (KaehlerDifferential.D R S)`.
See `KaehlerDifferential.kerTotal_eq` and `KaehlerDifferential.total_surjective`.
-/
noncomputable def KaehlerDifferential.kerTotal : Submodule S (S →₀ S) :=
Submodule.span S
(((Set.range fun x : S × S => single x.1 1 + single x.2 1 - single (x.1 + x.2) 1) ∪
Set.range fun x : S × S => single x.2 x.1 + single x.1 x.2 - single (x.1 * x.2) 1) ∪
Set.range fun x : R => single (algebraMap R S x) 1)
unsuppress_compilation in
-- Porting note: was `local notation x "𝖣" y => (KaehlerDifferential.kerTotal R S).mkQ (single y x)`
-- but not having `DFunLike.coe` leads to `kerTotal_mkQ_single_smul` failing.
local notation3 x "𝖣" y => DFunLike.coe (KaehlerDifferential.kerTotal R S).mkQ (single y x)
theorem KaehlerDifferential.kerTotal_mkQ_single_add (x y z) : (z𝖣x + y) = (z𝖣x) + z𝖣y := by
rw [← map_add, eq_comm, ← sub_eq_zero, ← map_sub (Submodule.mkQ (kerTotal R S)),
Submodule.mkQ_apply, Submodule.Quotient.mk_eq_zero]
simp_rw [← Finsupp.smul_single_one _ z, ← smul_add, ← smul_sub]
exact Submodule.smul_mem _ _ (Submodule.subset_span (Or.inl <| Or.inl <| ⟨⟨_, _⟩, rfl⟩))
theorem KaehlerDifferential.kerTotal_mkQ_single_mul (x y z) :
(z𝖣x * y) = ((z * x)𝖣y) + (z * y)𝖣x := by
rw [← map_add, eq_comm, ← sub_eq_zero, ← map_sub (Submodule.mkQ (kerTotal R S)),
Submodule.mkQ_apply, Submodule.Quotient.mk_eq_zero]
simp_rw [← Finsupp.smul_single_one _ z, ← @smul_eq_mul _ _ z, ← Finsupp.smul_single, ← smul_add,
← smul_sub]
exact Submodule.smul_mem _ _ (Submodule.subset_span (Or.inl <| Or.inr <| ⟨⟨_, _⟩, rfl⟩))
theorem KaehlerDifferential.kerTotal_mkQ_single_algebraMap (x y) : (y𝖣algebraMap R S x) = 0 := by
rw [Submodule.mkQ_apply, Submodule.Quotient.mk_eq_zero, ← Finsupp.smul_single_one _ y]
exact Submodule.smul_mem _ _ (Submodule.subset_span (Or.inr <| ⟨_, rfl⟩))
theorem KaehlerDifferential.kerTotal_mkQ_single_algebraMap_one (x) : (x𝖣1) = 0 := by
rw [← (algebraMap R S).map_one, KaehlerDifferential.kerTotal_mkQ_single_algebraMap]
theorem KaehlerDifferential.kerTotal_mkQ_single_smul (r : R) (x y) : (y𝖣r • x) = r • y𝖣x := by
letI : SMulZeroClass R S := inferInstance
rw [Algebra.smul_def, KaehlerDifferential.kerTotal_mkQ_single_mul,
KaehlerDifferential.kerTotal_mkQ_single_algebraMap, add_zero, ← LinearMap.map_smul_of_tower,
Finsupp.smul_single, mul_comm, Algebra.smul_def]
/-- The (universal) derivation into `(S →₀ S) ⧸ KaehlerDifferential.kerTotal R S`. -/
noncomputable def KaehlerDifferential.derivationQuotKerTotal :
Derivation R S ((S →₀ S) ⧸ KaehlerDifferential.kerTotal R S) where
toFun x := 1𝖣x
map_add' x y := KaehlerDifferential.kerTotal_mkQ_single_add _ _ _ _ _
map_smul' r s := KaehlerDifferential.kerTotal_mkQ_single_smul _ _ _ _ _
map_one_eq_zero' := KaehlerDifferential.kerTotal_mkQ_single_algebraMap_one _ _ _
leibniz' a b :=
(KaehlerDifferential.kerTotal_mkQ_single_mul _ _ _ _ _).trans
(by simp_rw [← Finsupp.smul_single_one _ (1 * _ : S)]; dsimp; simp)
theorem KaehlerDifferential.derivationQuotKerTotal_apply (x) :
KaehlerDifferential.derivationQuotKerTotal R S x = 1𝖣x :=
rfl
theorem KaehlerDifferential.derivationQuotKerTotal_lift_comp_total :
(KaehlerDifferential.derivationQuotKerTotal R S).liftKaehlerDifferential.comp
(Finsupp.total S (Ω[S⁄R]) S (KaehlerDifferential.D R S)) =
Submodule.mkQ _ := by
apply Finsupp.lhom_ext
intro a b
conv_rhs => rw [← Finsupp.smul_single_one a b, LinearMap.map_smul]
simp [KaehlerDifferential.derivationQuotKerTotal_apply]
theorem KaehlerDifferential.kerTotal_eq :
LinearMap.ker (Finsupp.total S (Ω[S⁄R]) S (KaehlerDifferential.D R S)) =
KaehlerDifferential.kerTotal R S := by
apply le_antisymm
· conv_rhs => rw [← (KaehlerDifferential.kerTotal R S).ker_mkQ]
rw [← KaehlerDifferential.derivationQuotKerTotal_lift_comp_total]
exact LinearMap.ker_le_ker_comp _ _
· rw [KaehlerDifferential.kerTotal, Submodule.span_le]
rintro _ ((⟨⟨x, y⟩, rfl⟩ | ⟨⟨x, y⟩, rfl⟩) | ⟨x, rfl⟩) <;> dsimp <;> simp [LinearMap.mem_ker]
theorem KaehlerDifferential.total_surjective :
Function.Surjective (Finsupp.total S (Ω[S⁄R]) S (KaehlerDifferential.D R S)) := by
rw [← LinearMap.range_eq_top, Finsupp.range_total, KaehlerDifferential.span_range_derivation]
/-- `Ω[S⁄R]` is isomorphic to `S` copies of `S` with kernel `KaehlerDifferential.kerTotal`. -/
@[simps!]
noncomputable def KaehlerDifferential.quotKerTotalEquiv :
((S →₀ S) ⧸ KaehlerDifferential.kerTotal R S) ≃ₗ[S] Ω[S⁄R] :=
{ (KaehlerDifferential.kerTotal R S).liftQ
(Finsupp.total S (Ω[S⁄R]) S (KaehlerDifferential.D R S))
(KaehlerDifferential.kerTotal_eq R S).ge with
invFun := (KaehlerDifferential.derivationQuotKerTotal R S).liftKaehlerDifferential
left_inv := by
intro x
obtain ⟨x, rfl⟩ := Submodule.mkQ_surjective _ x
exact
LinearMap.congr_fun (KaehlerDifferential.derivationQuotKerTotal_lift_comp_total R S : _) x
right_inv := by
intro x
obtain ⟨x, rfl⟩ := KaehlerDifferential.total_surjective R S x
have := LinearMap.congr_fun (KaehlerDifferential.derivationQuotKerTotal_lift_comp_total R S) x
rw [LinearMap.comp_apply] at this
rw [this]
rfl }
theorem KaehlerDifferential.quotKerTotalEquiv_symm_comp_D :
(KaehlerDifferential.quotKerTotalEquiv R S).symm.toLinearMap.compDer
(KaehlerDifferential.D R S) =
KaehlerDifferential.derivationQuotKerTotal R S := by
convert (KaehlerDifferential.derivationQuotKerTotal R S).liftKaehlerDifferential_comp
end Presentation
section ExactSequence
/- We have the commutative diagram
```
A --→ B
↑ ↑
| |
R --→ S
```
-/
variable (A B : Type*) [CommRing A] [CommRing B] [Algebra R A]
variable [Algebra A B] [Algebra S B]
unsuppress_compilation in
-- The map `(A →₀ A) →ₗ[A] (B →₀ B)`
local macro "finsupp_map" : term =>
`((Finsupp.mapRange.linearMap (Algebra.linearMap A B)).comp
(Finsupp.lmapDomain A A (algebraMap A B)))
/--
Given the commutative diagram
```
A --→ B
↑ ↑
| |
R --→ S
```
The kernel of the presentation `⊕ₓ B dx ↠ Ω_{B/S}` is spanned by the image of the
kernel of `⊕ₓ A dx ↠ Ω_{A/R}` and all `ds` with `s : S`.
See `kerTotal_map'` for the special case where `R = S`.
-/
theorem KaehlerDifferential.kerTotal_map [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B]
(h : Function.Surjective (algebraMap A B)) :
(KaehlerDifferential.kerTotal R A).map finsupp_map ⊔
Submodule.span A (Set.range fun x : S => .single (algebraMap S B x) (1 : B)) =
(KaehlerDifferential.kerTotal S B).restrictScalars _ := by
rw [KaehlerDifferential.kerTotal, Submodule.map_span, KaehlerDifferential.kerTotal,
Submodule.restrictScalars_span _ _ h]
simp_rw [Set.image_union, Submodule.span_union, ← Set.image_univ, Set.image_image, Set.image_univ,
map_sub, map_add]
simp only [LinearMap.comp_apply, Finsupp.lmapDomain_apply, Finsupp.mapDomain_single,
Finsupp.mapRange.linearMap_apply, Finsupp.mapRange_single, Algebra.linearMap_apply,
map_one, map_add, map_mul]
simp_rw [sup_assoc, ← (h.prodMap h).range_comp]
congr!
-- Porting note: new
simp_rw [← IsScalarTower.algebraMap_apply R A B]
rw [sup_eq_right]
apply Submodule.span_mono
simp_rw [IsScalarTower.algebraMap_apply R S B]
exact Set.range_comp_subset_range (algebraMap R S)
fun x => Finsupp.single (algebraMap S B x) (1 : B)
/--
This is a special case of `kerTotal_map` where `R = S`.
The kernel of the presentation `⊕ₓ B dx ↠ Ω_{B/R}` is spanned by the image of the
kernel of `⊕ₓ A dx ↠ Ω_{A/R}` and all `da` with `a : A`.
-/
theorem KaehlerDifferential.kerTotal_map' [Algebra R B]
[IsScalarTower R A B] (h : Function.Surjective (algebraMap A B)) :
(KaehlerDifferential.kerTotal R A ⊔
Submodule.span A (Set.range fun x ↦ .single (algebraMap R A x) 1)).map finsupp_map =
(KaehlerDifferential.kerTotal R B).restrictScalars _ := by
rw [Submodule.map_sup, ← kerTotal_map R R A B h, Submodule.map_span, ← Set.range_comp]
congr
refine congr_arg Set.range ?_
ext; simp [IsScalarTower.algebraMap_eq R A B]
variable [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B] [SMulCommClass S A B]
/-- The map `Ω[A⁄R] →ₗ[A] Ω[B⁄S]` given a square
```
A --→ B
↑ ↑
| |
R --→ S
```
-/
def KaehlerDifferential.map : Ω[A⁄R] →ₗ[A] Ω[B⁄S] :=
Derivation.liftKaehlerDifferential
(((KaehlerDifferential.D S B).restrictScalars R).compAlgebraMap A)
theorem KaehlerDifferential.map_compDer :
(KaehlerDifferential.map R S A B).compDer (KaehlerDifferential.D R A) =
((KaehlerDifferential.D S B).restrictScalars R).compAlgebraMap A :=
Derivation.liftKaehlerDifferential_comp _
@[simp]
theorem KaehlerDifferential.map_D (x : A) :
KaehlerDifferential.map R S A B (KaehlerDifferential.D R A x) =
KaehlerDifferential.D S B (algebraMap A B x) :=
Derivation.congr_fun (KaehlerDifferential.map_compDer R S A B) x
theorem KaehlerDifferential.ker_map :
LinearMap.ker (KaehlerDifferential.map R S A B) =
(((kerTotal S B).restrictScalars A).comap finsupp_map).map
(Finsupp.total A (Ω[A⁄R]) A (D R A)) := by
rw [← Submodule.map_comap_eq_of_surjective (total_surjective R A) (LinearMap.ker _)]
congr 1
ext x
simp only [Submodule.mem_comap, LinearMap.mem_ker, Finsupp.apply_total, ← kerTotal_eq,
Submodule.restrictScalars_mem]
simp only [Finsupp.total_apply, Function.comp_apply, LinearMap.coe_comp, Finsupp.lmapDomain_apply,
Finsupp.mapRange.linearMap_apply]
rw [Finsupp.sum_mapRange_index, Finsupp.sum_mapDomain_index]
· simp [ofId]
· simp
· simp [add_smul]
· simp
lemma KaehlerDifferential.ker_map_of_surjective (h : Function.Surjective (algebraMap A B)) :
LinearMap.ker (map R R A B) =
(LinearMap.ker finsupp_map).map (Finsupp.total A _ A (D R A)) := by
rw [ker_map, ← kerTotal_map' R A B h, Submodule.comap_map_eq, Submodule.map_sup,
Submodule.map_sup, ← kerTotal_eq, ← Submodule.comap_bot,
Submodule.map_comap_eq_of_surjective (total_surjective _ _),
bot_sup_eq, Submodule.map_span, ← Set.range_comp]
convert bot_sup_eq _
rw [Submodule.span_eq_bot]; simp
open IsScalarTower (toAlgHom)
theorem KaehlerDifferential.map_surjective_of_surjective
(h : Function.Surjective (algebraMap A B)) :
Function.Surjective (KaehlerDifferential.map R S A B) := by
rw [← LinearMap.range_eq_top, _root_.eq_top_iff, ← @Submodule.restrictScalars_top A B,
← KaehlerDifferential.span_range_derivation, Submodule.restrictScalars_span _ _ h,
Submodule.span_le]
rintro _ ⟨x, rfl⟩
obtain ⟨y, rfl⟩ := h x
rw [← KaehlerDifferential.map_D R S A B]
exact ⟨_, rfl⟩
theorem KaehlerDifferential.map_surjective :
Function.Surjective (KaehlerDifferential.map R S B B) :=
map_surjective_of_surjective R S B B Function.surjective_id
/-- The lift of the map `Ω[A⁄R] →ₗ[A] Ω[B⁄R]` to the base change along `A → B`.
This is the first map in the exact sequence `B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → Ω[B⁄A] → 0`. -/
noncomputable def KaehlerDifferential.mapBaseChange : B ⊗[A] Ω[A⁄R] →ₗ[B] Ω[B⁄R] :=
(TensorProduct.isBaseChange A (Ω[A⁄R]) B).lift (KaehlerDifferential.map R R A B)
@[simp]
theorem KaehlerDifferential.mapBaseChange_tmul (x : B) (y : Ω[A⁄R]) :
KaehlerDifferential.mapBaseChange R A B (x ⊗ₜ y) = x • KaehlerDifferential.map R R A B y := by
conv_lhs => rw [← mul_one x, ← smul_eq_mul, ← TensorProduct.smul_tmul', LinearMap.map_smul]
congr 1
exact IsBaseChange.lift_eq _ _ _
lemma KaehlerDifferential.range_mapBaseChange :
LinearMap.range (mapBaseChange R A B) = LinearMap.ker (map R A B B) := by
apply le_antisymm
· rintro _ ⟨x, rfl⟩
induction' x with r s
· simp
· obtain ⟨x, rfl⟩ := total_surjective _ _ s
simp only [mapBaseChange_tmul, LinearMap.mem_ker, map_smul]
induction x using Finsupp.induction_linear
· simp
· simp [smul_add, *]
· simp
· rw [map_add]; exact add_mem ‹_› ‹_›
· convert_to (kerTotal A B).map (Finsupp.total B (Ω[B⁄R]) B (D R B)) ≤ _
· rw [KaehlerDifferential.ker_map]
congr 1
convert Submodule.comap_id _
· ext; simp
rw [Submodule.map_le_iff_le_comap, kerTotal, Submodule.span_le]
rintro f ((⟨⟨x, y⟩, rfl⟩|⟨⟨x, y⟩, rfl⟩)|⟨x, rfl⟩)
· use 0; simp
· use 0; simp
· use 1 ⊗ₜ D _ _ x; simp
/-- The sequence `B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → Ω[B⁄A] → 0` is exact.
Also see `KaehlerDifferential.map_surjective`. -/
lemma KaehlerDifferential.exact_mapBaseChange_map :
Function.Exact (mapBaseChange R A B) (map R A B B) :=
SetLike.ext_iff.mp (range_mapBaseChange R A B).symm
/-- The map `I → B ⊗[A] B ⊗[A] Ω[A⁄R]` where `I = ker(A → B)`. -/
@[simps]
noncomputable
def KaehlerDifferential.kerToTensor :
RingHom.ker (algebraMap A B) →ₗ[A] B ⊗[A] Ω[A⁄R] where
toFun x := 1 ⊗ₜ D R A x
map_add' x y := by simp only [AddSubmonoid.coe_add, Submodule.coe_toAddSubmonoid, map_add,
TensorProduct.tmul_add]
map_smul' r x := by simp only [SetLike.val_smul, Derivation.leibniz, RingHom.id_apply,
TensorProduct.tmul_smul, TensorProduct.smul_tmul', add_zero, ← Algebra.algebraMap_eq_smul_one,
TensorProduct.zero_tmul, smul_eq_mul, TensorProduct.tmul_add, (RingHom.mem_ker _).mp x.prop]
/-- The map `I/I² → B ⊗[A] B ⊗[A] Ω[A⁄R]` where `I = ker(A → B)`. -/
noncomputable
def KaehlerDifferential.kerCotangentToTensor :
(RingHom.ker (algebraMap A B)).Cotangent →ₗ[A] B ⊗[A] Ω[A⁄R] :=
Submodule.liftQ _ (kerToTensor R A B) (iSup_le_iff.mpr (by
simp only [Submodule.map_le_iff_le_comap, Subtype.forall]
rintro x hx y -
simp only [Submodule.mem_comap, LinearMap.lsmul_apply, LinearMap.mem_ker, map_smul,
kerToTensor_apply, TensorProduct.smul_tmul', ← algebraMap_eq_smul_one,
(RingHom.mem_ker _).mp hx, TensorProduct.zero_tmul]))
@[simp]
lemma KaehlerDifferential.kerCotangentToTensor_toCotangent (x) :
kerCotangentToTensor R A B (Ideal.toCotangent _ x) = 1 ⊗ₜ D _ _ x.1 := rfl
theorem KaehlerDifferential.range_kerCotangentToTensor
(h : Function.Surjective (algebraMap A B)) :
LinearMap.range (kerCotangentToTensor R A B) =
(LinearMap.ker (KaehlerDifferential.mapBaseChange R A B)).restrictScalars A := by
classical
ext x
constructor
· rintro ⟨x, rfl⟩
obtain ⟨x, rfl⟩ := Ideal.toCotangent_surjective _ x
simp [kerCotangentToTensor_toCotangent, (RingHom.mem_ker _).mp x.2]
· intro hx
obtain ⟨x, rfl⟩ := LinearMap.rTensor_surjective (Ω[A⁄R]) (g := Algebra.linearMap A B) h x
obtain ⟨x, rfl⟩ := (TensorProduct.lid _ _).symm.surjective x
replace hx : x ∈ LinearMap.ker (KaehlerDifferential.map R R A B) := by simpa using hx
rw [KaehlerDifferential.ker_map_of_surjective R A B h] at hx
obtain ⟨x, hx, rfl⟩ := hx
simp only [TensorProduct.lid_symm_apply, LinearMap.rTensor_tmul,
Algebra.linearMap_apply, _root_.map_one]
rw [← Finsupp.sum_single x, Finsupp.sum, ← Finset.sum_fiberwise_of_maps_to
(fun _ ↦ Finset.mem_image_of_mem (algebraMap A B))]
simp only [Function.comp_apply, map_sum (s := x.support.image (algebraMap A B)),
TensorProduct.tmul_sum]
apply sum_mem
intro c _
simp only [Finset.filter_congr_decidable, TensorProduct.lid_symm_apply, LinearMap.rTensor_tmul,
AlgHom.toLinearMap_apply, _root_.map_one, LinearMap.mem_range]
simp only [map_sum, Finsupp.total_single]
have : (x.support.filter (algebraMap A B · = c)).sum x ∈ RingHom.ker (algebraMap A B) := by
simpa [Finsupp.mapDomain, Finsupp.sum, Finsupp.finset_sum_apply, RingHom.mem_ker,
Finsupp.single_apply, ← Finset.sum_filter] using DFunLike.congr_fun hx c
obtain ⟨a, ha⟩ := h c
use (x.support.filter (algebraMap A B · = c)).attach.sum
fun i ↦ x i • Ideal.toCotangent _ ⟨i - a, ?_⟩; swap
· have : x i ≠ 0 ∧ algebraMap A B i = c := by simpa using i.prop
simp [RingHom.mem_ker, ha, this.2]
· simp only [map_sum, LinearMapClass.map_smul, kerCotangentToTensor_toCotangent, map_sub]
simp_rw [← TensorProduct.tmul_smul]
simp only [smul_sub, TensorProduct.tmul_sub, Finset.sum_sub_distrib, ← TensorProduct.tmul_sum,
← Finset.sum_smul, Finset.sum_attach, sub_eq_self,
Finset.sum_attach (f := fun i ↦ x i • KaehlerDifferential.D R A i)]
rw [← TensorProduct.smul_tmul, ← Algebra.algebraMap_eq_smul_one, (RingHom.mem_ker _).mp this,
TensorProduct.zero_tmul]
theorem KaehlerDifferential.exact_kerCotangentToTensor_mapBaseChange
(h : Function.Surjective (algebraMap A B)) :
Function.Exact (kerCotangentToTensor R A B) (KaehlerDifferential.mapBaseChange R A B) :=
SetLike.ext_iff.mp (range_kerCotangentToTensor R A B h).symm
lemma KaehlerDifferential.mapBaseChange_surjective
(h : Function.Surjective (algebraMap A B)) :
Function.Surjective (KaehlerDifferential.mapBaseChange R A B) := by
have := subsingleton_of_surjective A B h
rw [← LinearMap.range_eq_top, KaehlerDifferential.range_mapBaseChange, ← top_le_iff]
exact fun x _ ↦ Subsingleton.elim _ _
end ExactSequence
end KaehlerDifferential
|
RingTheory\Kaehler\CotangentComplex.lean | /-
Copyright (c) 2024 Andrew Yang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Andrew Yang
-/
import Mathlib.RingTheory.Kaehler.Polynomial
import Mathlib.RingTheory.Generators
/-!
# Naive cotangent complex associated to a presentation.
Given a presentation `0 → I → R[x₁,...,xₙ] → S → 0` (or equivalently a closed embedding `S ↪ Aⁿ`
defined by `I`), we may define the (naive) cotangent complex `I/I² → ⨁ᵢ S dxᵢ → Ω[S/R] → 0`.
## Main results
- `Algebra.Generators.Cotangent`: The conormal space `I/I²`. (Defined in `Generators/Basic`)
- `Algebra.Generators.CotangentSpace`: The cotangent space `⨁ᵢ S dxᵢ`.
- `Algebra.Generators.CotangentComplex`: The map `I/I² → ⨁ᵢ S dxᵢ`.
- `Algebra.Generators.toKaehler`: The projection `⨁ᵢ S dxᵢ → Ω[S/R]`.
- `Algebra.Generators.toKaehler_surjective`: The map `⨁ᵢ S dxᵢ → Ω[S/R]` is surjective.
- `Algebra.Generators.exact_cotangentComplex_toKaehler`: `I/I² → ⨁ᵢ S dxᵢ → Ω[S/R]` is exact.
- `Algebra.Generators.Hom.Sub`: If `f` and `g` are two maps between presentations, `f - g` induces
a map `⨁ᵢ S dxᵢ → I/I²` that makes `f` and `g` homotopic.
-/
open KaehlerDifferential TensorProduct MvPolynomial
namespace Algebra
universe w u v
variable {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S]
namespace Generators
variable (P : Generators.{w} R S)
/--
The cotangent space on `P = R[X]`.
This is isomorphic to `Sⁿ` with `n` being the number of variables of `P`.
-/
abbrev CotangentSpace : Type _ := S ⊗[P.Ring] Ω[P.Ring⁄R]
/-- The canonical basis on the `CotangentSpace`. -/
noncomputable
def cotangentSpaceBasis : Basis P.vars S P.CotangentSpace :=
(mvPolynomialBasis _ _).baseChange _
@[simp]
lemma cotangentSpaceBasis_repr_tmul (r x i) :
P.cotangentSpaceBasis.repr (r ⊗ₜ .D _ _ x) i = r * aeval P.val (pderiv i x) := by
classical
simp only [cotangentSpaceBasis, Basis.baseChange_repr_tmul, mvPolynomialBasis_repr_apply,
Algebra.smul_def, mul_comm r, algebraMap_apply]
lemma cotangentSpaceBasis_repr_one_tmul (x i) :
P.cotangentSpaceBasis.repr (1 ⊗ₜ .D _ _ x) i = aeval P.val (pderiv i x) := by
rw [cotangentSpaceBasis_repr_tmul, one_mul]
lemma cotangentSpaceBasis_apply i :
P.cotangentSpaceBasis i = 1 ⊗ₜ .D _ _ (.X i) := by
simp [cotangentSpaceBasis]
/-- The cotangent complex given by a presentation `R[X] → S` (i.e. a closed embedding `S ↪ Aⁿ`). -/
noncomputable
def cotangentComplex : P.Cotangent →ₗ[S] P.CotangentSpace :=
letI f : P.Cotangent ≃ₗ[P.Ring] P.ker.Cotangent :=
{ __ := AddEquiv.refl _, map_smul' := Cotangent.val_smul' }
(kerCotangentToTensor R P.Ring S ∘ₗ f).extendScalarsOfSurjective P.algebraMap_surjective
@[simp]
lemma cotangentComplex_mk (x) : P.cotangentComplex (.mk x) = 1 ⊗ₜ .D _ _ x :=
kerCotangentToTensor_toCotangent _ _ _ _
universe w' u' v'
variable {R' : Type u'} {S' : Type v'} [CommRing R'] [CommRing S'] [Algebra R' S']
variable (P' : Generators.{w'} R' S')
variable [Algebra R R'] [Algebra S S'] [Algebra R S'] [IsScalarTower R R' S'] [IsScalarTower R S S']
attribute [local instance] SMulCommClass.of_commMonoid
variable {P P'}
namespace CotangentSpace
/--
This is the map on the cotangent space associated to a map of presentation.
The matrix associated to this map is the Jacobian matrix. See `CotangentSpace.repr_map`.
-/
protected noncomputable
def map (f : Hom P P') : P.CotangentSpace →ₗ[S] P'.CotangentSpace := by
letI := ((algebraMap S S').comp (algebraMap P.Ring S)).toAlgebra
haveI : IsScalarTower P.Ring S S' := IsScalarTower.of_algebraMap_eq' rfl
letI := f.toAlgHom.toAlgebra
haveI : IsScalarTower P.Ring P'.Ring S' :=
IsScalarTower.of_algebraMap_eq (fun x ↦ (f.algebraMap_toAlgHom x).symm)
apply LinearMap.liftBaseChange
refine (TensorProduct.mk _ _ _ 1).restrictScalars _ ∘ₗ KaehlerDifferential.map R R' P.Ring P'.Ring
@[simp]
lemma map_tmul (f : Hom P P') (x y) :
CotangentSpace.map f (x ⊗ₜ .D _ _ y) = (algebraMap _ _ x) ⊗ₜ .D _ _ (f.toAlgHom y) := by
simp only [CotangentSpace.map, AlgHom.toRingHom_eq_coe, LinearMap.liftBaseChange_tmul,
LinearMap.coe_comp, LinearMap.coe_restrictScalars, Function.comp_apply, map_D, mk_apply]
rw [smul_tmul', ← Algebra.algebraMap_eq_smul_one]
rfl
@[simp]
lemma repr_map (f : Hom P P') (i j) :
P'.cotangentSpaceBasis.repr (CotangentSpace.map f (P.cotangentSpaceBasis i)) j =
aeval P'.val (pderiv j (f.val i)) := by
simp only [cotangentSpaceBasis_apply, map_tmul, _root_.map_one, Hom.toAlgHom_X,
cotangentSpaceBasis_repr_one_tmul]
universe w'' u'' v''
variable {R'' : Type u''} {S'' : Type v''} [CommRing R''] [CommRing S''] [Algebra R'' S'']
variable {P'' : Generators.{w''} R'' S''}
variable [Algebra R R''] [Algebra S S''] [Algebra R S'']
[IsScalarTower R R'' S''] [IsScalarTower R S S'']
variable [Algebra R' R''] [Algebra S' S''] [Algebra R' S'']
[IsScalarTower R' R'' S''] [IsScalarTower R' S' S'']
variable [IsScalarTower R R' R''] [IsScalarTower S S' S'']
@[simp]
lemma map_id :
CotangentSpace.map (.id P) = LinearMap.id := by
apply P.cotangentSpaceBasis.ext
intro i
simp only [cotangentSpaceBasis_apply, map_tmul, _root_.map_one, Hom.toAlgHom_X, Hom.id_val,
LinearMap.id_coe, id_eq]
lemma map_comp (f : Hom P P') (g : Hom P' P'') :
CotangentSpace.map (g.comp f) =
(CotangentSpace.map g).restrictScalars S ∘ₗ CotangentSpace.map f := by
apply P.cotangentSpaceBasis.ext
intro i
simp only [cotangentSpaceBasis_apply, map_tmul, _root_.map_one, Hom.toAlgHom_X, Hom.comp_val,
LinearMap.coe_comp, LinearMap.coe_restrictScalars, Function.comp_apply]
rfl
lemma map_comp_apply (f : Hom P P') (g : Hom P' P'') (x) :
CotangentSpace.map (g.comp f) x = .map g (.map f x) :=
DFunLike.congr_fun (map_comp f g) x
lemma map_cotangentComplex (f : Hom P P') (x) :
CotangentSpace.map f (P.cotangentComplex x) = P'.cotangentComplex (.map f x) := by
obtain ⟨x, rfl⟩ := Cotangent.mk_surjective x
rw [cotangentComplex_mk, map_tmul, _root_.map_one, Cotangent.map_mk,
cotangentComplex_mk]
lemma map_comp_cotangentComplex (f : Hom P P') :
CotangentSpace.map f ∘ₗ P.cotangentComplex =
P'.cotangentComplex.restrictScalars S ∘ₗ Cotangent.map f := by
ext x; exact map_cotangentComplex f x
end CotangentSpace
universe uT
variable {T : Type uT} [CommRing T] [Algebra R T] [Algebra S T] [IsScalarTower R S T]
lemma Hom.sub_aux (f g : Hom P P') (x y) :
letI := ((algebraMap S S').comp (algebraMap P.Ring S)).toAlgebra
f.toAlgHom (x * y) - g.toAlgHom (x * y) -
(P'.σ ((algebraMap P.Ring S') x) * (f.toAlgHom y - g.toAlgHom y) +
P'.σ ((algebraMap P.Ring S') y) * (f.toAlgHom x - g.toAlgHom x)) ∈
P'.ker ^ 2 := by
letI := ((algebraMap S S').comp (algebraMap P.Ring S)).toAlgebra
have :
(f.toAlgHom x - P'.σ (algebraMap P.Ring S' x)) * (f.toAlgHom y - g.toAlgHom y) +
(g.toAlgHom y - P'.σ (algebraMap P.Ring S' y)) * (f.toAlgHom x - g.toAlgHom x)
∈ P'.ker ^ 2 := by
rw [pow_two]
refine Ideal.add_mem _ (Ideal.mul_mem_mul ?_ ?_) (Ideal.mul_mem_mul ?_ ?_) <;>
simp only [RingHom.algebraMap_toAlgebra, AlgHom.toRingHom_eq_coe, RingHom.coe_comp,
RingHom.coe_coe, Function.comp_apply, map_aeval, ← IsScalarTower.algebraMap_eq,
coe_eval₂Hom, ← aeval_def, ker, RingHom.mem_ker, map_sub, algebraMap_toAlgHom, aeval_val_σ,
sub_self]
convert this using 1
simp only [_root_.map_mul]
ring
/--
If `f` and `g` are two maps `P → P'` between presentations,
then the image of `f - g` is in the kernel of `P' → S`.
-/
@[simps! apply_coe]
noncomputable
def Hom.subToKer (f g : Hom P P') : P.Ring →ₗ[R] P'.ker := by
refine ((f.toAlgHom.toLinearMap - g.toAlgHom.toLinearMap).codRestrict
(P'.ker.restrictScalars R) ?_)
intro x
simp only [LinearMap.sub_apply, AlgHom.toLinearMap_apply, ker, algebraMap_eq,
Submodule.restrictScalars_mem, RingHom.mem_ker, map_sub, RingHom.coe_coe, algebraMap_toAlgHom,
map_aeval, coe_eval₂Hom, sub_self]
/--
If `f` and `g` are two maps `P → P'` between presentations,
their difference induces a map `P.CotangentSpace →ₗ[S] P'.Cotangent` that makes two maps
between the cotangent complexes homotopic.
-/
noncomputable
def Hom.sub (f g : Hom P P') : P.CotangentSpace →ₗ[S] P'.Cotangent := by
letI := ((algebraMap S S').comp (algebraMap P.Ring S)).toAlgebra
haveI : IsScalarTower P.Ring S S' := IsScalarTower.of_algebraMap_eq' rfl
letI := f.toAlgHom.toAlgebra
haveI : IsScalarTower P.Ring P'.Ring S' :=
IsScalarTower.of_algebraMap_eq fun x ↦ (f.algebraMap_toAlgHom x).symm
haveI : IsScalarTower R P.Ring S' :=
IsScalarTower.of_algebraMap_eq fun x ↦
show algebraMap R S' x = algebraMap S S' (algebraMap P.Ring S (algebraMap R P.Ring x)) by
rw [← IsScalarTower.algebraMap_apply R P.Ring S, ← IsScalarTower.algebraMap_apply]
refine (Derivation.liftKaehlerDifferential ?_).liftBaseChange S
refine
{ __ := Cotangent.mk.restrictScalars R ∘ₗ f.subToKer g
map_one_eq_zero' := ?_
leibniz' := ?_ }
· ext
simp only [LinearMap.coe_comp, LinearMap.coe_restrictScalars, Function.comp_apply,
Cotangent.val_mk, Cotangent.val_zero, Ideal.toCotangent_eq_zero]
erw [LinearMap.codRestrict_apply]
simp only [LinearMap.sub_apply, AlgHom.toLinearMap_apply, _root_.map_one, sub_self,
Submodule.zero_mem]
· intro x y
ext
simp only [LinearMap.coe_comp, LinearMap.coe_restrictScalars, Function.comp_apply,
Cotangent.val_mk, Cotangent.val_add, Cotangent.val_smul''', ← map_smul, ← map_add,
Ideal.toCotangent_eq, AddSubmonoid.coe_add, Submodule.coe_toAddSubmonoid,
SetLike.val_smul, smul_eq_mul]
exact Hom.sub_aux f g x y
lemma Hom.sub_one_tmul (f g : Hom P P') (x) :
f.sub g (1 ⊗ₜ .D _ _ x) = Cotangent.mk (f.subToKer g x) := by
simp only [sub, LinearMap.liftBaseChange_tmul, Derivation.liftKaehlerDifferential_comp_D,
Derivation.mk_coe, LinearMap.coe_comp, LinearMap.coe_restrictScalars, Function.comp_apply,
one_smul]
@[simp]
lemma Hom.sub_tmul (f g : Hom P P') (r x) :
f.sub g (r ⊗ₜ .D _ _ x) = r • Cotangent.mk (f.subToKer g x) := by
simp only [sub, LinearMap.liftBaseChange_tmul, Derivation.liftKaehlerDifferential_comp_D,
Derivation.mk_coe, LinearMap.coe_comp, LinearMap.coe_restrictScalars, Function.comp_apply]
lemma CotangentSpace.map_sub_map (f g : Hom P P') :
CotangentSpace.map f - CotangentSpace.map g =
P'.cotangentComplex.restrictScalars S ∘ₗ (f.sub g) := by
apply P.cotangentSpaceBasis.ext
intro i
simp only [cotangentSpaceBasis_apply, LinearMap.sub_apply, map_tmul, _root_.map_one,
Hom.toAlgHom_X, LinearMap.coe_comp, LinearMap.coe_restrictScalars, Function.comp_apply,
Hom.sub_one_tmul, cotangentComplex_mk, Hom.subToKer_apply_coe, map_sub, tmul_sub]
lemma Cotangent.map_sub_map (f g : Hom P P') :
map f - map g = (f.sub g) ∘ₗ P.cotangentComplex := by
ext x
obtain ⟨x, rfl⟩ := mk_surjective x
simp only [LinearMap.sub_apply, map_mk, LinearMap.coe_comp, Function.comp_apply,
cotangentComplex_mk, Hom.sub_tmul, one_smul, val_mk]
apply (Ideal.cotangentEquivIdeal _).injective
ext
simp only [val_sub, val_mk, map_sub, AddSubgroupClass.coe_sub, Ideal.cotangentEquivIdeal_apply,
Ideal.toCotangent_to_quotient_square, Submodule.mkQ_apply, Ideal.Quotient.mk_eq_mk,
Hom.subToKer_apply_coe]
variable (P) in
/-- The projection map from the relative cotangent space to the module of differentials. -/
noncomputable
abbrev toKaehler : P.CotangentSpace →ₗ[S] Ω[S⁄R] := mapBaseChange _ _ _
@[simp]
lemma toKaehler_cotangentSpaceBasis (i) :
P.toKaehler (P.cotangentSpaceBasis i) = D R S (P.val i) := by
simp [cotangentSpaceBasis_apply]
lemma toKaehler_surjective : Function.Surjective P.toKaehler :=
mapBaseChange_surjective _ _ _ P.algebraMap_surjective
lemma exact_cotangentComplex_toKaehler : Function.Exact P.cotangentComplex P.toKaehler :=
exact_kerCotangentToTensor_mapBaseChange _ _ _ P.algebraMap_surjective
end Generators
end Algebra
|
RingTheory\Kaehler\Polynomial.lean | /-
Copyright (c) 2024 Andrew Yang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Andrew Yang
-/
import Mathlib.RingTheory.Kaehler.Basic
import Mathlib.Algebra.MvPolynomial.PDeriv
import Mathlib.Algebra.Polynomial.Derivation
/-!
# The Kaehler differential module of polynomial algebras
-/
open scoped TensorProduct
open Algebra
universe u v
variable (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S]
suppress_compilation
section MvPolynomial
/-- The relative differential module of a polynomial algebra `R[σ]` is the free module generated by
`{ dx | x ∈ σ }`. Also see `KaehlerDifferential.mvPolynomialBasis`. -/
def KaehlerDifferential.mvPolynomialEquiv (σ : Type*) :
Ω[MvPolynomial σ R⁄R] ≃ₗ[MvPolynomial σ R] σ →₀ MvPolynomial σ R where
__ := (MvPolynomial.mkDerivation _ (Finsupp.single · 1)).liftKaehlerDifferential
invFun := Finsupp.total σ _ _ (fun x ↦ D _ _ (MvPolynomial.X x))
right_inv := by
intro x
induction' x using Finsupp.induction_linear with _ _ _ _ a b
· simp only [AddHom.toFun_eq_coe, LinearMap.coe_toAddHom]; rw [map_zero, map_zero]
· simp only [AddHom.toFun_eq_coe, LinearMap.coe_toAddHom, map_add] at *; simp only [*]
· simp [LinearMap.map_smul, -map_smul]
left_inv := by
intro x
obtain ⟨x, rfl⟩ := total_surjective _ _ x
induction' x using Finsupp.induction_linear with _ _ _ _ a b
· simp only [AddHom.toFun_eq_coe, LinearMap.coe_toAddHom]; rw [map_zero, map_zero, map_zero]
· simp only [map_add, AddHom.toFun_eq_coe, LinearMap.coe_toAddHom] at *; simp only [*]
· simp only [AddHom.toFun_eq_coe, LinearMap.coe_toAddHom, Finsupp.total_single,
LinearMap.map_smul, Derivation.liftKaehlerDifferential_comp_D]
congr 1
induction a using MvPolynomial.induction_on
· simp only [MvPolynomial.derivation_C, map_zero]
· simp only [map_add, *]
· simp [*]
/-- `{ dx | x ∈ σ }` forms a basis of the relative differential module
of a polynomial algebra `R[σ]`. -/
def KaehlerDifferential.mvPolynomialBasis (σ) :
Basis σ (MvPolynomial σ R) (Ω[MvPolynomial σ R⁄R]) :=
⟨mvPolynomialEquiv R σ⟩
lemma KaehlerDifferential.mvPolynomialBasis_repr_comp_D (σ) :
(mvPolynomialBasis R σ).repr.toLinearMap.compDer (D _ _) =
MvPolynomial.mkDerivation _ (Finsupp.single · 1) :=
Derivation.liftKaehlerDifferential_comp _
lemma KaehlerDifferential.mvPolynomialBasis_repr_D (σ) (x) :
(mvPolynomialBasis R σ).repr (D _ _ x) =
MvPolynomial.mkDerivation R (Finsupp.single · (1 : MvPolynomial σ R)) x :=
Derivation.congr_fun (mvPolynomialBasis_repr_comp_D R σ) x
@[simp]
lemma KaehlerDifferential.mvPolynomialBasis_repr_D_X (σ) (i) :
(mvPolynomialBasis R σ).repr (D _ _ (.X i)) = Finsupp.single i 1 := by
simp [mvPolynomialBasis_repr_D]
@[simp]
lemma KaehlerDifferential.mvPolynomialBasis_repr_apply (σ) (x) (i) :
(mvPolynomialBasis R σ).repr (D _ _ x) i = MvPolynomial.pderiv i x := by
classical
suffices ((Finsupp.lapply i).comp
(mvPolynomialBasis R σ).repr.toLinearMap).compDer (D _ _) = MvPolynomial.pderiv i by
rw [← this]; rfl
apply MvPolynomial.derivation_ext
intro j
simp [Finsupp.single_apply, Pi.single_apply]
lemma KaehlerDifferential.mvPolynomialBasis_repr_symm_single (σ) (i) (x) :
(mvPolynomialBasis R σ).repr.symm (Finsupp.single i x) = x • D R (MvPolynomial σ R) (.X i) := by
apply (mvPolynomialBasis R σ).repr.injective; simp [LinearEquiv.map_smul, -map_smul]
@[simp]
lemma KaehlerDifferential.mvPolynomialBasis_apply (σ) (i) :
mvPolynomialBasis R σ i = D R (MvPolynomial σ R) (.X i) :=
(mvPolynomialBasis_repr_symm_single R σ i 1).trans (one_smul _ _)
instance (σ) : Module.Free (MvPolynomial σ R) (Ω[MvPolynomial σ R⁄R]) :=
.of_basis (KaehlerDifferential.mvPolynomialBasis R σ)
end MvPolynomial
section Polynomial
open Polynomial
lemma KaehlerDifferential.polynomial_D_apply (P : R[X]) :
D R R[X] P = derivative P • D R R[X] X := by
rw [← aeval_X_left_apply P, (D R R[X]).map_aeval, aeval_X_left_apply, aeval_X_left_apply]
/-- The relative differential module of the univariate polynomial algebra `R[X]` is isomorphic to
`R[X]` as an `R[X]`-module. -/
def KaehlerDifferential.polynomialEquiv : Ω[R[X]⁄R] ≃ₗ[R[X]] R[X] where
__ := derivative'.liftKaehlerDifferential
invFun := (Algebra.lsmul R R _).toLinearMap.flip (D R R[X] X)
left_inv := by
intro x
obtain ⟨x, rfl⟩ := total_surjective _ _ x
induction' x using Finsupp.induction_linear with x y hx hy x y
· simp
· simp only [map_add, AddHom.toFun_eq_coe, LinearMap.coe_toAddHom, LinearMap.flip_apply,
AlgHom.toLinearMap_apply, lsmul_coe] at *; simp only [*]
· simp [polynomial_D_apply _ x]
right_inv x := by simp
lemma KaehlerDifferential.polynomialEquiv_comp_D :
(polynomialEquiv R).compDer (D R R[X]) = derivative' :=
Derivation.liftKaehlerDifferential_comp _
@[simp]
lemma KaehlerDifferential.polynomialEquiv_D (P) :
polynomialEquiv R (D R R[X] P) = derivative P :=
Derivation.congr_fun (polynomialEquiv_comp_D R) P
@[simp]
lemma KaehlerDifferential.polynomialEquiv_symm (P) :
(polynomialEquiv R).symm P = P • D R R[X] X := rfl
end Polynomial
|
RingTheory\Localization\Algebra.lean | /-
Copyright (c) 2024 Christian Merten. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Christian Merten
-/
import Mathlib.Algebra.Module.LocalizedModule
import Mathlib.RingTheory.Ideal.Maps
import Mathlib.RingTheory.Localization.Basic
import Mathlib.RingTheory.Localization.Ideal
/-!
# Localization of algebra maps
In this file we provide constructors to localize algebra maps. Also we show that
localization commutes with taking kernels for ring homomorphisms.
## Implementation detail
The proof that localization commutes with taking kernels does not use the result for linear maps,
as the translation is currently tedious and can be unified easily after the localization refactor.
-/
universe u u' v v' w w'
variable {R S P : Type*} (Q : Type*) [CommSemiring R] [CommSemiring S] [CommSemiring P]
[CommSemiring Q]
{M : Submonoid R} {T : Submonoid P}
[Algebra R S] [Algebra P Q] [IsLocalization M S] [IsLocalization T Q]
(g : R →+* P)
open IsLocalization in
variable (M S) in
/-- The span of `I` in a localization of `R` at `M` is the localization of `I` at `M`. -/
instance Algebra.idealMap_isLocalizedModule (I : Ideal R) :
IsLocalizedModule M (Algebra.idealMap I (S := S)) where
map_units x :=
(Module.End_isUnit_iff _).mpr ⟨fun a b e ↦ Subtype.ext ((map_units S x).mul_right_injective
(by simpa [Algebra.smul_def] using congr(($e).1))),
fun a ↦ ⟨⟨_, Ideal.mul_mem_left _ (map_units S x).unit⁻¹.1 a.2⟩,
Subtype.ext (by simp [Algebra.smul_def, ← mul_assoc])⟩⟩
surj' y :=
have ⟨x, hx⟩ := (mem_map_algebraMap_iff M S).mp y.property
⟨x, Subtype.ext (by simp [Submonoid.smul_def, Algebra.smul_def, mul_comm, hx])⟩
exists_of_eq h := ⟨_, Subtype.ext (exists_of_eq congr(($h).1)).choose_spec⟩
lemma IsLocalization.ker_map (hT : Submonoid.map g M = T) :
RingHom.ker (IsLocalization.map Q g (hT.symm ▸ M.le_comap_map) : S →+* Q) =
(RingHom.ker g).map (algebraMap R S) := by
ext x
obtain ⟨x, s, rfl⟩ := IsLocalization.mk'_surjective M x
simp [RingHom.mem_ker, IsLocalization.map_mk', IsLocalization.mk'_eq_zero_iff,
IsLocalization.mk'_mem_map_algebraMap_iff, ← hT]
variable (S) in
/-- The canonical linear map from the kernel of `g` to the kernel of its localization. -/
def RingHom.toKerIsLocalization (hy : M ≤ Submonoid.comap g T) :
RingHom.ker g →ₗ[R] RingHom.ker (IsLocalization.map Q g hy : S →+* Q) where
toFun x := ⟨algebraMap R S x, by simp [RingHom.mem_ker, (RingHom.mem_ker g).mp x.property]⟩
map_add' x y := by
simp only [AddSubmonoid.coe_add, Submodule.coe_toAddSubmonoid, map_add, AddSubmonoid.mk_add_mk]
map_smul' a x := by
simp only [SetLike.val_smul, smul_eq_mul, map_mul, RingHom.id_apply,
SetLike.mk_smul_of_tower_mk, Algebra.smul_def]
@[simp]
lemma RingHom.toKerIsLocalization_apply (hy : M ≤ Submonoid.comap g T) (r : RingHom.ker g) :
(RingHom.toKerIsLocalization S Q g hy r).val = algebraMap R S r :=
rfl
/-- The canonical linear map from the kernel of `g` to the kernel of its localization
is localizing. In other words, localization commutes with taking kernels. -/
lemma RingHom.toKerIsLocalization_isLocalizedModule (hT : Submonoid.map g M = T) :
IsLocalizedModule M (toKerIsLocalization S Q g (hT.symm ▸ Submonoid.le_comap_map M)) := by
let e := LinearEquiv.ofEq _ _ (IsLocalization.ker_map (S := S) Q g hT).symm
convert_to IsLocalizedModule M ((e.restrictScalars R).toLinearMap ∘ₗ
Algebra.idealMap S (RingHom.ker g))
apply IsLocalizedModule.of_linearEquiv
section Algebra
open Algebra
variable {R : Type u} [CommRing R] (M : Submonoid R)
variable {A : Type v} [CommRing A] [Algebra R A]
variable {B : Type w} [CommRing B] [Algebra R B]
variable (Rₚ : Type u') [CommRing Rₚ] [Algebra R Rₚ] [IsLocalization M Rₚ]
variable (Aₚ : Type v') [CommRing Aₚ] [Algebra R Aₚ] [Algebra A Aₚ] [IsScalarTower R A Aₚ]
[IsLocalization (Algebra.algebraMapSubmonoid A M) Aₚ]
variable (Bₚ : Type v') [CommRing Bₚ] [Algebra R Bₚ] [Algebra B Bₚ] [IsScalarTower R B Bₚ]
[IsLocalization (Algebra.algebraMapSubmonoid B M) Bₚ]
variable [Algebra Rₚ Aₚ] [Algebra Rₚ Bₚ] [IsScalarTower R Rₚ Aₚ] [IsScalarTower R Rₚ Bₚ]
namespace IsLocalization
instance isLocalization_algebraMapSubmonoid_map_algHom (f : A →ₐ[R] B) :
IsLocalization ((algebraMapSubmonoid A M).map f.toRingHom) Bₚ := by
erw [algebraMapSubmonoid_map_eq M f]
infer_instance
/-- An algebra map `A →ₐ[R] B` induces an algebra map on localizations `Aₚ →ₐ[Rₚ] Bₚ`. -/
noncomputable def mapₐ (f : A →ₐ[R] B) : Aₚ →ₐ[Rₚ] Bₚ :=
⟨IsLocalization.map Bₚ f.toRingHom (Algebra.algebraMapSubmonoid_le_comap M f), fun r ↦ by
obtain ⟨a, m, rfl⟩ := IsLocalization.mk'_surjective M r
simp [algebraMap_mk' (S := A), algebraMap_mk' (S := B), map_mk']⟩
@[simp]
lemma mapₐ_coe (f : A →ₐ[R] B) :
(mapₐ M Rₚ Aₚ Bₚ f : Aₚ → Bₚ) = map Bₚ f.toRingHom (algebraMapSubmonoid_le_comap M f) :=
rfl
lemma mapₐ_injective_of_injective (f : A →ₐ[R] B) (hf : Function.Injective f) :
Function.Injective (mapₐ M Rₚ Aₚ Bₚ f) :=
IsLocalization.map_injective_of_injective _ _ _ hf
lemma mapₐ_surjective_of_surjective (f : A →ₐ[R] B) (hf : Function.Surjective f) :
Function.Surjective (mapₐ M Rₚ Aₚ Bₚ f) :=
IsLocalization.map_surjective_of_surjective _ _ _ hf
end IsLocalization
open IsLocalization
/-- The canonical linear map from the kernel of an algebra homomorphism to its localization. -/
def AlgHom.toKerIsLocalization (f : A →ₐ[R] B) :
RingHom.ker f →ₗ[A] RingHom.ker (mapₐ M Rₚ Aₚ Bₚ f) :=
RingHom.toKerIsLocalization Aₚ Bₚ f.toRingHom (algebraMapSubmonoid_le_comap M f)
@[simp]
lemma AlgHom.toKerIsLocalization_apply (f : A →ₐ[R] B) (x : RingHom.ker f) :
AlgHom.toKerIsLocalization M Rₚ Aₚ Bₚ f x =
RingHom.toKerIsLocalization Aₚ Bₚ f.toRingHom (algebraMapSubmonoid_le_comap M f) x :=
rfl
/-- The canonical linear map from the kernel of an algebra homomorphism to its localization
is localizing. -/
lemma AlgHom.toKerIsLocalization_isLocalizedModule (f : A →ₐ[R] B) :
IsLocalizedModule (Algebra.algebraMapSubmonoid A M)
(AlgHom.toKerIsLocalization M Rₚ Aₚ Bₚ f) :=
RingHom.toKerIsLocalization_isLocalizedModule Bₚ f.toRingHom
(algebraMapSubmonoid_map_eq M f)
end Algebra
|
RingTheory\Localization\AsSubring.lean | /-
Copyright (c) 2022 Adam Topaz. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Adam Topaz, Junyan Xu
-/
import Mathlib.RingTheory.Localization.LocalizationLocalization
/-!
# Localizations of domains as subalgebras of the fraction field.
Given a domain `A` with fraction field `K`, and a submonoid `S` of `A` which
does not contain zero, this file constructs the localization of `A` at `S`
as a subalgebra of the field `K` over `A`.
-/
namespace Localization
open nonZeroDivisors
variable {A : Type*} (K : Type*) [CommRing A] (S : Submonoid A) (hS : S ≤ A⁰)
section CommRing
variable [CommRing K] [Algebra A K] [IsFractionRing A K]
theorem map_isUnit_of_le (hS : S ≤ A⁰) (s : S) : IsUnit (algebraMap A K s) := by
apply IsLocalization.map_units K (⟨s.1, hS s.2⟩ : A⁰)
/-- The canonical map from a localization of `A` at `S` to the fraction ring
of `A`, given that `S ≤ A⁰`. -/
noncomputable def mapToFractionRing (B : Type*) [CommRing B] [Algebra A B] [IsLocalization S B]
(hS : S ≤ A⁰) : B →ₐ[A] K :=
{ IsLocalization.lift (map_isUnit_of_le K S hS) with commutes' := fun a => by simp }
@[simp]
theorem mapToFractionRing_apply {B : Type*} [CommRing B] [Algebra A B] [IsLocalization S B]
(hS : S ≤ A⁰) (b : B) :
mapToFractionRing K S B hS b = IsLocalization.lift (map_isUnit_of_le K S hS) b :=
rfl
theorem mem_range_mapToFractionRing_iff (B : Type*) [CommRing B] [Algebra A B] [IsLocalization S B]
(hS : S ≤ A⁰) (x : K) :
x ∈ (mapToFractionRing K S B hS).range ↔
∃ (a s : A) (hs : s ∈ S), x = IsLocalization.mk' K a ⟨s, hS hs⟩ :=
⟨by
rintro ⟨x, rfl⟩
obtain ⟨a, s, rfl⟩ := IsLocalization.mk'_surjective S x
use a, s, s.2
apply IsLocalization.lift_mk', by
rintro ⟨a, s, hs, rfl⟩
use IsLocalization.mk' _ a ⟨s, hs⟩
apply IsLocalization.lift_mk'⟩
instance isLocalization_range_mapToFractionRing (B : Type*) [CommRing B] [Algebra A B]
[IsLocalization S B] (hS : S ≤ A⁰) : IsLocalization S (mapToFractionRing K S B hS).range :=
IsLocalization.isLocalization_of_algEquiv S <|
show B ≃ₐ[A] _ from AlgEquiv.ofBijective (mapToFractionRing K S B hS).rangeRestrict (by
refine ⟨fun a b h => ?_, Set.surjective_onto_range⟩
refine (IsLocalization.lift_injective_iff _).2 (fun a b => ?_) (Subtype.ext_iff.1 h)
exact ⟨fun h => congr_arg _ (IsLocalization.injective _ hS h),
fun h => congr_arg _ (IsFractionRing.injective A K h)⟩)
instance isFractionRing_range_mapToFractionRing (B : Type*) [CommRing B] [Algebra A B]
[IsLocalization S B] (hS : S ≤ A⁰) : IsFractionRing (mapToFractionRing K S B hS).range K :=
IsFractionRing.isFractionRing_of_isLocalization S _ _ hS
/-- Given a commutative ring `A` with fraction ring `K`, and a submonoid `S` of `A` which
contains no zero divisor, this is the localization of `A` at `S`, considered as
a subalgebra of `K` over `A`.
The carrier of this subalgebra is defined as the set of all `x : K` of the form
`IsLocalization.mk' K a ⟨s, _⟩`, where `s ∈ S`.
-/
noncomputable def subalgebra (hS : S ≤ A⁰) : Subalgebra A K :=
(mapToFractionRing K S (Localization S) hS).range.copy
{ x | ∃ (a s : A) (hs : s ∈ S), x = IsLocalization.mk' K a ⟨s, hS hs⟩ } <| by
ext
symm
apply mem_range_mapToFractionRing_iff
namespace subalgebra
instance isLocalization_subalgebra : IsLocalization S (subalgebra K S hS) := by
dsimp only [Localization.subalgebra]
rw [Subalgebra.copy_eq]
infer_instance
instance isFractionRing : IsFractionRing (subalgebra K S hS) K :=
IsFractionRing.isFractionRing_of_isLocalization S _ _ hS
end subalgebra
end CommRing
section Field
variable [Field K] [Algebra A K] [IsFractionRing A K]
namespace subalgebra
theorem mem_range_mapToFractionRing_iff_ofField (B : Type*) [CommRing B] [Algebra A B]
[IsLocalization S B] (x : K) :
x ∈ (mapToFractionRing K S B hS).range ↔
∃ (a s : A) (_ : s ∈ S), x = algebraMap A K a * (algebraMap A K s)⁻¹ := by
rw [mem_range_mapToFractionRing_iff]
convert Iff.rfl
congr
rw [Units.val_inv_eq_inv_val]
rfl
/-- Given a domain `A` with fraction field `K`, and a submonoid `S` of `A` which
contains no zero divisor, this is the localization of `A` at `S`, considered as
a subalgebra of `K` over `A`.
The carrier of this subalgebra is defined as the set of all `x : K` of the form
`algebraMap A K a * (algebraMap A K s)⁻¹` where `a s : A` and `s ∈ S`.
-/
noncomputable def ofField : Subalgebra A K :=
(mapToFractionRing K S (Localization S) hS).range.copy
{ x | ∃ (a s : A) (_ : s ∈ S), x = algebraMap A K a * (algebraMap A K s)⁻¹ } <| by
ext
symm
apply mem_range_mapToFractionRing_iff_ofField
instance isLocalization_ofField : IsLocalization S (subalgebra.ofField K S hS) := by
dsimp only [Localization.subalgebra.ofField]
rw [Subalgebra.copy_eq]
infer_instance
instance isFractionRing_ofField : IsFractionRing (subalgebra.ofField K S hS) K :=
IsFractionRing.isFractionRing_of_isLocalization S _ _ hS
end subalgebra
end Field
end Localization
|
RingTheory\Localization\AtPrime.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.Localization.Ideal
import Mathlib.RingTheory.LocalRing.MaximalIdeal.Basic
import Mathlib.RingTheory.LocalRing.RingHom.Defs
/-!
# Localizations of commutative rings at the complement of a prime ideal
## Main definitions
* `IsLocalization.AtPrime (P : Ideal R) [IsPrime P] (S : Type*)` expresses that `S` is a
localization at (the complement of) a prime ideal `P`, as an abbreviation of
`IsLocalization P.prime_compl S`
## Main results
* `IsLocalization.AtPrime.localRing`: a theorem (not an instance) stating a localization at the
complement of a prime ideal is a local ring
## Implementation notes
See `RingTheory.Localization.Basic` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
variable {R : Type*} [CommSemiring R] (M : Submonoid R) (S : Type*) [CommSemiring S]
variable [Algebra R S] {P : Type*} [CommSemiring P]
section AtPrime
variable (P : Ideal R) [hp : P.IsPrime]
namespace Ideal
/-- The complement of a prime ideal `P ⊆ R` is a submonoid of `R`. -/
def primeCompl : Submonoid R where
carrier := (Pᶜ : Set R)
one_mem' := by convert P.ne_top_iff_one.1 hp.1
mul_mem' {x y} hnx hny hxy := Or.casesOn (hp.mem_or_mem hxy) hnx hny
theorem primeCompl_le_nonZeroDivisors [NoZeroDivisors R] : P.primeCompl ≤ nonZeroDivisors R :=
le_nonZeroDivisors_of_noZeroDivisors <| not_not_intro P.zero_mem
end Ideal
/-- Given a prime ideal `P`, the typeclass `IsLocalization.AtPrime S P` states that `S` is
isomorphic to the localization of `R` at the complement of `P`. -/
protected abbrev IsLocalization.AtPrime :=
IsLocalization P.primeCompl S
/-- Given a prime ideal `P`, `Localization.AtPrime P` is a localization of
`R` at the complement of `P`, as a quotient type. -/
protected abbrev Localization.AtPrime :=
Localization P.primeCompl
namespace IsLocalization
theorem AtPrime.Nontrivial [IsLocalization.AtPrime S P] : Nontrivial S :=
nontrivial_of_ne (0 : S) 1 fun hze => by
rw [← (algebraMap R S).map_one, ← (algebraMap R S).map_zero] at hze
obtain ⟨t, ht⟩ := (eq_iff_exists P.primeCompl S).1 hze
have htz : (t : R) = 0 := by simpa using ht.symm
exact t.2 (htz.symm ▸ P.zero_mem : ↑t ∈ P)
theorem AtPrime.localRing [IsLocalization.AtPrime S P] : LocalRing S :=
-- Porting note: since I couldn't get local instance running, I just specify it manually
letI := AtPrime.Nontrivial S P
LocalRing.of_nonunits_add
(by
intro x y hx hy hu
cases' isUnit_iff_exists_inv.1 hu with z hxyz
have : ∀ {r : R} {s : P.primeCompl}, mk' S r s ∈ nonunits S → r ∈ P := fun {r s} =>
not_imp_comm.1 fun nr => isUnit_iff_exists_inv.2 ⟨mk' S ↑s (⟨r, nr⟩ : P.primeCompl),
mk'_mul_mk'_eq_one' _ _ <| show r ∈ P.primeCompl from nr⟩
rcases mk'_surjective P.primeCompl x with ⟨rx, sx, hrx⟩
rcases mk'_surjective P.primeCompl y with ⟨ry, sy, hry⟩
rcases mk'_surjective P.primeCompl z with ⟨rz, sz, hrz⟩
rw [← hrx, ← hry, ← hrz, ← mk'_add, ← mk'_mul, ← mk'_self S P.primeCompl.one_mem] at hxyz
rw [← hrx] at hx
rw [← hry] at hy
obtain ⟨t, ht⟩ := IsLocalization.eq.1 hxyz
simp only [mul_one, one_mul, Submonoid.coe_mul, Subtype.coe_mk] at ht
suffices (t : R) * (sx * sy * sz) ∈ P from
not_or_of_not (mt hp.mem_or_mem <| not_or_of_not sx.2 sy.2) sz.2
(hp.mem_or_mem <| (hp.mem_or_mem this).resolve_left t.2)
rw [← ht]
exact
P.mul_mem_left _ <| P.mul_mem_right _ <|
P.add_mem (P.mul_mem_right _ <| this hx) <| P.mul_mem_right _ <| this hy)
end IsLocalization
namespace Localization
/-- The localization of `R` at the complement of a prime ideal is a local ring. -/
instance AtPrime.localRing : LocalRing (Localization P.primeCompl) :=
IsLocalization.AtPrime.localRing (Localization P.primeCompl) P
end Localization
end AtPrime
namespace IsLocalization
variable {A : Type*} [CommRing A] [IsDomain A]
/-- The localization of an integral domain at the complement of a prime ideal is an integral domain.
-/
instance isDomain_of_local_atPrime {P : Ideal A} (_ : P.IsPrime) :
IsDomain (Localization.AtPrime P) :=
isDomain_localization P.primeCompl_le_nonZeroDivisors
namespace AtPrime
variable (I : Ideal R) [hI : I.IsPrime] [IsLocalization.AtPrime S I]
/-- The prime ideals in the localization of a commutative ring at a prime ideal I are in
order-preserving bijection with the prime ideals contained in I. -/
def orderIsoOfPrime : { p : Ideal S // p.IsPrime } ≃o { p : Ideal R // p.IsPrime ∧ p ≤ I } :=
(IsLocalization.orderIsoOfPrime I.primeCompl S).trans <| .setCongr _ _ <| show setOf _ = setOf _
by ext; simp [Ideal.primeCompl, ← le_compl_iff_disjoint_left]
theorem isUnit_to_map_iff (x : R) : IsUnit ((algebraMap R S) x) ↔ x ∈ I.primeCompl :=
⟨fun h hx =>
(isPrime_of_isPrime_disjoint I.primeCompl S I hI disjoint_compl_left).ne_top <|
(Ideal.map (algebraMap R S) I).eq_top_of_isUnit_mem (Ideal.mem_map_of_mem _ hx) h,
fun h => map_units S ⟨x, h⟩⟩
-- Can't use typeclasses to infer the `LocalRing` instance, so use an `optParam` instead
-- (since `LocalRing` is a `Prop`, there should be no unification issues.)
theorem to_map_mem_maximal_iff (x : R) (h : LocalRing S := localRing S I) :
algebraMap R S x ∈ LocalRing.maximalIdeal S ↔ x ∈ I :=
not_iff_not.mp <| by
simpa only [LocalRing.mem_maximalIdeal, mem_nonunits_iff, Classical.not_not] using
isUnit_to_map_iff S I x
theorem comap_maximalIdeal (h : LocalRing S := localRing S I) :
(LocalRing.maximalIdeal S).comap (algebraMap R S) = I :=
Ideal.ext fun x => by simpa only [Ideal.mem_comap] using to_map_mem_maximal_iff _ I x
theorem isUnit_mk'_iff (x : R) (y : I.primeCompl) : IsUnit (mk' S x y) ↔ x ∈ I.primeCompl :=
⟨fun h hx => mk'_mem_iff.mpr ((to_map_mem_maximal_iff S I x).mpr hx) h, fun h =>
isUnit_iff_exists_inv.mpr ⟨mk' S ↑y ⟨x, h⟩, mk'_mul_mk'_eq_one ⟨x, h⟩ y⟩⟩
theorem mk'_mem_maximal_iff (x : R) (y : I.primeCompl) (h : LocalRing S := localRing S I) :
mk' S x y ∈ LocalRing.maximalIdeal S ↔ x ∈ I :=
not_iff_not.mp <| by
simpa only [LocalRing.mem_maximalIdeal, mem_nonunits_iff, Classical.not_not] using
isUnit_mk'_iff S I x y
end AtPrime
end IsLocalization
namespace Localization
open IsLocalization
attribute [local instance] Classical.propDecidable
variable (I : Ideal R) [hI : I.IsPrime]
variable {I}
/-- The unique maximal ideal of the localization at `I.primeCompl` lies over the ideal `I`. -/
theorem AtPrime.comap_maximalIdeal :
Ideal.comap (algebraMap R (Localization.AtPrime I))
(LocalRing.maximalIdeal (Localization I.primeCompl)) =
I :=
-- Porting note: need to provide full name
IsLocalization.AtPrime.comap_maximalIdeal _ _
/-- The image of `I` in the localization at `I.primeCompl` is a maximal ideal, and in particular
it is the unique maximal ideal given by the local ring structure `AtPrime.localRing` -/
theorem AtPrime.map_eq_maximalIdeal :
Ideal.map (algebraMap R (Localization.AtPrime I)) I =
LocalRing.maximalIdeal (Localization I.primeCompl) := by
convert congr_arg (Ideal.map (algebraMap R (Localization.AtPrime I)))
-- Porting note: `algebraMap R ...` can not be solve by unification
(AtPrime.comap_maximalIdeal (hI := hI)).symm
-- Porting note: can not find `hI`
rw [map_comap I.primeCompl]
theorem le_comap_primeCompl_iff {J : Ideal P} [hJ : J.IsPrime] {f : R →+* P} :
I.primeCompl ≤ J.primeCompl.comap f ↔ J.comap f ≤ I :=
⟨fun h x hx => by
contrapose! hx
exact h hx,
fun h x hx hfxJ => hx (h hfxJ)⟩
variable (I)
/-- For a ring hom `f : R →+* S` and a prime ideal `J` in `S`, the induced ring hom from the
localization of `R` at `J.comap f` to the localization of `S` at `J`.
To make this definition more flexible, we allow any ideal `I` of `R` as input, together with a proof
that `I = J.comap f`. This can be useful when `I` is not definitionally equal to `J.comap f`.
-/
noncomputable def localRingHom (J : Ideal P) [J.IsPrime] (f : R →+* P) (hIJ : I = J.comap f) :
Localization.AtPrime I →+* Localization.AtPrime J :=
IsLocalization.map (Localization.AtPrime J) f (le_comap_primeCompl_iff.mpr (ge_of_eq hIJ))
theorem localRingHom_to_map (J : Ideal P) [J.IsPrime] (f : R →+* P) (hIJ : I = J.comap f)
(x : R) : localRingHom I J f hIJ (algebraMap _ _ x) = algebraMap _ _ (f x) :=
map_eq _ _
theorem localRingHom_mk' (J : Ideal P) [J.IsPrime] (f : R →+* P) (hIJ : I = J.comap f) (x : R)
(y : I.primeCompl) :
localRingHom I J f hIJ (IsLocalization.mk' _ x y) =
IsLocalization.mk' (Localization.AtPrime J) (f x)
(⟨f y, le_comap_primeCompl_iff.mpr (ge_of_eq hIJ) y.2⟩ : J.primeCompl) :=
map_mk' _ _ _
instance isLocalRingHom_localRingHom (J : Ideal P) [hJ : J.IsPrime] (f : R →+* P)
(hIJ : I = J.comap f) : IsLocalRingHom (localRingHom I J f hIJ) :=
IsLocalRingHom.mk fun x hx => by
rcases IsLocalization.mk'_surjective I.primeCompl x with ⟨r, s, rfl⟩
rw [localRingHom_mk'] at hx
rw [AtPrime.isUnit_mk'_iff] at hx ⊢
exact fun hr => hx ((SetLike.ext_iff.mp hIJ r).mp hr)
theorem localRingHom_unique (J : Ideal P) [J.IsPrime] (f : R →+* P) (hIJ : I = J.comap f)
{j : Localization.AtPrime I →+* Localization.AtPrime J}
(hj : ∀ x : R, j (algebraMap _ _ x) = algebraMap _ _ (f x)) : localRingHom I J f hIJ = j :=
map_unique _ _ hj
@[simp]
theorem localRingHom_id : localRingHom I I (RingHom.id R) (Ideal.comap_id I).symm = RingHom.id _ :=
localRingHom_unique _ _ _ _ fun _ => rfl
-- Porting note: simplifier won't pick up this lemma, so deleted @[simp]
theorem localRingHom_comp {S : Type*} [CommSemiring S] (J : Ideal S) [hJ : J.IsPrime] (K : Ideal P)
[hK : K.IsPrime] (f : R →+* S) (hIJ : I = J.comap f) (g : S →+* P) (hJK : J = K.comap g) :
localRingHom I K (g.comp f) (by rw [hIJ, hJK, Ideal.comap_comap f g]) =
(localRingHom J K g hJK).comp (localRingHom I J f hIJ) :=
localRingHom_unique _ _ _ _ fun r => by
simp only [Function.comp_apply, RingHom.coe_comp, localRingHom_to_map]
end Localization
|
RingTheory\Localization\BaseChange.lean | /-
Copyright (c) 2022 Jujian Zhang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Andrew Yang, Jujian Zhang
-/
import Mathlib.RingTheory.IsTensorProduct
import Mathlib.RingTheory.Localization.Module
/-!
# Localized Module
Given a commutative semiring `R`, a multiplicative subset `S ⊆ R` and an `R`-module `M`, we can
localize `M` by `S`. This gives us a `Localization S`-module.
## Main definition
* `isLocalizedModule_iff_isBaseChange` : A localization of modules corresponds to a base change.
-/
variable {R : Type*} [CommSemiring R] (S : Submonoid R)
(A : Type*) [CommRing A] [Algebra R A] [IsLocalization S A]
{M : Type*} [AddCommMonoid M] [Module R M] [Module A M] [IsScalarTower R A M]
{M' : Type*} [AddCommMonoid M'] [Module R M'] [Module A M'] [IsScalarTower R A M']
(f : M →ₗ[R] M')
/-- The forward direction of `isLocalizedModule_iff_isBaseChange`. It is also used to prove the
other direction. -/
theorem IsLocalizedModule.isBaseChange [IsLocalizedModule S f] : IsBaseChange A f :=
.of_lift_unique _ fun Q _ _ _ _ g ↦ by
obtain ⟨ℓ, rfl, h₂⟩ := IsLocalizedModule.is_universal S f g fun s ↦ by
rw [← (Algebra.lsmul R (A := A) R Q).commutes]; exact (IsLocalization.map_units A s).map _
refine ⟨ℓ.extendScalarsOfIsLocalization S A, by simp, fun g'' h ↦ ?_⟩
cases h₂ (LinearMap.restrictScalars R g'') h; rfl
/-- The map `(f : M →ₗ[R] M')` is a localization of modules iff the map
`(Localization S) × M → N, (s, m) ↦ s • f m` is the tensor product (insomuch as it is the universal
bilinear map).
In particular, there is an isomorphism between `LocalizedModule S M` and `(Localization S) ⊗[R] M`
given by `m/s ↦ (1/s) ⊗ₜ m`.
-/
theorem isLocalizedModule_iff_isBaseChange : IsLocalizedModule S f ↔ IsBaseChange A f := by
refine ⟨fun _ ↦ IsLocalizedModule.isBaseChange S A f, fun h ↦ ?_⟩
have : IsBaseChange A (LocalizedModule.mkLinearMap S M) := IsLocalizedModule.isBaseChange S A _
let e := (this.equiv.symm.trans h.equiv).restrictScalars R
convert IsLocalizedModule.of_linearEquiv S (LocalizedModule.mkLinearMap S M) e
ext
rw [LinearMap.coe_comp, LinearEquiv.coe_coe, Function.comp_apply,
LinearEquiv.restrictScalars_apply, LinearEquiv.trans_apply, IsBaseChange.equiv_symm_apply,
IsBaseChange.equiv_tmul, one_smul]
|
RingTheory\Localization\Basic.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.Algebra.Algebra.Tower
import Mathlib.Algebra.GroupWithZero.NonZeroDivisors
import Mathlib.GroupTheory.MonoidLocalization.MonoidWithZero
import Mathlib.RingTheory.OreLocalization.Ring
import Mathlib.RingTheory.Ideal.Basic
/-!
# Localizations of commutative rings
We characterize the localization of a commutative ring `R` at a submonoid `M` up to
isomorphism; that is, a commutative ring `S` is the localization of `R` at `M` iff we can find a
ring homomorphism `f : R →+* S` satisfying 3 properties:
1. For all `y ∈ M`, `f y` is a unit;
2. For all `z : S`, there exists `(x, y) : R × M` such that `z * f y = f x`;
3. For all `x, y : R` such that `f x = f y`, there exists `c ∈ M` such that `x * c = y * c`.
(The converse is a consequence of 1.)
In the following, let `R, P` be commutative rings, `S, Q` be `R`- and `P`-algebras
and `M, T` be submonoids of `R` and `P` respectively, e.g.:
```
variable (R S P Q : Type*) [CommRing R] [CommRing S] [CommRing P] [CommRing Q]
variable [Algebra R S] [Algebra P Q] (M : Submonoid R) (T : Submonoid P)
```
## Main definitions
* `IsLocalization (M : Submonoid R) (S : Type*)` is a typeclass expressing that `S` is a
localization of `R` at `M`, i.e. the canonical map `algebraMap R S : R →+* S` is a
localization map (satisfying the above properties).
* `IsLocalization.mk' S` is a surjection sending `(x, y) : R × M` to `f x * (f y)⁻¹`
* `IsLocalization.lift` is the ring homomorphism from `S` induced by a homomorphism from `R`
which maps elements of `M` to invertible elements of the codomain.
* `IsLocalization.map S Q` is the ring homomorphism from `S` to `Q` which maps elements
of `M` to elements of `T`
* `IsLocalization.ringEquivOfRingEquiv`: if `R` and `P` are isomorphic by an isomorphism
sending `M` to `T`, then `S` and `Q` are isomorphic
* `IsLocalization.algEquiv`: if `Q` is another localization of `R` at `M`, then `S` and `Q`
are isomorphic as `R`-algebras
## Main results
* `Localization M S`, a construction of the localization as a quotient type, defined in
`GroupTheory.MonoidLocalization`, has `CommRing`, `Algebra R` and `IsLocalization M`
instances if `R` is a ring. `Localization.Away`, `Localization.AtPrime` and `FractionRing`
are abbreviations for `Localization`s and have their corresponding `IsLocalization` instances
## Implementation notes
In maths it is natural to reason up to isomorphism, but in Lean we cannot naturally `rewrite` one
structure with an isomorphic one; one way around this is to isolate a predicate characterizing
a structure up to isomorphism, and reason about things that satisfy the predicate.
A previous version of this file used a fully bundled type of ring localization maps,
then used a type synonym `f.codomain` for `f : LocalizationMap M S` to instantiate the
`R`-algebra structure on `S`. This results in defining ad-hoc copies for everything already
defined on `S`. By making `IsLocalization` a predicate on the `algebraMap R S`,
we can ensure the localization map commutes nicely with other `algebraMap`s.
To prove most lemmas about a localization map `algebraMap R S` in this file we invoke the
corresponding proof for the underlying `CommMonoid` localization map
`IsLocalization.toLocalizationMap M S`, which can be found in `GroupTheory.MonoidLocalization`
and the namespace `Submonoid.LocalizationMap`.
To reason about the localization as a quotient type, use `mk_eq_of_mk'` and associated lemmas.
These show the quotient map `mk : R → M → Localization M` equals the surjection
`LocalizationMap.mk'` induced by the map `algebraMap : R →+* Localization M`.
The lemma `mk_eq_of_mk'` hence gives you access to the results in the rest of the file,
which are about the `LocalizationMap.mk'` induced by any localization map.
The proof that "a `CommRing` `K` which is the localization of an integral domain `R` at `R \ {0}`
is a field" is a `def` rather than an `instance`, so if you want to reason about a field of
fractions `K`, assume `[Field K]` instead of just `[CommRing K]`.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
open Function
section CommSemiring
variable {R : Type*} [CommSemiring R] (M : Submonoid R) (S : Type*) [CommSemiring S]
variable [Algebra R S] {P : Type*} [CommSemiring P]
/-- The typeclass `IsLocalization (M : Submonoid R) S` where `S` is an `R`-algebra
expresses that `S` is isomorphic to the localization of `R` at `M`. -/
@[mk_iff] class IsLocalization : Prop where
-- Porting note: add ' to fields, and made new versions of these with either `S` or `M` explicit.
/-- Everything in the image of `algebraMap` is a unit -/
map_units' : ∀ y : M, IsUnit (algebraMap R S y)
/-- The `algebraMap` is surjective -/
surj' : ∀ z : S, ∃ x : R × M, z * algebraMap R S x.2 = algebraMap R S x.1
/-- The kernel of `algebraMap` is contained in the annihilator of `M`;
it is then equal to the annihilator by `map_units'` -/
exists_of_eq : ∀ {x y}, algebraMap R S x = algebraMap R S y → ∃ c : M, ↑c * x = ↑c * y
variable {M}
namespace IsLocalization
section IsLocalization
variable [IsLocalization M S]
section
@[inherit_doc IsLocalization.map_units']
theorem map_units : ∀ y : M, IsUnit (algebraMap R S y) :=
IsLocalization.map_units'
variable (M) {S}
@[inherit_doc IsLocalization.surj']
theorem surj : ∀ z : S, ∃ x : R × M, z * algebraMap R S x.2 = algebraMap R S x.1 :=
IsLocalization.surj'
variable (S)
@[inherit_doc IsLocalization.exists_of_eq]
theorem eq_iff_exists {x y} : algebraMap R S x = algebraMap R S y ↔ ∃ c : M, ↑c * x = ↑c * y :=
Iff.intro IsLocalization.exists_of_eq fun ⟨c, h⟩ ↦ by
apply_fun algebraMap R S at h
rw [map_mul, map_mul] at h
exact (IsLocalization.map_units S c).mul_right_inj.mp h
variable {S}
theorem of_le (N : Submonoid R) (h₁ : M ≤ N) (h₂ : ∀ r ∈ N, IsUnit (algebraMap R S r)) :
IsLocalization N S where
map_units' r := h₂ r r.2
surj' s :=
have ⟨⟨x, y, hy⟩, H⟩ := IsLocalization.surj M s
⟨⟨x, y, h₁ hy⟩, H⟩
exists_of_eq {x y} := by
rw [IsLocalization.eq_iff_exists M]
rintro ⟨c, hc⟩
exact ⟨⟨c, h₁ c.2⟩, hc⟩
variable (S)
/-- `IsLocalization.toLocalizationWithZeroMap M S` shows `S` is the monoid localization of
`R` at `M`. -/
@[simps]
def toLocalizationWithZeroMap : Submonoid.LocalizationWithZeroMap M S where
__ := algebraMap R S
toFun := algebraMap R S
map_units' := IsLocalization.map_units _
surj' := IsLocalization.surj _
exists_of_eq _ _ := IsLocalization.exists_of_eq
/-- `IsLocalization.toLocalizationMap M S` shows `S` is the monoid localization of `R` at `M`. -/
abbrev toLocalizationMap : Submonoid.LocalizationMap M S :=
(toLocalizationWithZeroMap M S).toLocalizationMap
@[simp]
theorem toLocalizationMap_toMap : (toLocalizationMap M S).toMap = (algebraMap R S : R →*₀ S) :=
rfl
theorem toLocalizationMap_toMap_apply (x) : (toLocalizationMap M S).toMap x = algebraMap R S x :=
rfl
theorem surj₂ : ∀ z w : S, ∃ z' w' : R, ∃ d : M,
(z * algebraMap R S d = algebraMap R S z') ∧ (w * algebraMap R S d = algebraMap R S w') :=
(toLocalizationMap M S).surj₂
end
variable (M) {S}
/-- Given a localization map `f : M →* N`, a section function sending `z : N` to some
`(x, y) : M × S` such that `f x * (f y)⁻¹ = z`. -/
noncomputable def sec (z : S) : R × M :=
Classical.choose <| IsLocalization.surj _ z
@[simp]
theorem toLocalizationMap_sec : (toLocalizationMap M S).sec = sec M :=
rfl
/-- Given `z : S`, `IsLocalization.sec M z` is defined to be a pair `(x, y) : R × M` such
that `z * f y = f x` (so this lemma is true by definition). -/
theorem sec_spec (z : S) :
z * algebraMap R S (IsLocalization.sec M z).2 = algebraMap R S (IsLocalization.sec M z).1 :=
Classical.choose_spec <| IsLocalization.surj _ z
/-- Given `z : S`, `IsLocalization.sec M z` is defined to be a pair `(x, y) : R × M` such
that `z * f y = f x`, so this lemma is just an application of `S`'s commutativity. -/
theorem sec_spec' (z : S) :
algebraMap R S (IsLocalization.sec M z).1 = algebraMap R S (IsLocalization.sec M z).2 * z := by
rw [mul_comm, sec_spec]
variable {M}
/-- If `M` contains `0` then the localization at `M` is trivial. -/
theorem subsingleton (h : 0 ∈ M) : Subsingleton S := (toLocalizationMap M S).subsingleton h
theorem map_right_cancel {x y} {c : M} (h : algebraMap R S (c * x) = algebraMap R S (c * y)) :
algebraMap R S x = algebraMap R S y :=
(toLocalizationMap M S).map_right_cancel h
theorem map_left_cancel {x y} {c : M} (h : algebraMap R S (x * c) = algebraMap R S (y * c)) :
algebraMap R S x = algebraMap R S y :=
(toLocalizationMap M S).map_left_cancel h
theorem eq_zero_of_fst_eq_zero {z x} {y : M} (h : z * algebraMap R S y = algebraMap R S x)
(hx : x = 0) : z = 0 := by
rw [hx, (algebraMap R S).map_zero] at h
exact (IsUnit.mul_left_eq_zero (IsLocalization.map_units S y)).1 h
variable (M S)
theorem map_eq_zero_iff (r : R) : algebraMap R S r = 0 ↔ ∃ m : M, ↑m * r = 0 := by
constructor
· intro h
obtain ⟨m, hm⟩ := (IsLocalization.eq_iff_exists M S).mp ((algebraMap R S).map_zero.trans h.symm)
exact ⟨m, by simpa using hm.symm⟩
· rintro ⟨m, hm⟩
rw [← (IsLocalization.map_units S m).mul_right_inj, mul_zero, ← RingHom.map_mul, hm,
RingHom.map_zero]
variable {M}
/-- `IsLocalization.mk' S` is the surjection sending `(x, y) : R × M` to
`f x * (f y)⁻¹`. -/
noncomputable def mk' (x : R) (y : M) : S :=
(toLocalizationMap M S).mk' x y
@[simp]
theorem mk'_sec (z : S) : mk' S (IsLocalization.sec M z).1 (IsLocalization.sec M z).2 = z :=
(toLocalizationMap M S).mk'_sec _
theorem mk'_mul (x₁ x₂ : R) (y₁ y₂ : M) : mk' S (x₁ * x₂) (y₁ * y₂) = mk' S x₁ y₁ * mk' S x₂ y₂ :=
(toLocalizationMap M S).mk'_mul _ _ _ _
theorem mk'_one (x) : mk' S x (1 : M) = algebraMap R S x :=
(toLocalizationMap M S).mk'_one _
@[simp]
theorem mk'_spec (x) (y : M) : mk' S x y * algebraMap R S y = algebraMap R S x :=
(toLocalizationMap M S).mk'_spec _ _
@[simp]
theorem mk'_spec' (x) (y : M) : algebraMap R S y * mk' S x y = algebraMap R S x :=
(toLocalizationMap M S).mk'_spec' _ _
@[simp]
theorem mk'_spec_mk (x) (y : R) (hy : y ∈ M) :
mk' S x ⟨y, hy⟩ * algebraMap R S y = algebraMap R S x :=
mk'_spec S x ⟨y, hy⟩
@[simp]
theorem mk'_spec'_mk (x) (y : R) (hy : y ∈ M) :
algebraMap R S y * mk' S x ⟨y, hy⟩ = algebraMap R S x :=
mk'_spec' S x ⟨y, hy⟩
variable {S}
theorem eq_mk'_iff_mul_eq {x} {y : M} {z} :
z = mk' S x y ↔ z * algebraMap R S y = algebraMap R S x :=
(toLocalizationMap M S).eq_mk'_iff_mul_eq
theorem mk'_eq_iff_eq_mul {x} {y : M} {z} :
mk' S x y = z ↔ algebraMap R S x = z * algebraMap R S y :=
(toLocalizationMap M S).mk'_eq_iff_eq_mul
theorem mk'_add_eq_iff_add_mul_eq_mul {x} {y : M} {z₁ z₂} :
mk' S x y + z₁ = z₂ ↔ algebraMap R S x + z₁ * algebraMap R S y = z₂ * algebraMap R S y := by
rw [← mk'_spec S x y, ← IsUnit.mul_left_inj (IsLocalization.map_units S y), right_distrib]
theorem mk'_pow (x : R) (y : M) (n : ℕ) : mk' S (x ^ n) (y ^ n) = mk' S x y ^ n := by
simp_rw [IsLocalization.mk'_eq_iff_eq_mul, SubmonoidClass.coe_pow, map_pow, ← mul_pow]
simp
variable (M)
theorem mk'_surjective (z : S) : ∃ (x : _) (y : M), mk' S x y = z :=
let ⟨r, hr⟩ := IsLocalization.surj _ z
⟨r.1, r.2, (eq_mk'_iff_mul_eq.2 hr).symm⟩
variable (S)
/-- The localization of a `Fintype` is a `Fintype`. Cannot be an instance. -/
noncomputable def fintype' [Fintype R] : Fintype S :=
have := Classical.propDecidable
Fintype.ofSurjective (Function.uncurry <| IsLocalization.mk' S) fun a =>
Prod.exists'.mpr <| IsLocalization.mk'_surjective M a
variable {M S}
/-- Localizing at a submonoid with 0 inside it leads to the trivial ring. -/
def uniqueOfZeroMem (h : (0 : R) ∈ M) : Unique S :=
uniqueOfZeroEqOne <| by simpa using IsLocalization.map_units S ⟨0, h⟩
theorem mk'_eq_iff_eq {x₁ x₂} {y₁ y₂ : M} :
mk' S x₁ y₁ = mk' S x₂ y₂ ↔ algebraMap R S (y₂ * x₁) = algebraMap R S (y₁ * x₂) :=
(toLocalizationMap M S).mk'_eq_iff_eq
theorem mk'_eq_iff_eq' {x₁ x₂} {y₁ y₂ : M} :
mk' S x₁ y₁ = mk' S x₂ y₂ ↔ algebraMap R S (x₁ * y₂) = algebraMap R S (x₂ * y₁) :=
(toLocalizationMap M S).mk'_eq_iff_eq'
theorem mk'_mem_iff {x} {y : M} {I : Ideal S} : mk' S x y ∈ I ↔ algebraMap R S x ∈ I := by
constructor <;> intro h
· rw [← mk'_spec S x y, mul_comm]
exact I.mul_mem_left ((algebraMap R S) y) h
· rw [← mk'_spec S x y] at h
obtain ⟨b, hb⟩ := isUnit_iff_exists_inv.1 (map_units S y)
have := I.mul_mem_left b h
rwa [mul_comm, mul_assoc, hb, mul_one] at this
protected theorem eq {a₁ b₁} {a₂ b₂ : M} :
mk' S a₁ a₂ = mk' S b₁ b₂ ↔ ∃ c : M, ↑c * (↑b₂ * a₁) = c * (a₂ * b₁) :=
(toLocalizationMap M S).eq
theorem mk'_eq_zero_iff (x : R) (s : M) : mk' S x s = 0 ↔ ∃ m : M, ↑m * x = 0 := by
rw [← (map_units S s).mul_left_inj, mk'_spec, zero_mul, map_eq_zero_iff M]
@[simp]
theorem mk'_zero (s : M) : IsLocalization.mk' S 0 s = 0 := by
rw [eq_comm, IsLocalization.eq_mk'_iff_mul_eq, zero_mul, map_zero]
theorem ne_zero_of_mk'_ne_zero {x : R} {y : M} (hxy : IsLocalization.mk' S x y ≠ 0) : x ≠ 0 := by
rintro rfl
exact hxy (IsLocalization.mk'_zero _)
section Ext
variable [Algebra R P] [IsLocalization M P]
theorem eq_iff_eq {x y} :
algebraMap R S x = algebraMap R S y ↔ algebraMap R P x = algebraMap R P y :=
(toLocalizationMap M S).eq_iff_eq (toLocalizationMap M P)
theorem mk'_eq_iff_mk'_eq {x₁ x₂} {y₁ y₂ : M} :
mk' S x₁ y₁ = mk' S x₂ y₂ ↔ mk' P x₁ y₁ = mk' P x₂ y₂ :=
(toLocalizationMap M S).mk'_eq_iff_mk'_eq (toLocalizationMap M P)
theorem mk'_eq_of_eq {a₁ b₁ : R} {a₂ b₂ : M} (H : ↑a₂ * b₁ = ↑b₂ * a₁) :
mk' S a₁ a₂ = mk' S b₁ b₂ :=
(toLocalizationMap M S).mk'_eq_of_eq H
theorem mk'_eq_of_eq' {a₁ b₁ : R} {a₂ b₂ : M} (H : b₁ * ↑a₂ = a₁ * ↑b₂) :
mk' S a₁ a₂ = mk' S b₁ b₂ :=
(toLocalizationMap M S).mk'_eq_of_eq' H
theorem mk'_cancel (a : R) (b c : M) :
mk' S (a * c) (b * c) = mk' S a b := (toLocalizationMap M S).mk'_cancel _ _ _
variable (S)
@[simp]
theorem mk'_self {x : R} (hx : x ∈ M) : mk' S x ⟨x, hx⟩ = 1 :=
(toLocalizationMap M S).mk'_self _ hx
@[simp]
theorem mk'_self' {x : M} : mk' S (x : R) x = 1 :=
(toLocalizationMap M S).mk'_self' _
theorem mk'_self'' {x : M} : mk' S x.1 x = 1 :=
mk'_self' _
end Ext
theorem mul_mk'_eq_mk'_of_mul (x y : R) (z : M) :
(algebraMap R S) x * mk' S y z = mk' S (x * y) z :=
(toLocalizationMap M S).mul_mk'_eq_mk'_of_mul _ _ _
theorem mk'_eq_mul_mk'_one (x : R) (y : M) : mk' S x y = (algebraMap R S) x * mk' S 1 y :=
((toLocalizationMap M S).mul_mk'_one_eq_mk' _ _).symm
@[simp]
theorem mk'_mul_cancel_left (x : R) (y : M) : mk' S (y * x : R) y = (algebraMap R S) x :=
(toLocalizationMap M S).mk'_mul_cancel_left _ _
theorem mk'_mul_cancel_right (x : R) (y : M) : mk' S (x * y) y = (algebraMap R S) x :=
(toLocalizationMap M S).mk'_mul_cancel_right _ _
@[simp]
theorem mk'_mul_mk'_eq_one (x y : M) : mk' S (x : R) y * mk' S (y : R) x = 1 := by
rw [← mk'_mul, mul_comm]; exact mk'_self _ _
theorem mk'_mul_mk'_eq_one' (x : R) (y : M) (h : x ∈ M) : mk' S x y * mk' S (y : R) ⟨x, h⟩ = 1 :=
mk'_mul_mk'_eq_one ⟨x, h⟩ _
theorem smul_mk' (x y : R) (m : M) : x • mk' S y m = mk' S (x * y) m := by
nth_rw 2 [← one_mul m]
rw [mk'_mul, mk'_one, Algebra.smul_def]
@[simp] theorem smul_mk'_one (x : R) (m : M) : x • mk' S 1 m = mk' S x m := by
rw [smul_mk', mul_one]
@[simp] lemma smul_mk'_self {m : M} {r : R} :
(m : R) • mk' S r m = algebraMap R S r := by
rw [smul_mk', mk'_mul_cancel_left]
@[simps]
instance invertible_mk'_one (s : M) : Invertible (IsLocalization.mk' S (1 : R) s) where
invOf := algebraMap R S s
invOf_mul_self := by simp
mul_invOf_self := by simp
section
variable (M)
theorem isUnit_comp (j : S →+* P) (y : M) : IsUnit (j.comp (algebraMap R S) y) :=
(toLocalizationMap M S).isUnit_comp j.toMonoidHom _
end
/-- Given a localization map `f : R →+* S` for a submonoid `M ⊆ R` and a map of `CommSemiring`s
`g : R →+* P` such that `g(M) ⊆ Units P`, `f x = f y → g x = g y` for all `x y : R`. -/
theorem eq_of_eq {g : R →+* P} (hg : ∀ y : M, IsUnit (g y)) {x y}
(h : (algebraMap R S) x = (algebraMap R S) y) : g x = g y :=
Submonoid.LocalizationMap.eq_of_eq (toLocalizationMap M S) (g := g.toMonoidHom) hg h
theorem mk'_add (x₁ x₂ : R) (y₁ y₂ : M) :
mk' S (x₁ * y₂ + x₂ * y₁) (y₁ * y₂) = mk' S x₁ y₁ + mk' S x₂ y₂ :=
mk'_eq_iff_eq_mul.2 <|
Eq.symm
(by
rw [mul_comm (_ + _), mul_add, mul_mk'_eq_mk'_of_mul, mk'_add_eq_iff_add_mul_eq_mul,
mul_comm (_ * _), ← mul_assoc, add_comm, ← map_mul, mul_mk'_eq_mk'_of_mul,
mk'_add_eq_iff_add_mul_eq_mul]
simp only [map_add, Submonoid.coe_mul, map_mul]
ring)
theorem mul_add_inv_left {g : R →+* P} (h : ∀ y : M, IsUnit (g y)) (y : M) (w z₁ z₂ : P) :
w * ↑(IsUnit.liftRight (g.toMonoidHom.restrict M) h y)⁻¹ + z₁ =
z₂ ↔ w + g y * z₁ = g y * z₂ := by
rw [mul_comm, ← one_mul z₁, ← Units.inv_mul (IsUnit.liftRight (g.toMonoidHom.restrict M) h y),
mul_assoc, ← mul_add, Units.inv_mul_eq_iff_eq_mul, Units.inv_mul_cancel_left,
IsUnit.coe_liftRight]
simp [RingHom.toMonoidHom_eq_coe, MonoidHom.restrict_apply]
theorem lift_spec_mul_add {g : R →+* P} (hg : ∀ y : M, IsUnit (g y)) (z w w' v) :
((toLocalizationWithZeroMap M S).lift g.toMonoidWithZeroHom hg) z * w + w' = v ↔
g ((toLocalizationMap M S).sec z).1 * w + g ((toLocalizationMap M S).sec z).2 * w' =
g ((toLocalizationMap M S).sec z).2 * v := by
erw [mul_comm, ← mul_assoc, mul_add_inv_left hg, mul_comm]
rfl
/-- Given a localization map `f : R →+* S` for a submonoid `M ⊆ R` and a map of `CommSemiring`s
`g : R →+* P` such that `g y` is invertible for all `y : M`, the homomorphism induced from
`S` to `P` sending `z : S` to `g x * (g y)⁻¹`, where `(x, y) : R × M` are such that
`z = f x * (f y)⁻¹`. -/
noncomputable def lift {g : R →+* P} (hg : ∀ y : M, IsUnit (g y)) : S →+* P :=
{ Submonoid.LocalizationWithZeroMap.lift (toLocalizationWithZeroMap M S)
g.toMonoidWithZeroHom hg with
map_add' := by
intro x y
erw [(toLocalizationMap M S).lift_spec, mul_add, mul_comm, eq_comm, lift_spec_mul_add,
add_comm, mul_comm, mul_assoc, mul_comm, mul_assoc, lift_spec_mul_add]
simp_rw [← mul_assoc]
show g _ * g _ * g _ + g _ * g _ * g _ = g _ * g _ * g _
simp_rw [← map_mul g, ← map_add g]
apply eq_of_eq (S := S) hg
simp only [sec_spec', toLocalizationMap_sec, map_add, map_mul]
ring }
variable {g : R →+* P} (hg : ∀ y : M, IsUnit (g y))
/-- Given a localization map `f : R →+* S` for a submonoid `M ⊆ R` and a map of `CommSemiring`s
`g : R →* P` such that `g y` is invertible for all `y : M`, the homomorphism induced from
`S` to `P` maps `f x * (f y)⁻¹` to `g x * (g y)⁻¹` for all `x : R, y ∈ M`. -/
theorem lift_mk' (x y) :
lift hg (mk' S x y) = g x * ↑(IsUnit.liftRight (g.toMonoidHom.restrict M) hg y)⁻¹ :=
(toLocalizationMap M S).lift_mk' _ _ _
theorem lift_mk'_spec (x v) (y : M) : lift hg (mk' S x y) = v ↔ g x = g y * v :=
(toLocalizationMap M S).lift_mk'_spec _ _ _ _
@[simp]
theorem lift_eq (x : R) : lift hg ((algebraMap R S) x) = g x :=
(toLocalizationMap M S).lift_eq _ _
theorem lift_eq_iff {x y : R × M} :
lift hg (mk' S x.1 x.2) = lift hg (mk' S y.1 y.2) ↔ g (x.1 * y.2) = g (y.1 * x.2) :=
(toLocalizationMap M S).lift_eq_iff _
@[simp]
theorem lift_comp : (lift hg).comp (algebraMap R S) = g :=
RingHom.ext <| (DFunLike.ext_iff (F := MonoidHom _ _)).1 <| (toLocalizationMap M S).lift_comp _
@[simp]
theorem lift_of_comp (j : S →+* P) : lift (isUnit_comp M j) = j :=
RingHom.ext <| (DFunLike.ext_iff (F := MonoidHom _ _)).1 <|
(toLocalizationMap M S).lift_of_comp j.toMonoidHom
variable (M)
/-- See note [partially-applied ext lemmas] -/
theorem monoidHom_ext ⦃j k : S →* P⦄
(h : j.comp (algebraMap R S : R →* S) = k.comp (algebraMap R S)) : j = k :=
Submonoid.LocalizationMap.epic_of_localizationMap (toLocalizationMap M S) <| DFunLike.congr_fun h
/-- See note [partially-applied ext lemmas] -/
theorem ringHom_ext ⦃j k : S →+* P⦄ (h : j.comp (algebraMap R S) = k.comp (algebraMap R S)) :
j = k :=
RingHom.coe_monoidHom_injective <| monoidHom_ext M <| MonoidHom.ext <| RingHom.congr_fun h
/- This is not an instance because the submonoid `M` would become a metavariable
in typeclass search. -/
theorem algHom_subsingleton [Algebra R P] : Subsingleton (S →ₐ[R] P) :=
⟨fun f g =>
AlgHom.coe_ringHom_injective <|
IsLocalization.ringHom_ext M <| by rw [f.comp_algebraMap, g.comp_algebraMap]⟩
/-- To show `j` and `k` agree on the whole localization, it suffices to show they agree
on the image of the base ring, if they preserve `1` and `*`. -/
protected theorem ext (j k : S → P) (hj1 : j 1 = 1) (hk1 : k 1 = 1)
(hjm : ∀ a b, j (a * b) = j a * j b) (hkm : ∀ a b, k (a * b) = k a * k b)
(h : ∀ a, j (algebraMap R S a) = k (algebraMap R S a)) : j = k :=
let j' : MonoidHom S P :=
{ toFun := j, map_one' := hj1, map_mul' := hjm }
let k' : MonoidHom S P :=
{ toFun := k, map_one' := hk1, map_mul' := hkm }
have : j' = k' := monoidHom_ext M (MonoidHom.ext h)
show j'.toFun = k'.toFun by rw [this]
variable {M}
theorem lift_unique {j : S →+* P} (hj : ∀ x, j ((algebraMap R S) x) = g x) : lift hg = j :=
RingHom.ext <|
(DFunLike.ext_iff (F := MonoidHom _ _)).1 <|
Submonoid.LocalizationMap.lift_unique (toLocalizationMap M S) (g := g.toMonoidHom) hg
(j := j.toMonoidHom) hj
@[simp]
theorem lift_id (x) : lift (map_units S : ∀ _ : M, IsUnit _) x = x :=
(toLocalizationMap M S).lift_id _
theorem lift_surjective_iff :
Surjective (lift hg : S → P) ↔ ∀ v : P, ∃ x : R × M, v * g x.2 = g x.1 :=
(toLocalizationMap M S).lift_surjective_iff hg
theorem lift_injective_iff :
Injective (lift hg : S → P) ↔ ∀ x y, algebraMap R S x = algebraMap R S y ↔ g x = g y :=
(toLocalizationMap M S).lift_injective_iff hg
section Map
variable {T : Submonoid P} {Q : Type*} [CommSemiring Q] (hy : M ≤ T.comap g)
variable [Algebra P Q] [IsLocalization T Q]
section
variable (Q)
/-- Map a homomorphism `g : R →+* P` to `S →+* Q`, where `S` and `Q` are
localizations of `R` and `P` at `M` and `T` respectively,
such that `g(M) ⊆ T`.
We send `z : S` to `algebraMap P Q (g x) * (algebraMap P Q (g y))⁻¹`, where
`(x, y) : R × M` are such that `z = f x * (f y)⁻¹`. -/
noncomputable def map (g : R →+* P) (hy : M ≤ T.comap g) : S →+* Q :=
lift (M := M) (g := (algebraMap P Q).comp g) fun y => map_units _ ⟨g y, hy y.2⟩
end
-- Porting note: added `simp` attribute, since it proves very similar lemmas marked `simp`
@[simp]
theorem map_eq (x) : map Q g hy ((algebraMap R S) x) = algebraMap P Q (g x) :=
lift_eq (fun y => map_units _ ⟨g y, hy y.2⟩) x
@[simp]
theorem map_comp : (map Q g hy).comp (algebraMap R S) = (algebraMap P Q).comp g :=
lift_comp fun y => map_units _ ⟨g y, hy y.2⟩
theorem map_mk' (x) (y : M) : map Q g hy (mk' S x y) = mk' Q (g x) ⟨g y, hy y.2⟩ :=
Submonoid.LocalizationMap.map_mk' (toLocalizationMap M S) (g := g.toMonoidHom)
(fun y => hy y.2) (k := toLocalizationMap T Q) ..
@[simp]
theorem map_id_mk' {Q : Type*} [CommSemiring Q] [Algebra R Q] [IsLocalization M Q] (x) (y : M) :
map Q (RingHom.id R) (le_refl M) (mk' S x y) = mk' Q x y :=
map_mk' ..
@[simp]
theorem map_id (z : S) (h : M ≤ M.comap (RingHom.id R) := le_refl M) :
map S (RingHom.id _) h z = z :=
lift_id _
theorem map_unique (j : S →+* Q) (hj : ∀ x : R, j (algebraMap R S x) = algebraMap P Q (g x)) :
map Q g hy = j :=
lift_unique (fun y => map_units _ ⟨g y, hy y.2⟩) hj
/-- If `CommSemiring` homs `g : R →+* P, l : P →+* A` induce maps of localizations, the composition
of the induced maps equals the map of localizations induced by `l ∘ g`. -/
theorem map_comp_map {A : Type*} [CommSemiring A] {U : Submonoid A} {W} [CommSemiring W]
[Algebra A W] [IsLocalization U W] {l : P →+* A} (hl : T ≤ U.comap l) :
(map W l hl).comp (map Q g hy : S →+* _) = map W (l.comp g) fun _ hx => hl (hy hx) :=
RingHom.ext fun x =>
Submonoid.LocalizationMap.map_map (P := P) (toLocalizationMap M S) (fun y => hy y.2)
(toLocalizationMap U W) (fun w => hl w.2) x
/-- If `CommSemiring` homs `g : R →+* P, l : P →+* A` induce maps of localizations, the composition
of the induced maps equals the map of localizations induced by `l ∘ g`. -/
theorem map_map {A : Type*} [CommSemiring A] {U : Submonoid A} {W} [CommSemiring W] [Algebra A W]
[IsLocalization U W] {l : P →+* A} (hl : T ≤ U.comap l) (x : S) :
map W l hl (map Q g hy x) = map W (l.comp g) (fun x hx => hl (hy hx)) x := by
rw [← map_comp_map (Q := Q) hy hl]; rfl
theorem map_smul (x : S) (z : R) : map Q g hy (z • x : S) = g z • map Q g hy x := by
rw [Algebra.smul_def, Algebra.smul_def, RingHom.map_mul, map_eq]
section
variable (S Q)
/-- If `S`, `Q` are localizations of `R` and `P` at submonoids `M, T` respectively, an
isomorphism `j : R ≃+* P` such that `j(M) = T` induces an isomorphism of localizations
`S ≃+* Q`. -/
@[simps]
noncomputable def ringEquivOfRingEquiv (h : R ≃+* P) (H : M.map h.toMonoidHom = T) : S ≃+* Q :=
have H' : T.map h.symm.toMonoidHom = M := by
rw [← M.map_id, ← H, Submonoid.map_map]
congr
ext
apply h.symm_apply_apply
{ map Q (h : R →+* P) (M.le_comap_of_map_le (le_of_eq H)) with
toFun := map Q (h : R →+* P) (M.le_comap_of_map_le (le_of_eq H))
invFun := map S (h.symm : P →+* R) (T.le_comap_of_map_le (le_of_eq H'))
left_inv := fun x => by
rw [map_map, map_unique _ (RingHom.id _), RingHom.id_apply]
simp
right_inv := fun x => by
rw [map_map, map_unique _ (RingHom.id _), RingHom.id_apply]
simp }
end
theorem ringEquivOfRingEquiv_eq_map {j : R ≃+* P} (H : M.map j.toMonoidHom = T) :
(ringEquivOfRingEquiv S Q j H : S →+* Q) =
map Q (j : R →+* P) (M.le_comap_of_map_le (le_of_eq H)) :=
rfl
-- Porting note (#10618): removed `simp`, `simp` can prove it
theorem ringEquivOfRingEquiv_eq {j : R ≃+* P} (H : M.map j.toMonoidHom = T) (x) :
ringEquivOfRingEquiv S Q j H ((algebraMap R S) x) = algebraMap P Q (j x) := by
simp
theorem ringEquivOfRingEquiv_mk' {j : R ≃+* P} (H : M.map j.toMonoidHom = T) (x : R) (y : M) :
ringEquivOfRingEquiv S Q j H (mk' S x y) =
mk' Q (j x) ⟨j y, show j y ∈ T from H ▸ Set.mem_image_of_mem j y.2⟩ := by
simp [map_mk']
end Map
section AlgEquiv
variable {Q : Type*} [CommSemiring Q] [Algebra R Q] [IsLocalization M Q]
section
variable (M S Q)
/-- If `S`, `Q` are localizations of `R` at the submonoid `M` respectively,
there is an isomorphism of localizations `S ≃ₐ[R] Q`. -/
@[simps!]
noncomputable def algEquiv : S ≃ₐ[R] Q :=
{ ringEquivOfRingEquiv S Q (RingEquiv.refl R) M.map_id with
commutes' := ringEquivOfRingEquiv_eq _ }
end
-- Porting note (#10618): removed `simp`, `simp` can prove it
theorem algEquiv_mk' (x : R) (y : M) : algEquiv M S Q (mk' S x y) = mk' Q x y := by
simp
-- Porting note (#10618): removed `simp`, `simp` can prove it
theorem algEquiv_symm_mk' (x : R) (y : M) : (algEquiv M S Q).symm (mk' Q x y) = mk' S x y := by simp
variable (M) in
protected lemma bijective (f : S →+* Q) (hf : f.comp (algebraMap R S) = algebraMap R Q) :
Function.Bijective f :=
(show f = IsLocalization.algEquiv M S Q by
apply IsLocalization.ringHom_ext M; rw [hf]; ext; simp) ▸
(IsLocalization.algEquiv M S Q).toEquiv.bijective
end AlgEquiv
section at_units
lemma at_units {R : Type*} [CommSemiring R] (S : Submonoid R)
(hS : S ≤ IsUnit.submonoid R) : IsLocalization S R where
map_units' y := hS y.prop
surj' := fun s ↦ ⟨⟨s, 1⟩, by simp⟩
exists_of_eq := fun {x y} (e : x = y) ↦ ⟨1, e ▸ rfl⟩
variable (R M)
/-- The localization at a module of units is isomorphic to the ring. -/
noncomputable def atUnits (H : M ≤ IsUnit.submonoid R) : R ≃ₐ[R] S := by
refine AlgEquiv.ofBijective (Algebra.ofId R S) ⟨?_, ?_⟩
· intro x y hxy
obtain ⟨c, eq⟩ := (IsLocalization.eq_iff_exists M S).mp hxy
obtain ⟨u, hu⟩ := H c.prop
rwa [← hu, Units.mul_right_inj] at eq
· intro y
obtain ⟨⟨x, s⟩, eq⟩ := IsLocalization.surj M y
obtain ⟨u, hu⟩ := H s.prop
use x * u.inv
dsimp [Algebra.ofId, RingHom.toFun_eq_coe, AlgHom.coe_mks]
rw [RingHom.map_mul, ← eq, ← hu, mul_assoc, ← RingHom.map_mul]
simp
end at_units
section
variable (M S) (Q : Type*) [CommSemiring Q] [Algebra P Q]
/-- Injectivity of a map descends to the map induced on localizations. -/
theorem map_injective_of_injective (h : Function.Injective g) [IsLocalization (M.map g) Q] :
Function.Injective (map Q g M.le_comap_map : S → Q) :=
(toLocalizationMap M S).map_injective_of_injective h (toLocalizationMap (M.map g) Q)
/-- Surjectivity of a map descends to the map induced on localizations. -/
theorem map_surjective_of_surjective (h : Function.Surjective g) [IsLocalization (M.map g) Q] :
Function.Surjective (map Q g M.le_comap_map : S → Q) :=
(toLocalizationMap M S).map_surjective_of_surjective h (toLocalizationMap (M.map g) Q)
end
end IsLocalization
section
variable (M) {S}
theorem isLocalization_of_algEquiv [Algebra R P] [IsLocalization M S] (h : S ≃ₐ[R] P) :
IsLocalization M P := by
constructor
· intro y
convert (IsLocalization.map_units S y).map h.toAlgHom.toRingHom.toMonoidHom
exact (h.commutes y).symm
· intro y
obtain ⟨⟨x, s⟩, e⟩ := IsLocalization.surj M (h.symm y)
apply_fun (show S → P from h) at e
simp only [map_mul, h.apply_symm_apply, h.commutes] at e
exact ⟨⟨x, s⟩, e⟩
· intro x y
rw [← h.symm.toEquiv.injective.eq_iff, ← IsLocalization.eq_iff_exists M S, ← h.symm.commutes, ←
h.symm.commutes]
exact id
theorem isLocalization_iff_of_algEquiv [Algebra R P] (h : S ≃ₐ[R] P) :
IsLocalization M S ↔ IsLocalization M P :=
⟨fun _ => isLocalization_of_algEquiv M h, fun _ => isLocalization_of_algEquiv M h.symm⟩
theorem isLocalization_iff_of_ringEquiv (h : S ≃+* P) :
IsLocalization M S ↔
haveI := (h.toRingHom.comp <| algebraMap R S).toAlgebra; IsLocalization M P :=
letI := (h.toRingHom.comp <| algebraMap R S).toAlgebra
isLocalization_iff_of_algEquiv M { h with commutes' := fun _ => rfl }
variable (S)
theorem isLocalization_of_base_ringEquiv [IsLocalization M S] (h : R ≃+* P) :
haveI := ((algebraMap R S).comp h.symm.toRingHom).toAlgebra
IsLocalization (M.map h.toMonoidHom) S := by
letI : Algebra P S := ((algebraMap R S).comp h.symm.toRingHom).toAlgebra
constructor
· rintro ⟨_, ⟨y, hy, rfl⟩⟩
convert IsLocalization.map_units S ⟨y, hy⟩
dsimp only [RingHom.algebraMap_toAlgebra, RingHom.comp_apply]
exact congr_arg _ (h.symm_apply_apply _)
· intro y
obtain ⟨⟨x, s⟩, e⟩ := IsLocalization.surj M y
refine ⟨⟨h x, _, _, s.prop, rfl⟩, ?_⟩
dsimp only [RingHom.algebraMap_toAlgebra, RingHom.comp_apply] at e ⊢
convert e <;> exact h.symm_apply_apply _
· intro x y
rw [RingHom.algebraMap_toAlgebra, RingHom.comp_apply, RingHom.comp_apply,
IsLocalization.eq_iff_exists M S]
simp_rw [← h.toEquiv.apply_eq_iff_eq]
change (∃ c : M, h (c * h.symm x) = h (c * h.symm y)) → _
simp only [RingEquiv.apply_symm_apply, RingEquiv.map_mul]
exact fun ⟨c, e⟩ ↦ ⟨⟨_, _, c.prop, rfl⟩, e⟩
theorem isLocalization_iff_of_base_ringEquiv (h : R ≃+* P) :
IsLocalization M S ↔
haveI := ((algebraMap R S).comp h.symm.toRingHom).toAlgebra
IsLocalization (M.map h.toMonoidHom) S := by
letI : Algebra P S := ((algebraMap R S).comp h.symm.toRingHom).toAlgebra
refine ⟨fun _ => isLocalization_of_base_ringEquiv M S h, ?_⟩
intro H
convert isLocalization_of_base_ringEquiv (Submonoid.map (RingEquiv.toMonoidHom h) M) S h.symm
· erw [Submonoid.map_equiv_eq_comap_symm, Submonoid.comap_map_eq_of_injective]
exact h.toEquiv.injective
rw [RingHom.algebraMap_toAlgebra, RingHom.comp_assoc]
simp only [RingHom.comp_id, RingEquiv.symm_symm, RingEquiv.symm_toRingHom_comp_toRingHom]
apply Algebra.algebra_ext
intro r
rw [RingHom.algebraMap_toAlgebra]
end
variable (M)
theorem nonZeroDivisors_le_comap [IsLocalization M S] :
nonZeroDivisors R ≤ (nonZeroDivisors S).comap (algebraMap R S) := by
rintro a ha b (e : b * algebraMap R S a = 0)
obtain ⟨x, s, rfl⟩ := mk'_surjective M b
rw [← @mk'_one R _ M, ← mk'_mul, ← (algebraMap R S).map_zero, ← @mk'_one R _ M,
IsLocalization.eq] at e
obtain ⟨c, e⟩ := e
rw [mul_zero, mul_zero, Submonoid.coe_one, one_mul, ← mul_assoc] at e
rw [mk'_eq_zero_iff]
exact ⟨c, ha _ e⟩
theorem map_nonZeroDivisors_le [IsLocalization M S] :
(nonZeroDivisors R).map (algebraMap R S) ≤ nonZeroDivisors S :=
Submonoid.map_le_iff_le_comap.mpr (nonZeroDivisors_le_comap M S)
end IsLocalization
namespace Localization
open IsLocalization
/-! ### Constructing a localization at a given submonoid -/
section
instance instUniqueLocalization [Subsingleton R] : Unique (Localization M) where
uniq a := by
with_unfolding_all show a = mk 1 1
exact Localization.induction_on a fun _ => by
congr <;> apply Subsingleton.elim
theorem add_mk (a b c d) : (mk a b : Localization M) + mk c d =
mk ((b : R) * c + (d : R) * a) (b * d) := by
rw [add_comm (b * c) (d * a), mul_comm b d]
exact OreLocalization.oreDiv_add_oreDiv
theorem add_mk_self (a b c) : (mk a b : Localization M) + mk c b = mk (a + c) b := by
rw [add_mk, mk_eq_mk_iff, r_eq_r']
refine (r' M).symm ⟨1, ?_⟩
simp only [Submonoid.coe_one, Submonoid.coe_mul]
ring
/-- For any given denominator `b : M`, the map `a ↦ a / b` is an `AddMonoidHom` from `R` to
`Localization M`-/
@[simps]
def mkAddMonoidHom (b : M) : R →+ Localization M where
toFun a := mk a b
map_zero' := mk_zero _
map_add' _ _ := (add_mk_self _ _ _).symm
theorem mk_sum {ι : Type*} (f : ι → R) (s : Finset ι) (b : M) :
mk (∑ i ∈ s, f i) b = ∑ i ∈ s, mk (f i) b :=
map_sum (mkAddMonoidHom b) f s
theorem mk_list_sum (l : List R) (b : M) : mk l.sum b = (l.map fun a => mk a b).sum :=
map_list_sum (mkAddMonoidHom b) l
theorem mk_multiset_sum (l : Multiset R) (b : M) : mk l.sum b = (l.map fun a => mk a b).sum :=
(mkAddMonoidHom b).map_multiset_sum l
instance isLocalization : IsLocalization M (Localization M) where
map_units' := (Localization.monoidOf M).map_units
surj' := (Localization.monoidOf M).surj
exists_of_eq := (Localization.monoidOf M).eq_iff_exists.mp
end
@[simp]
theorem toLocalizationMap_eq_monoidOf : toLocalizationMap M (Localization M) = monoidOf M :=
rfl
theorem monoidOf_eq_algebraMap (x) : (monoidOf M).toMap x = algebraMap R (Localization M) x :=
rfl
theorem mk_one_eq_algebraMap (x) : mk x 1 = algebraMap R (Localization M) x :=
rfl
theorem mk_eq_mk'_apply (x y) : mk x y = IsLocalization.mk' (Localization M) x y := by
rw [mk_eq_monoidOf_mk'_apply, mk', toLocalizationMap_eq_monoidOf]
-- Porting note: removed `simp`. Left hand side can be simplified; not clear what normal form should
--be.
theorem mk_eq_mk' : (mk : R → M → Localization M) = IsLocalization.mk' (Localization M) :=
mk_eq_monoidOf_mk'
theorem mk_algebraMap {A : Type*} [CommSemiring A] [Algebra A R] (m : A) :
mk (algebraMap A R m) 1 = algebraMap A (Localization M) m := by
rw [mk_eq_mk', mk'_eq_iff_eq_mul, Submonoid.coe_one, map_one, mul_one]; rfl
theorem mk_natCast (m : ℕ) : (mk m 1 : Localization M) = m := by
simpa using mk_algebraMap (R := R) (A := ℕ) _
@[deprecated (since := "2024-04-17")]
alias mk_nat_cast := mk_natCast
variable [IsLocalization M S]
section
variable (M)
/-- The localization of `R` at `M` as a quotient type is isomorphic to any other localization. -/
@[simps!]
noncomputable def algEquiv : Localization M ≃ₐ[R] S :=
IsLocalization.algEquiv M _ _
/-- The localization of a singleton is a singleton. Cannot be an instance due to metavariables. -/
noncomputable def _root_.IsLocalization.unique (R Rₘ) [CommSemiring R] [CommSemiring Rₘ]
(M : Submonoid R) [Subsingleton R] [Algebra R Rₘ] [IsLocalization M Rₘ] : Unique Rₘ :=
have : Inhabited Rₘ := ⟨1⟩
(algEquiv M Rₘ).symm.injective.unique
end
-- Porting note (#10618): removed `simp`, `simp` can prove it
nonrec theorem algEquiv_mk' (x : R) (y : M) : algEquiv M S (mk' (Localization M) x y) = mk' S x y :=
algEquiv_mk' _ _
-- Porting note (#10618): removed `simp`, `simp` can prove it
nonrec theorem algEquiv_symm_mk' (x : R) (y : M) :
(algEquiv M S).symm (mk' S x y) = mk' (Localization M) x y :=
algEquiv_symm_mk' _ _
theorem algEquiv_mk (x y) : algEquiv M S (mk x y) = mk' S x y := by rw [mk_eq_mk', algEquiv_mk']
theorem algEquiv_symm_mk (x : R) (y : M) : (algEquiv M S).symm (mk' S x y) = mk x y := by
rw [mk_eq_mk', algEquiv_symm_mk']
lemma coe_algEquiv :
(Localization.algEquiv M S : Localization M →+* S) =
IsLocalization.map (M := M) (T := M) _ (RingHom.id R) le_rfl := rfl
lemma coe_algEquiv_symm :
((Localization.algEquiv M S).symm : S →+* Localization M) =
IsLocalization.map (M := M) (T := M) _ (RingHom.id R) le_rfl := rfl
end Localization
end CommSemiring
section CommRing
variable {R : Type*} [CommRing R] {M : Submonoid R} (S : Type*) [CommRing S]
variable [Algebra R S] {P : Type*} [CommRing P]
namespace Localization
theorem neg_mk (a b) : -(mk a b : Localization M) = mk (-a) b := OreLocalization.neg_def _ _
theorem sub_mk (a c) (b d) : (mk a b : Localization M) - mk c d =
mk ((d : R) * a - b * c) (b * d) := by
rw [sub_eq_add_neg, neg_mk, add_mk, add_comm, mul_neg, ← sub_eq_add_neg]
theorem mk_intCast (m : ℤ) : (mk m 1 : Localization M) = m := by
simpa using mk_algebraMap (R := R) (A := ℤ) _
@[deprecated (since := "2024-04-17")]
alias mk_int_cast := mk_intCast
end Localization
namespace IsLocalization
variable {K : Type*} [IsLocalization M S]
theorem to_map_eq_zero_iff {x : R} (hM : M ≤ nonZeroDivisors R) : algebraMap R S x = 0 ↔ x = 0 := by
rw [← (algebraMap R S).map_zero]
constructor <;> intro h
· cases' (eq_iff_exists M S).mp h with c hc
rw [mul_zero, mul_comm] at hc
exact hM c.2 x hc
· rw [h]
protected theorem injective (hM : M ≤ nonZeroDivisors R) : Injective (algebraMap R S) := by
rw [injective_iff_map_eq_zero (algebraMap R S)]
intro a ha
rwa [to_map_eq_zero_iff S hM] at ha
protected theorem to_map_ne_zero_of_mem_nonZeroDivisors [Nontrivial R] (hM : M ≤ nonZeroDivisors R)
{x : R} (hx : x ∈ nonZeroDivisors R) : algebraMap R S x ≠ 0 :=
show (algebraMap R S).toMonoidWithZeroHom x ≠ 0 from
map_ne_zero_of_mem_nonZeroDivisors (algebraMap R S) (IsLocalization.injective S hM) hx
variable {S}
theorem sec_snd_ne_zero [Nontrivial R] (hM : M ≤ nonZeroDivisors R) (x : S) :
((sec M x).snd : R) ≠ 0 :=
nonZeroDivisors.coe_ne_zero ⟨(sec M x).snd.val, hM (sec M x).snd.property⟩
theorem sec_fst_ne_zero [Nontrivial R] [NoZeroDivisors S] (hM : M ≤ nonZeroDivisors R) {x : S}
(hx : x ≠ 0) : (sec M x).fst ≠ 0 := by
have hsec := sec_spec M x
intro hfst
rw [hfst, map_zero, mul_eq_zero, _root_.map_eq_zero_iff] at hsec
· exact Or.elim hsec hx (sec_snd_ne_zero hM x)
· exact IsLocalization.injective S hM
variable {Q : Type*} [CommRing Q] {g : R →+* P} [Algebra P Q]
variable (A : Type*) [CommRing A] [IsDomain A]
/-- A `CommRing` `S` which is the localization of a ring `R` without zero divisors at a subset of
non-zero elements does not have zero divisors. -/
theorem noZeroDivisors_of_le_nonZeroDivisors [Algebra A S] {M : Submonoid A} [IsLocalization M S]
(hM : M ≤ nonZeroDivisors A) : NoZeroDivisors S :=
{ eq_zero_or_eq_zero_of_mul_eq_zero := by
intro z w h
cases' surj M z with x hx
cases' surj M w with y hy
have :
z * w * algebraMap A S y.2 * algebraMap A S x.2 = algebraMap A S x.1 * algebraMap A S y.1 :=
by rw [mul_assoc z, hy, ← hx]; ring
rw [h, zero_mul, zero_mul, ← (algebraMap A S).map_mul] at this
cases' eq_zero_or_eq_zero_of_mul_eq_zero ((to_map_eq_zero_iff S hM).mp this.symm) with H H
· exact Or.inl (eq_zero_of_fst_eq_zero hx H)
· exact Or.inr (eq_zero_of_fst_eq_zero hy H) }
/-- A `CommRing` `S` which is the localization of an integral domain `R` at a subset of
non-zero elements is an integral domain. -/
theorem isDomain_of_le_nonZeroDivisors [Algebra A S] {M : Submonoid A} [IsLocalization M S]
(hM : M ≤ nonZeroDivisors A) : IsDomain S := by
apply @NoZeroDivisors.to_isDomain _ _ (id _) (id _)
· exact
⟨⟨(algebraMap A S) 0, (algebraMap A S) 1, fun h =>
zero_ne_one (IsLocalization.injective S hM h)⟩⟩
· exact noZeroDivisors_of_le_nonZeroDivisors _ hM
variable {A}
/-- The localization of an integral domain to a set of non-zero elements is an integral domain. -/
theorem isDomain_localization {M : Submonoid A} (hM : M ≤ nonZeroDivisors A) :
IsDomain (Localization M) :=
isDomain_of_le_nonZeroDivisors _ hM
end IsLocalization
open IsLocalization
/-- If `R` is a field, then localizing at a submonoid not containing `0` adds no new elements. -/
theorem IsField.localization_map_bijective {R Rₘ : Type*} [CommRing R] [CommRing Rₘ]
{M : Submonoid R} (hM : (0 : R) ∉ M) (hR : IsField R) [Algebra R Rₘ] [IsLocalization M Rₘ] :
Function.Bijective (algebraMap R Rₘ) := by
letI := hR.toField
replace hM := le_nonZeroDivisors_of_noZeroDivisors hM
refine ⟨IsLocalization.injective _ hM, fun x => ?_⟩
obtain ⟨r, ⟨m, hm⟩, rfl⟩ := mk'_surjective M x
obtain ⟨n, hn⟩ := hR.mul_inv_cancel (nonZeroDivisors.ne_zero <| hM hm)
exact ⟨r * n, by erw [eq_mk'_iff_mul_eq, ← map_mul, mul_assoc, _root_.mul_comm n, hn, mul_one]⟩
/-- If `R` is a field, then localizing at a submonoid not containing `0` adds no new elements. -/
theorem Field.localization_map_bijective {K Kₘ : Type*} [Field K] [CommRing Kₘ] {M : Submonoid K}
(hM : (0 : K) ∉ M) [Algebra K Kₘ] [IsLocalization M Kₘ] :
Function.Bijective (algebraMap K Kₘ) :=
(Field.toIsField K).localization_map_bijective hM
-- this looks weird due to the `letI` inside the above lemma, but trying to do it the other
-- way round causes issues with defeq of instances, so this is actually easier.
section Algebra
variable {S} {Rₘ Sₘ : Type*} [CommRing Rₘ] [CommRing Sₘ]
variable [Algebra R Rₘ] [IsLocalization M Rₘ]
variable [Algebra S Sₘ] [i : IsLocalization (Algebra.algebraMapSubmonoid S M) Sₘ]
section
variable (S M)
/-- Definition of the natural algebra induced by the localization of an algebra.
Given an algebra `R → S`, a submonoid `R` of `M`, and a localization `Rₘ` for `M`,
let `Sₘ` be the localization of `S` to the image of `M` under `algebraMap R S`.
Then this is the natural algebra structure on `Rₘ → Sₘ`, such that the entire square commutes,
where `localization_map.map_comp` gives the commutativity of the underlying maps.
This instance can be helpful if you define `Sₘ := Localization (Algebra.algebraMapSubmonoid S M)`,
however we will instead use the hypotheses `[Algebra Rₘ Sₘ] [IsScalarTower R Rₘ Sₘ]` in lemmas
since the algebra structure may arise in different ways.
-/
noncomputable def localizationAlgebra : Algebra Rₘ Sₘ :=
(map Sₘ (algebraMap R S)
(show _ ≤ (Algebra.algebraMapSubmonoid S M).comap _ from M.le_comap_map) :
Rₘ →+* Sₘ).toAlgebra
end
section
variable [Algebra Rₘ Sₘ] [Algebra R Sₘ] [IsScalarTower R Rₘ Sₘ] [IsScalarTower R S Sₘ]
variable (S Rₘ Sₘ)
theorem IsLocalization.map_units_map_submonoid
[IsLocalization (Algebra.algebraMapSubmonoid S M) Sₘ] (y : M) :
IsUnit (algebraMap R Sₘ y) := by
rw [IsScalarTower.algebraMap_apply _ S]
exact IsLocalization.map_units Sₘ ⟨algebraMap R S y, Algebra.mem_algebraMapSubmonoid_of_mem y⟩
-- can't be simp, as `S` only appears on the RHS
theorem IsLocalization.algebraMap_mk' (x : R) (y : M) :
algebraMap Rₘ Sₘ (IsLocalization.mk' Rₘ x y) =
IsLocalization.mk' Sₘ (algebraMap R S x)
⟨algebraMap R S y, Algebra.mem_algebraMapSubmonoid_of_mem y⟩ := by
rw [IsLocalization.eq_mk'_iff_mul_eq, Subtype.coe_mk, ← IsScalarTower.algebraMap_apply, ←
IsScalarTower.algebraMap_apply, IsScalarTower.algebraMap_apply R Rₘ Sₘ,
IsScalarTower.algebraMap_apply R Rₘ Sₘ, ← _root_.map_mul, mul_comm,
IsLocalization.mul_mk'_eq_mk'_of_mul]
exact congr_arg (algebraMap Rₘ Sₘ) (IsLocalization.mk'_mul_cancel_left x y)
variable (M)
/-- If the square below commutes, the bottom map is uniquely specified:
```
R → S
↓ ↓
Rₘ → Sₘ
```
-/
theorem IsLocalization.algebraMap_eq_map_map_submonoid :
algebraMap Rₘ Sₘ =
map Sₘ (algebraMap R S)
(show _ ≤ (Algebra.algebraMapSubmonoid S M).comap _ from M.le_comap_map) :=
Eq.symm <|
IsLocalization.map_unique _ (algebraMap Rₘ Sₘ) fun x => by
rw [← IsScalarTower.algebraMap_apply R S Sₘ, ← IsScalarTower.algebraMap_apply R Rₘ Sₘ]
/-- If the square below commutes, the bottom map is uniquely specified:
```
R → S
↓ ↓
Rₘ → Sₘ
```
-/
theorem IsLocalization.algebraMap_apply_eq_map_map_submonoid (x) :
algebraMap Rₘ Sₘ x =
map Sₘ (algebraMap R S)
(show _ ≤ (Algebra.algebraMapSubmonoid S M).comap _ from M.le_comap_map) x :=
DFunLike.congr_fun (IsLocalization.algebraMap_eq_map_map_submonoid _ _ _ _) x
theorem IsLocalization.lift_algebraMap_eq_algebraMap :
IsLocalization.lift (M := M) (IsLocalization.map_units_map_submonoid S Sₘ) =
algebraMap Rₘ Sₘ :=
IsLocalization.lift_unique _ fun _ => (IsScalarTower.algebraMap_apply _ _ _ _).symm
end
variable (Rₘ Sₘ)
/-- Injectivity of the underlying `algebraMap` descends to the algebra induced by localization. -/
theorem localizationAlgebra_injective (hRS : Function.Injective (algebraMap R S)) :
Function.Injective (@algebraMap Rₘ Sₘ _ _ (localizationAlgebra M S)) :=
have : IsLocalization (M.map (algebraMap R S)) Sₘ := i
IsLocalization.map_injective_of_injective _ _ _ hRS
end Algebra
end CommRing
|
RingTheory\Localization\Cardinality.lean | /-
Copyright (c) 2022 Eric Rodriguez. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Eric Rodriguez
-/
import Mathlib.SetTheory.Cardinal.Ordinal
import Mathlib.RingTheory.Artinian
/-!
# Cardinality of localizations
In this file, we establish the cardinality of localizations. In most cases, a localization has
cardinality equal to the base ring. If there are zero-divisors, however, this is no longer true -
for example, `ZMod 6` localized at `{2, 4}` is equal to `ZMod 3`, and if you have zero in your
submonoid, then your localization is trivial (see `IsLocalization.uniqueOfZeroMem`).
## Main statements
* `IsLocalization.card_le`: A localization has cardinality no larger than the base ring.
* `IsLocalization.card`: If you don't localize at zero-divisors, the localization of a ring has
cardinality equal to its base ring,
-/
open Cardinal nonZeroDivisors
universe u v
namespace IsLocalization
variable {R : Type u} [CommRing R] {L : Type u} [CommRing L] [Algebra R L]
/-- A localization always has cardinality less than or equal to the base ring. -/
theorem card_le (S : Submonoid R) [IsLocalization S L] : #L ≤ #R := by
classical
cases fintypeOrInfinite R
· exact Cardinal.mk_le_of_surjective (IsArtinianRing.localization_surjective S _)
erw [← Cardinal.mul_eq_self <| Cardinal.aleph0_le_mk R]
set f : R × R → L := fun aa => IsLocalization.mk' _ aa.1 (if h : aa.2 ∈ S then ⟨aa.2, h⟩ else 1)
refine @Cardinal.mk_le_of_surjective _ _ f fun a => ?_
obtain ⟨x, y, h⟩ := IsLocalization.mk'_surjective S a
use (x, y)
dsimp [f]
rwa [dif_pos <| show ↑y ∈ S from y.2, SetLike.eta]
variable (L)
/-- If you do not localize at any zero-divisors, localization preserves cardinality. -/
theorem card (S : Submonoid R) [IsLocalization S L] (hS : S ≤ R⁰) : #R = #L :=
(Cardinal.mk_le_of_injective (IsLocalization.injective L hS)).antisymm (card_le S)
end IsLocalization
|
RingTheory\Localization\Finiteness.lean | /-
Copyright (c) 2024 Christian Merten. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Christian Merten
-/
import Mathlib.Algebra.Module.LocalizedModuleIntegers
import Mathlib.RingTheory.Localization.Algebra
import Mathlib.RingTheory.LocalProperties
/-!
# Finiteness properties under localization
In this file we establish behaviour of `Module.Finite` under localizations.
## Main results
- `Module.Finite.of_isLocalizedModule`: If `M` is a finite `R`-module,
`S` is a submonoid of `R`, `Rₚ` is the localization of `R` at `S`
and `Mₚ` is the localization of `M` at `S`, then `Mₚ` is a finite
`Rₚ`-module.
- `Module.Finite.of_localizationSpan_finite`: If `M` is an `R`-module
and `{ r }` is a finite set generating the unit ideal such that
`Mᵣ` is a finite `Rᵣ`-module for each `r`, then `M` is a finite `R`-module.
-/
universe u v w t
namespace Module.Finite
section
variable {R : Type u} [CommSemiring R] (S : Submonoid R)
variable {Rₚ : Type v} [CommSemiring Rₚ] [Algebra R Rₚ] [IsLocalization S Rₚ]
variable {M : Type w} [AddCommMonoid M] [Module R M]
variable {Mₚ : Type t} [AddCommMonoid Mₚ] [Module R Mₚ] [Module Rₚ Mₚ] [IsScalarTower R Rₚ Mₚ]
variable (f : M →ₗ[R] Mₚ) [IsLocalizedModule S f]
lemma of_isLocalizedModule [Module.Finite R M] : Module.Finite Rₚ Mₚ := by
classical
obtain ⟨T, hT⟩ := ‹Module.Finite R M›
use T.image f
rw [eq_top_iff]
rintro x -
obtain ⟨⟨y, m⟩, (hyx : IsLocalizedModule.mk' f y m = x)⟩ :=
IsLocalizedModule.mk'_surjective S f x
have hy : y ∈ Submodule.span R T := by rw [hT]; trivial
have : f y ∈ Submodule.map f (Submodule.span R T) := Submodule.mem_map_of_mem hy
rw [Submodule.map_span] at this
have H : Submodule.span R (f '' T) ≤
(Submodule.span Rₚ (f '' T)).restrictScalars R := by
rw [Submodule.span_le]; exact Submodule.subset_span
convert (Submodule.span Rₚ (f '' T)).smul_mem
(IsLocalization.mk' Rₚ (1 : R) m) (H this) using 1
· rw [← hyx, ← IsLocalizedModule.mk'_one S, IsLocalizedModule.mk'_smul_mk']
simp
· simp
end
variable {R : Type u} [CommRing R] (S : Submonoid R) {M : Type w} [AddCommMonoid M] [Module R M]
/--
If there exists a finite set `{ r }` of `R` such that `Mᵣ` is `Rᵣ`-finite for each `r`,
then `M` is a finite `R`-module.
General version for any modules `Mᵣ` and rings `Rᵣ` satisfying the correct universal properties.
See `Module.Finite.of_localizationSpan_finite` for the specialized version.
See `of_localizationSpan'` for a version without the finite set assumption.
-/
theorem of_localizationSpan_finite' (t : Finset R) (ht : Ideal.span (t : Set R) = ⊤)
{Mₚ : ∀ (_ : t), Type*} [∀ (g : t), AddCommMonoid (Mₚ g)] [∀ (g : t), Module R (Mₚ g)]
{Rₚ : ∀ (_ : t), Type u} [∀ (g : t), CommRing (Rₚ g)] [∀ (g : t), Algebra R (Rₚ g)]
[∀ (g : t), IsLocalization.Away g.val (Rₚ g)]
[∀ (g : t), Module (Rₚ g) (Mₚ g)] [∀ (g : t), IsScalarTower R (Rₚ g) (Mₚ g)]
(f : ∀ (g : t), M →ₗ[R] Mₚ g) [∀ (g : t), IsLocalizedModule (Submonoid.powers g.val) (f g)]
(H : ∀ (g : t), Module.Finite (Rₚ g) (Mₚ g)) :
Module.Finite R M := by
classical
constructor
choose s₁ s₂ using (fun g ↦ (H g).1)
let sf := fun x : t ↦
IsLocalizedModule.finsetIntegerMultiple (Submonoid.powers x.val) (f x) (s₁ x)
use t.attach.biUnion sf
rw [Submodule.span_attach_biUnion, eq_top_iff]
rintro x -
refine Submodule.mem_of_span_eq_top_of_smul_pow_mem _ (t : Set R) ht _ (fun r ↦ ?_)
set S : Submonoid R := Submonoid.powers r.val
obtain ⟨⟨_, n₁, rfl⟩, hn₁⟩ := multiple_mem_span_of_mem_localization_span S (Rₚ r)
(s₁ r : Set (Mₚ r)) (IsLocalizedModule.mk' (f r) x (1 : S)) (by rw [s₂ r]; trivial)
rw [Submonoid.smul_def, ← IsLocalizedModule.mk'_smul, IsLocalizedModule.mk'_one] at hn₁
obtain ⟨⟨_, n₂, rfl⟩, hn₂⟩ := IsLocalizedModule.smul_mem_finsetIntegerMultiple_span
S (f r) _ (s₁ r) hn₁
rw [Submonoid.smul_def] at hn₂
use n₂ + n₁
apply le_iSup (fun x : t ↦ Submodule.span R (sf x : Set M)) r
rw [pow_add, mul_smul]
exact hn₂
/--
If there exists a set `{ r }` of `R` such that `Mᵣ` is `Rᵣ`-finite for each `r`,
then `M` is a finite `R`-module.
General version for any modules `Mᵣ` and rings `Rᵣ` satisfying the correct universal properties.
See `Module.Finite.of_localizationSpan_finite` for the specialized version.
-/
theorem of_localizationSpan' (t : Set R) (ht : Ideal.span t = ⊤)
{Mₚ : ∀ (_ : t), Type*} [∀ (g : t), AddCommMonoid (Mₚ g)] [∀ (g : t), Module R (Mₚ g)]
{Rₚ : ∀ (_ : t), Type u} [∀ (g : t), CommRing (Rₚ g)] [∀ (g : t), Algebra R (Rₚ g)]
[h₁ : ∀ (g : t), IsLocalization.Away g.val (Rₚ g)]
[∀ (g : t), Module (Rₚ g) (Mₚ g)] [∀ (g : t), IsScalarTower R (Rₚ g) (Mₚ g)]
(f : ∀ (g : t), M →ₗ[R] Mₚ g) [h₂ : ∀ (g : t), IsLocalizedModule (Submonoid.powers g.val) (f g)]
(H : ∀ (g : t), Module.Finite (Rₚ g) (Mₚ g)) :
Module.Finite R M := by
rw [Ideal.span_eq_top_iff_finite] at ht
obtain ⟨t', hc, ht'⟩ := ht
have (g : t') : IsLocalization.Away g.val (Rₚ ⟨g.val, hc g.property⟩) :=
h₁ ⟨g.val, hc g.property⟩
have (g : t') : IsLocalizedModule (Submonoid.powers g.val)
((fun g ↦ f ⟨g.val, hc g.property⟩) g) := h₂ ⟨g.val, hc g.property⟩
apply of_localizationSpan_finite' t' ht' (fun g ↦ f ⟨g.val, hc g.property⟩)
(fun g ↦ H ⟨g.val, hc g.property⟩)
/--
If there exists a finite set `{ r }` of `R` such that `Mᵣ` is `Rᵣ`-finite for each `r`,
then `M` is a finite `R`-module.
See `of_localizationSpan` for a version without the finite set assumption.
-/
theorem of_localizationSpan_finite (t : Finset R) (ht : Ideal.span (t : Set R) = ⊤)
(H : ∀ (g : t), Module.Finite (Localization.Away g.val)
(LocalizedModule (Submonoid.powers g.val) M)) :
Module.Finite R M :=
let f (g : t) : M →ₗ[R] LocalizedModule (Submonoid.powers g.val) M :=
LocalizedModule.mkLinearMap (Submonoid.powers g.val) M
of_localizationSpan_finite' t ht f H
/-- If there exists a set `{ r }` of `R` such that `Mᵣ` is `Rᵣ`-finite for each `r`,
then `M` is a finite `R`-module. -/
theorem of_localizationSpan (t : Set R) (ht : Ideal.span t = ⊤)
(H : ∀ (g : t), Module.Finite (Localization.Away g.val)
(LocalizedModule (Submonoid.powers g.val) M)) :
Module.Finite R M :=
let f (g : t) : M →ₗ[R] LocalizedModule (Submonoid.powers g.val) M :=
LocalizedModule.mkLinearMap (Submonoid.powers g.val) M
of_localizationSpan' t ht f H
end Finite
end Module
namespace Ideal
variable {R : Type u} [CommRing R]
/-- If `I` is an ideal such that there exists a set `{ r }` of `R` such
that the image of `I` in `Rᵣ` is finitely generated for each `r`, then `I` is finitely
generated. -/
lemma fg_of_localizationSpan {I : Ideal R} (t : Set R) (ht : Ideal.span t = ⊤)
(H : ∀ (g : t), (I.map (algebraMap R (Localization.Away g.val))).FG) : I.FG := by
apply Module.Finite.iff_fg.mp
let k (g : t) : I →ₗ[R] (I.map (algebraMap R (Localization.Away g.val))) :=
Algebra.idealMap I (S := Localization.Away g.val)
exact Module.Finite.of_localizationSpan' t ht k (fun g ↦ Module.Finite.iff_fg.mpr (H g))
end Ideal
variable {R : Type u} [CommRing R] {S : Type v} [CommRing S] {f : R →+* S}
/--
To check that the kernel of a ring homomorphism is finitely generated,
it suffices to check this after localizing at a spanning set of the source.
-/
lemma RingHom.ker_fg_of_localizationSpan (t : Set R) (ht : Ideal.span t = ⊤)
(H : ∀ g : t, (RingHom.ker (Localization.awayMap f g.val)).FG) :
(RingHom.ker f).FG := by
apply Ideal.fg_of_localizationSpan t ht
intro g
rw [← IsLocalization.ker_map (Localization.Away (f g.val)) f (Submonoid.map_powers f g.val)]
exact H g
|
RingTheory\Localization\FractionRing.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.Algebra.Algebra.Tower
import Mathlib.RingTheory.Localization.Basic
import Mathlib.Algebra.Field.Equiv
/-!
# Fraction ring / fraction field Frac(R) as localization
## Main definitions
* `IsFractionRing R K` expresses that `K` is a field of fractions of `R`, as an abbreviation of
`IsLocalization (NonZeroDivisors R) K`
## Main results
* `IsFractionRing.field`: a definition (not an instance) stating the localization of an integral
domain `R` at `R \ {0}` is a field
* `Rat.isFractionRing` is an instance stating `ℚ` is the field of fractions of `ℤ`
## Implementation notes
See `RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
variable (R : Type*) [CommRing R] {M : Submonoid R} (S : Type*) [CommRing S]
variable [Algebra R S] {P : Type*} [CommRing P]
variable {A : Type*} [CommRing A] [IsDomain A] (K : Type*)
-- TODO: should this extend `Algebra` instead of assuming it?
/-- `IsFractionRing R K` states `K` is the field of fractions of an integral domain `R`. -/
abbrev IsFractionRing [CommRing K] [Algebra R K] :=
IsLocalization (nonZeroDivisors R) K
instance {R : Type*} [Field R] : IsFractionRing R R :=
IsLocalization.at_units _ (fun _ ↦ isUnit_of_mem_nonZeroDivisors)
/-- The cast from `Int` to `Rat` as a `FractionRing`. -/
instance Rat.isFractionRing : IsFractionRing ℤ ℚ where
map_units' := by
rintro ⟨x, hx⟩
rw [mem_nonZeroDivisors_iff_ne_zero] at hx
simpa only [eq_intCast, isUnit_iff_ne_zero, Int.cast_eq_zero, Ne, Subtype.coe_mk] using hx
surj' := by
rintro ⟨n, d, hd, h⟩
refine ⟨⟨n, ⟨d, ?_⟩⟩, Rat.mul_den_eq_num _⟩
rw [mem_nonZeroDivisors_iff_ne_zero, Int.natCast_ne_zero_iff_pos]
exact Nat.zero_lt_of_ne_zero hd
exists_of_eq {x y} := by
rw [eq_intCast, eq_intCast, Int.cast_inj]
rintro rfl
use 1
namespace IsFractionRing
open IsLocalization
variable {R K}
section CommRing
variable [CommRing K] [Algebra R K] [IsFractionRing R K] [Algebra A K] [IsFractionRing A K]
theorem to_map_eq_zero_iff {x : R} : algebraMap R K x = 0 ↔ x = 0 :=
IsLocalization.to_map_eq_zero_iff _ le_rfl
variable (R K)
protected theorem injective : Function.Injective (algebraMap R K) :=
IsLocalization.injective _ (le_of_eq rfl)
variable {R K}
@[norm_cast, simp]
-- Porting note: using `↑` didn't work, so I needed to explicitly put in the cast myself
theorem coe_inj {a b : R} : (Algebra.cast a : K) = Algebra.cast b ↔ a = b :=
(IsFractionRing.injective R K).eq_iff
instance (priority := 100) [NoZeroDivisors K] : NoZeroSMulDivisors R K :=
NoZeroSMulDivisors.of_algebraMap_injective <| IsFractionRing.injective R K
protected theorem to_map_ne_zero_of_mem_nonZeroDivisors [Nontrivial R] {x : R}
(hx : x ∈ nonZeroDivisors R) : algebraMap R K x ≠ 0 :=
IsLocalization.to_map_ne_zero_of_mem_nonZeroDivisors _ le_rfl hx
variable (A)
/-- A `CommRing` `K` which is the localization of an integral domain `R` at `R - {0}` is an
integral domain. -/
protected theorem isDomain : IsDomain K :=
isDomain_of_le_nonZeroDivisors _ (le_refl (nonZeroDivisors A))
/-- The inverse of an element in the field of fractions of an integral domain. -/
protected noncomputable irreducible_def inv (z : K) : K := open scoped Classical in
if h : z = 0 then 0
else
mk' K ↑(sec (nonZeroDivisors A) z).2
⟨(sec _ z).1,
mem_nonZeroDivisors_iff_ne_zero.2 fun h0 =>
h <| eq_zero_of_fst_eq_zero (sec_spec (nonZeroDivisors A) z) h0⟩
protected theorem mul_inv_cancel (x : K) (hx : x ≠ 0) : x * IsFractionRing.inv A x = 1 := by
rw [IsFractionRing.inv, dif_neg hx, ←
IsUnit.mul_left_inj
(map_units K
⟨(sec _ x).1,
mem_nonZeroDivisors_iff_ne_zero.2 fun h0 =>
hx <| eq_zero_of_fst_eq_zero (sec_spec (nonZeroDivisors A) x) h0⟩),
one_mul, mul_assoc]
rw [mk'_spec, ← eq_mk'_iff_mul_eq]
exact (mk'_sec _ x).symm
/-- A `CommRing` `K` which is the localization of an integral domain `R` at `R - {0}` is a field.
See note [reducible non-instances]. -/
noncomputable abbrev toField : Field K where
__ := IsFractionRing.isDomain A
mul_inv_cancel := IsFractionRing.mul_inv_cancel A
inv_zero := show IsFractionRing.inv A (0 : K) = 0 by rw [IsFractionRing.inv]; exact dif_pos rfl
nnqsmul := _
nnqsmul_def := fun q a => rfl
qsmul := _
qsmul_def := fun a x => rfl
lemma surjective_iff_isField [IsDomain R] : Function.Surjective (algebraMap R K) ↔ IsField R where
mp h := (RingEquiv.ofBijective (algebraMap R K)
⟨IsFractionRing.injective R K, h⟩).toMulEquiv.isField (IsFractionRing.toField R).toIsField
mpr h :=
letI := h.toField
(IsLocalization.atUnits R _ (S := K)
(fun _ hx ↦ Ne.isUnit (mem_nonZeroDivisors_iff_ne_zero.mp hx))).surjective
end CommRing
variable {B : Type*} [CommRing B] [IsDomain B] [Field K] {L : Type*} [Field L] [Algebra A K]
[IsFractionRing A K] {g : A →+* L}
theorem mk'_mk_eq_div {r s} (hs : s ∈ nonZeroDivisors A) :
mk' K r ⟨s, hs⟩ = algebraMap A K r / algebraMap A K s :=
mk'_eq_iff_eq_mul.2 <|
(div_mul_cancel₀ (algebraMap A K r)
(IsFractionRing.to_map_ne_zero_of_mem_nonZeroDivisors hs)).symm
@[simp]
theorem mk'_eq_div {r} (s : nonZeroDivisors A) : mk' K r s = algebraMap A K r / algebraMap A K s :=
mk'_mk_eq_div s.2
theorem div_surjective (z : K) :
∃ x y : A, y ∈ nonZeroDivisors A ∧ algebraMap _ _ x / algebraMap _ _ y = z :=
let ⟨x, ⟨y, hy⟩, h⟩ := mk'_surjective (nonZeroDivisors A) z
⟨x, y, hy, by rwa [mk'_eq_div] at h⟩
theorem isUnit_map_of_injective (hg : Function.Injective g) (y : nonZeroDivisors A) :
IsUnit (g y) :=
IsUnit.mk0 (g y) <|
show g.toMonoidWithZeroHom y ≠ 0 from map_ne_zero_of_mem_nonZeroDivisors g hg y.2
@[simp]
theorem mk'_eq_zero_iff_eq_zero [Algebra R K] [IsFractionRing R K] {x : R} {y : nonZeroDivisors R} :
mk' K x y = 0 ↔ x = 0 := by
refine ⟨fun hxy => ?_, fun h => by rw [h, mk'_zero]⟩
simp_rw [mk'_eq_zero_iff, mul_left_coe_nonZeroDivisors_eq_zero_iff] at hxy
exact (exists_const _).mp hxy
theorem mk'_eq_one_iff_eq {x : A} {y : nonZeroDivisors A} : mk' K x y = 1 ↔ x = y := by
refine ⟨?_, fun hxy => by rw [hxy, mk'_self']⟩
intro hxy
have hy : (algebraMap A K) ↑y ≠ (0 : K) :=
IsFractionRing.to_map_ne_zero_of_mem_nonZeroDivisors y.property
rw [IsFractionRing.mk'_eq_div, div_eq_one_iff_eq hy] at hxy
exact IsFractionRing.injective A K hxy
open Function
/-- Given an integral domain `A` with field of fractions `K`,
and an injective ring hom `g : A →+* L` where `L` is a field, we get a
field hom sending `z : K` to `g x * (g y)⁻¹`, where `(x, y) : A × (NonZeroDivisors A)` are
such that `z = f x * (f y)⁻¹`. -/
noncomputable def lift (hg : Injective g) : K →+* L :=
IsLocalization.lift fun y : nonZeroDivisors A => isUnit_map_of_injective hg y
/-- Given an integral domain `A` with field of fractions `K`,
and an injective ring hom `g : A →+* L` where `L` is a field,
the field hom induced from `K` to `L` maps `x` to `g x` for all
`x : A`. -/
@[simp]
theorem lift_algebraMap (hg : Injective g) (x) : lift hg (algebraMap A K x) = g x :=
lift_eq _ _
/-- Given an integral domain `A` with field of fractions `K`,
and an injective ring hom `g : A →+* L` where `L` is a field,
field hom induced from `K` to `L` maps `f x / f y` to `g x / g y` for all
`x : A, y ∈ NonZeroDivisors A`. -/
theorem lift_mk' (hg : Injective g) (x) (y : nonZeroDivisors A) :
lift hg (mk' K x y) = g x / g y := by simp only [mk'_eq_div, map_div₀, lift_algebraMap]
/-- Given integral domains `A, B` with fields of fractions `K`, `L`
and an injective ring hom `j : A →+* B`, we get a field hom
sending `z : K` to `g (j x) * (g (j y))⁻¹`, where `(x, y) : A × (NonZeroDivisors A)` are
such that `z = f x * (f y)⁻¹`. -/
noncomputable def map {A B K L : Type*} [CommRing A] [CommRing B] [IsDomain B] [CommRing K]
[Algebra A K] [IsFractionRing A K] [CommRing L] [Algebra B L] [IsFractionRing B L] {j : A →+* B}
(hj : Injective j) : K →+* L :=
IsLocalization.map L j
(show nonZeroDivisors A ≤ (nonZeroDivisors B).comap j from
nonZeroDivisors_le_comap_nonZeroDivisors_of_injective j hj)
/-- Given integral domains `A, B` and localization maps to their fields of fractions
`f : A →+* K, g : B →+* L`, an isomorphism `j : A ≃+* B` induces an isomorphism of
fields of fractions `K ≃+* L`. -/
noncomputable def fieldEquivOfRingEquiv [Algebra B L] [IsFractionRing B L] (h : A ≃+* B) :
K ≃+* L :=
ringEquivOfRingEquiv (M := nonZeroDivisors A) K (T := nonZeroDivisors B) L h
(by
ext b
show b ∈ h.toEquiv '' _ ↔ _
erw [h.toEquiv.image_eq_preimage, Set.preimage, Set.mem_setOf_eq,
mem_nonZeroDivisors_iff_ne_zero, mem_nonZeroDivisors_iff_ne_zero]
exact h.symm.map_ne_zero_iff)
theorem isFractionRing_iff_of_base_ringEquiv (h : R ≃+* P) :
IsFractionRing R S ↔
@IsFractionRing P _ S _ ((algebraMap R S).comp h.symm.toRingHom).toAlgebra := by
delta IsFractionRing
convert isLocalization_iff_of_base_ringEquiv (nonZeroDivisors R) S h
ext x
erw [Submonoid.map_equiv_eq_comap_symm]
simp only [MulEquiv.coe_toMonoidHom, RingEquiv.toMulEquiv_eq_coe, Submonoid.mem_comap]
constructor
· rintro hx z (hz : z * h.symm x = 0)
rw [← h.map_eq_zero_iff]
apply hx
simpa only [h.map_zero, h.apply_symm_apply, h.map_mul] using congr_arg h hz
· rintro (hx : h.symm x ∈ _) z hz
rw [← h.symm.map_eq_zero_iff]
apply hx
rw [← h.symm.map_mul, hz, h.symm.map_zero]
protected theorem nontrivial (R S : Type*) [CommRing R] [Nontrivial R] [CommRing S] [Algebra R S]
[IsFractionRing R S] : Nontrivial S := by
apply nontrivial_of_ne
· intro h
apply @zero_ne_one R
exact
IsLocalization.injective S (le_of_eq rfl)
(((algebraMap R S).map_zero.trans h).trans (algebraMap R S).map_one.symm)
end IsFractionRing
variable (A)
/-- The fraction ring of a commutative ring `R` as a quotient type.
We instantiate this definition as generally as possible, and assume that the
commutative ring `R` is an integral domain only when this is needed for proving.
In this generality, this construction is also known as the *total fraction ring* of `R`.
-/
abbrev FractionRing :=
Localization (nonZeroDivisors R)
namespace FractionRing
instance unique [Subsingleton R] : Unique (FractionRing R) := inferInstance
instance [Nontrivial R] : Nontrivial (FractionRing R) := inferInstance
/-- Porting note: if the fields of this instance are explicitly defined as they were
in mathlib3, the last instance in this file suffers a TC timeout -/
noncomputable instance field : Field (FractionRing A) := inferInstance
@[simp]
theorem mk_eq_div {r s} :
(Localization.mk r s : FractionRing A) =
(algebraMap _ _ r / algebraMap A _ s : FractionRing A) := by
rw [Localization.mk_eq_mk', IsFractionRing.mk'_eq_div]
/-- This is not an instance because it creates a diamond when `K = FractionRing R`.
Should usually be introduced locally along with `isScalarTower_liftAlgebra`
See note [reducible non-instances]. -/
noncomputable abbrev liftAlgebra [IsDomain R] [Field K] [Algebra R K]
[NoZeroSMulDivisors R K] : Algebra (FractionRing R) K :=
RingHom.toAlgebra (IsFractionRing.lift (NoZeroSMulDivisors.algebraMap_injective R _))
-- Porting note: had to fill in the `_` by hand for this instance
instance isScalarTower_liftAlgebra [IsDomain R] [Field K] [Algebra R K] [NoZeroSMulDivisors R K] :
by letI := liftAlgebra R K; exact IsScalarTower R (FractionRing R) K := by
letI := liftAlgebra R K
exact IsScalarTower.of_algebraMap_eq fun x =>
(IsFractionRing.lift_algebraMap (NoZeroSMulDivisors.algebraMap_injective R K) x).symm
/-- Given an integral domain `A` and a localization map to a field of fractions
`f : A →+* K`, we get an `A`-isomorphism between the field of fractions of `A` as a quotient
type and `K`. -/
noncomputable def algEquiv (K : Type*) [Field K] [Algebra A K] [IsFractionRing A K] :
FractionRing A ≃ₐ[A] K :=
Localization.algEquiv (nonZeroDivisors A) K
instance [Algebra R A] [NoZeroSMulDivisors R A] : NoZeroSMulDivisors R (FractionRing A) := by
apply NoZeroSMulDivisors.of_algebraMap_injective
rw [IsScalarTower.algebraMap_eq R A]
apply Function.Injective.comp (NoZeroSMulDivisors.algebraMap_injective A (FractionRing A))
(NoZeroSMulDivisors.algebraMap_injective R A)
end FractionRing
|
RingTheory\Localization\Ideal.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.Ideal.QuotientOperations
import Mathlib.RingTheory.Localization.Basic
/-!
# Ideals in localizations of commutative rings
## Implementation notes
See `Mathlib/RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
namespace IsLocalization
section CommSemiring
variable {R : Type*} [CommSemiring R] (M : Submonoid R) (S : Type*) [CommSemiring S]
variable [Algebra R S] [IsLocalization M S]
/-- Explicit characterization of the ideal given by `Ideal.map (algebraMap R S) I`.
In practice, this ideal differs only in that the carrier set is defined explicitly.
This definition is only meant to be used in proving `mem_map_algebraMap_iff`,
and any proof that needs to refer to the explicit carrier set should use that theorem. -/
private def map_ideal (I : Ideal R) : Ideal S where
carrier := { z : S | ∃ x : I × M, z * algebraMap R S x.2 = algebraMap R S x.1 }
zero_mem' := ⟨⟨0, 1⟩, by simp⟩
add_mem' := by
rintro a b ⟨a', ha⟩ ⟨b', hb⟩
let Z : { x // x ∈ I } := ⟨(a'.2 : R) * (b'.1 : R) + (b'.2 : R) * (a'.1 : R),
I.add_mem (I.mul_mem_left _ b'.1.2) (I.mul_mem_left _ a'.1.2)⟩
use ⟨Z, a'.2 * b'.2⟩
simp only [RingHom.map_add, Submodule.coe_mk, Submonoid.coe_mul, RingHom.map_mul]
rw [add_mul, ← mul_assoc a, ha, mul_comm (algebraMap R S a'.2) (algebraMap R S b'.2), ←
mul_assoc b, hb]
ring
smul_mem' := by
rintro c x ⟨x', hx⟩
obtain ⟨c', hc⟩ := IsLocalization.surj M c
let Z : { x // x ∈ I } := ⟨c'.1 * x'.1, I.mul_mem_left c'.1 x'.1.2⟩
use ⟨Z, c'.2 * x'.2⟩
simp only [← hx, ← hc, smul_eq_mul, Submodule.coe_mk, Submonoid.coe_mul, RingHom.map_mul]
ring
theorem mem_map_algebraMap_iff {I : Ideal R} {z} : z ∈ Ideal.map (algebraMap R S) I ↔
∃ x : I × M, z * algebraMap R S x.2 = algebraMap R S x.1 := by
constructor
· change _ → z ∈ map_ideal M S I
refine fun h => Ideal.mem_sInf.1 h fun z hz => ?_
obtain ⟨y, hy⟩ := hz
let Z : { x // x ∈ I } := ⟨y, hy.left⟩
use ⟨Z, 1⟩
simp [hy.right]
· rintro ⟨⟨a, s⟩, h⟩
rw [← Ideal.unit_mul_mem_iff_mem _ (map_units S s), mul_comm]
exact h.symm ▸ Ideal.mem_map_of_mem _ a.2
lemma mk'_mem_map_algebraMap_iff (I : Ideal R) (x : R) (s : M) :
IsLocalization.mk' S x s ∈ I.map (algebraMap R S) ↔ ∃ s ∈ M, s * x ∈ I := by
rw [← Ideal.unit_mul_mem_iff_mem _ (IsLocalization.map_units S s), IsLocalization.mk'_spec',
IsLocalization.mem_map_algebraMap_iff M]
simp_rw [← map_mul, IsLocalization.eq_iff_exists M, mul_comm x, ← mul_assoc, ← Submonoid.coe_mul]
exact ⟨fun ⟨⟨y, t⟩, c, h⟩ ↦ ⟨_, (c * t).2, h ▸ I.mul_mem_left c.1 y.2⟩, fun ⟨s, hs, h⟩ ↦
⟨⟨⟨_, h⟩, ⟨s, hs⟩⟩, 1, by simp⟩⟩
theorem map_comap [IsLocalization M S] (J : Ideal S) :
Ideal.map (algebraMap R S) (Ideal.comap (algebraMap R S) J) = J :=
le_antisymm (Ideal.map_le_iff_le_comap.2 le_rfl) fun x hJ => by
obtain ⟨r, s, hx⟩ := mk'_surjective M x
rw [← hx] at hJ ⊢
exact
Ideal.mul_mem_right _ _
(Ideal.mem_map_of_mem _
(show (algebraMap R S) r ∈ J from
mk'_spec S r s ▸ J.mul_mem_right ((algebraMap R S) s) hJ))
theorem comap_map_of_isPrime_disjoint (I : Ideal R) (hI : I.IsPrime) (hM : Disjoint (M : Set R) I) :
Ideal.comap (algebraMap R S) (Ideal.map (algebraMap R S) I) = I := by
refine le_antisymm ?_ Ideal.le_comap_map
refine (fun a ha => ?_)
obtain ⟨⟨b, s⟩, h⟩ := (mem_map_algebraMap_iff M S).1 (Ideal.mem_comap.1 ha)
replace h : algebraMap R S (s * a) = algebraMap R S b := by
simpa only [← map_mul, mul_comm] using h
obtain ⟨c, hc⟩ := (eq_iff_exists M S).1 h
have : ↑c * ↑s * a ∈ I := by
rw [mul_assoc, hc]
exact I.mul_mem_left c b.2
exact (hI.mem_or_mem this).resolve_left fun hsc => hM.le_bot ⟨(c * s).2, hsc⟩
/-- If `S` is the localization of `R` at a submonoid, the ordering of ideals of `S` is
embedded in the ordering of ideals of `R`. -/
def orderEmbedding : Ideal S ↪o Ideal R where
toFun J := Ideal.comap (algebraMap R S) J
inj' := Function.LeftInverse.injective (map_comap M S)
map_rel_iff' := by
rintro J₁ J₂
constructor
· exact fun hJ => (map_comap M S) J₁ ▸ (map_comap M S) J₂ ▸ Ideal.map_mono hJ
· exact fun hJ => Ideal.comap_mono hJ
/-- If `R` is a ring, then prime ideals in the localization at `M`
correspond to prime ideals in the original ring `R` that are disjoint from `M`.
This lemma gives the particular case for an ideal and its comap,
see `le_rel_iso_of_prime` for the more general relation isomorphism -/
theorem isPrime_iff_isPrime_disjoint (J : Ideal S) :
J.IsPrime ↔
(Ideal.comap (algebraMap R S) J).IsPrime ∧
Disjoint (M : Set R) ↑(Ideal.comap (algebraMap R S) J) := by
constructor
· refine fun h =>
⟨⟨?_, ?_⟩,
Set.disjoint_left.mpr fun m hm1 hm2 =>
h.ne_top (Ideal.eq_top_of_isUnit_mem _ hm2 (map_units S ⟨m, hm1⟩))⟩
· refine fun hJ => h.ne_top ?_
rw [eq_top_iff, ← (orderEmbedding M S).le_iff_le]
exact le_of_eq hJ.symm
· intro x y hxy
rw [Ideal.mem_comap, RingHom.map_mul] at hxy
exact h.mem_or_mem hxy
· refine fun h => ⟨fun hJ => h.left.ne_top (eq_top_iff.2 ?_), ?_⟩
· rwa [eq_top_iff, ← (orderEmbedding M S).le_iff_le] at hJ
· intro x y hxy
obtain ⟨a, s, ha⟩ := mk'_surjective M x
obtain ⟨b, t, hb⟩ := mk'_surjective M y
have : mk' S (a * b) (s * t) ∈ J := by rwa [mk'_mul, ha, hb]
rw [mk'_mem_iff, ← Ideal.mem_comap] at this
have this₂ := (h.1).mul_mem_iff_mem_or_mem.1 this
rw [Ideal.mem_comap, Ideal.mem_comap] at this₂
rwa [← ha, ← hb, mk'_mem_iff, mk'_mem_iff]
/-- If `R` is a ring, then prime ideals in the localization at `M`
correspond to prime ideals in the original ring `R` that are disjoint from `M`.
This lemma gives the particular case for an ideal and its map,
see `le_rel_iso_of_prime` for the more general relation isomorphism, and the reverse implication -/
theorem isPrime_of_isPrime_disjoint (I : Ideal R) (hp : I.IsPrime) (hd : Disjoint (M : Set R) ↑I) :
(Ideal.map (algebraMap R S) I).IsPrime := by
rw [isPrime_iff_isPrime_disjoint M S, comap_map_of_isPrime_disjoint M S I hp hd]
exact ⟨hp, hd⟩
/-- If `R` is a ring, then prime ideals in the localization at `M`
correspond to prime ideals in the original ring `R` that are disjoint from `M` -/
def orderIsoOfPrime :
{ p : Ideal S // p.IsPrime } ≃o { p : Ideal R // p.IsPrime ∧ Disjoint (M : Set R) ↑p } where
toFun p := ⟨Ideal.comap (algebraMap R S) p.1, (isPrime_iff_isPrime_disjoint M S p.1).1 p.2⟩
invFun p := ⟨Ideal.map (algebraMap R S) p.1, isPrime_of_isPrime_disjoint M S p.1 p.2.1 p.2.2⟩
left_inv J := Subtype.eq (map_comap M S J)
right_inv I := Subtype.eq (comap_map_of_isPrime_disjoint M S I.1 I.2.1 I.2.2)
map_rel_iff' := by
rintro I I'
constructor
· exact (fun h => show I.val ≤ I'.val from map_comap M S I.val ▸
map_comap M S I'.val ▸ Ideal.map_mono h)
exact fun h x hx => h hx
end CommSemiring
section CommRing
variable {R : Type*} [CommRing R] (M : Submonoid R) (S : Type*) [CommRing S]
variable [Algebra R S] [IsLocalization M S]
/-- `quotientMap` applied to maximal ideals of a localization is `surjective`.
The quotient by a maximal ideal is a field, so inverses to elements already exist,
and the localization necessarily maps the equivalence class of the inverse in the localization -/
theorem surjective_quotientMap_of_maximal_of_localization {I : Ideal S} [I.IsPrime] {J : Ideal R}
{H : J ≤ I.comap (algebraMap R S)} (hI : (I.comap (algebraMap R S)).IsMaximal) :
Function.Surjective (Ideal.quotientMap I (algebraMap R S) H) := by
intro s
obtain ⟨s, rfl⟩ := Ideal.Quotient.mk_surjective s
obtain ⟨r, ⟨m, hm⟩, rfl⟩ := mk'_surjective M s
by_cases hM : (Ideal.Quotient.mk (I.comap (algebraMap R S))) m = 0
· have : I = ⊤ := by
rw [Ideal.eq_top_iff_one]
rw [Ideal.Quotient.eq_zero_iff_mem, Ideal.mem_comap] at hM
convert I.mul_mem_right (mk' S (1 : R) ⟨m, hm⟩) hM
rw [← mk'_eq_mul_mk'_one, mk'_self]
exact ⟨0, eq_comm.1 (by simp [Ideal.Quotient.eq_zero_iff_mem, this])⟩
· rw [Ideal.Quotient.maximal_ideal_iff_isField_quotient] at hI
obtain ⟨n, hn⟩ := hI.3 hM
obtain ⟨rn, rfl⟩ := Ideal.Quotient.mk_surjective n
refine ⟨(Ideal.Quotient.mk J) (r * rn), ?_⟩
-- The rest of the proof is essentially just algebraic manipulations to prove the equality
replace hn := congr_arg (Ideal.quotientMap I (algebraMap R S) le_rfl) hn
rw [RingHom.map_one, RingHom.map_mul] at hn
rw [Ideal.quotientMap_mk, ← sub_eq_zero, ← RingHom.map_sub, Ideal.Quotient.eq_zero_iff_mem, ←
Ideal.Quotient.eq_zero_iff_mem, RingHom.map_sub, sub_eq_zero, mk'_eq_mul_mk'_one]
simp only [mul_eq_mul_left_iff, RingHom.map_mul]
refine
Or.inl
(mul_left_cancel₀ (M₀ := S ⧸ I)
(fun hn =>
hM
(Ideal.Quotient.eq_zero_iff_mem.2
(Ideal.mem_comap.2 (Ideal.Quotient.eq_zero_iff_mem.1 hn))))
(_root_.trans hn ?_))
-- Porting note (#10691): was `rw`, but this took extremely long.
refine Eq.trans ?_ (RingHom.map_mul (Ideal.Quotient.mk I) (algebraMap R S m) (mk' S 1 ⟨m, hm⟩))
rw [← mk'_eq_mul_mk'_one, mk'_self, RingHom.map_one]
open nonZeroDivisors
theorem bot_lt_comap_prime [IsDomain R] (hM : M ≤ R⁰) (p : Ideal S) [hpp : p.IsPrime]
(hp0 : p ≠ ⊥) : ⊥ < Ideal.comap (algebraMap R S) p := by
haveI : IsDomain S := isDomain_of_le_nonZeroDivisors _ hM
rw [← Ideal.comap_bot_of_injective (algebraMap R S) (IsLocalization.injective _ hM)]
convert (orderIsoOfPrime M S).lt_iff_lt.mpr (show (⟨⊥, Ideal.bot_prime⟩ :
{ p : Ideal S // p.IsPrime }) < ⟨p, hpp⟩ from hp0.bot_lt)
theorem ideal_eq_iInf_comap_map_away {S : Finset R} (hS : Ideal.span (α := R) S = ⊤) (I : Ideal R) :
I = ⨅ f ∈ S, (I.map (algebraMap R (Localization.Away f))).comap
(algebraMap R (Localization.Away f)) := by
apply le_antisymm
· simp only [le_iInf₂_iff, ← Ideal.map_le_iff_le_comap, le_refl, implies_true]
· intro x hx
apply Submodule.mem_of_span_eq_top_of_smul_pow_mem _ _ hS
rintro ⟨s, hs⟩
simp only [Ideal.mem_iInf, Ideal.mem_comap] at hx
obtain ⟨⟨y, ⟨_, n, rfl⟩⟩, e⟩ :=
(IsLocalization.mem_map_algebraMap_iff (.powers s) _).mp (hx s hs)
dsimp only at e
rw [← map_mul, IsLocalization.eq_iff_exists (.powers s)] at e
obtain ⟨⟨_, m, rfl⟩, e⟩ := e
use m + n
dsimp at e ⊢
rw [pow_add, mul_assoc, ← mul_comm x, e]
exact I.mul_mem_left _ y.2
end CommRing
end IsLocalization
|
RingTheory\Localization\Integer.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.Localization.Basic
/-!
# Integer elements of a localization
## Main definitions
* `IsLocalization.IsInteger` is a predicate stating that `x : S` is in the image of `R`
## Implementation notes
See `RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
variable {R : Type*} [CommSemiring R] {M : Submonoid R} {S : Type*} [CommSemiring S]
variable [Algebra R S] {P : Type*} [CommSemiring P]
open Function
namespace IsLocalization
section
variable (R)
-- TODO: define a subalgebra of `IsInteger`s
/-- Given `a : S`, `S` a localization of `R`, `IsInteger R a` iff `a` is in the image of
the localization map from `R` to `S`. -/
def IsInteger (a : S) : Prop :=
a ∈ (algebraMap R S).rangeS
end
theorem isInteger_zero : IsInteger R (0 : S) :=
Subsemiring.zero_mem _
theorem isInteger_one : IsInteger R (1 : S) :=
Subsemiring.one_mem _
theorem isInteger_add {a b : S} (ha : IsInteger R a) (hb : IsInteger R b) : IsInteger R (a + b) :=
Subsemiring.add_mem _ ha hb
theorem isInteger_mul {a b : S} (ha : IsInteger R a) (hb : IsInteger R b) : IsInteger R (a * b) :=
Subsemiring.mul_mem _ ha hb
theorem isInteger_smul {a : R} {b : S} (hb : IsInteger R b) : IsInteger R (a • b) := by
rcases hb with ⟨b', hb⟩
use a * b'
rw [← hb, (algebraMap R S).map_mul, Algebra.smul_def]
variable (M)
variable [IsLocalization M S]
/-- Each element `a : S` has an `M`-multiple which is an integer.
This version multiplies `a` on the right, matching the argument order in `LocalizationMap.surj`.
-/
theorem exists_integer_multiple' (a : S) : ∃ b : M, IsInteger R (a * algebraMap R S b) :=
let ⟨⟨Num, denom⟩, h⟩ := IsLocalization.surj _ a
⟨denom, Set.mem_range.mpr ⟨Num, h.symm⟩⟩
/-- Each element `a : S` has an `M`-multiple which is an integer.
This version multiplies `a` on the left, matching the argument order in the `SMul` instance.
-/
theorem exists_integer_multiple (a : S) : ∃ b : M, IsInteger R ((b : R) • a) := by
simp_rw [Algebra.smul_def, mul_comm _ a]
apply exists_integer_multiple'
/-- We can clear the denominators of a `Finset`-indexed family of fractions. -/
theorem exist_integer_multiples {ι : Type*} (s : Finset ι) (f : ι → S) :
∃ b : M, ∀ i ∈ s, IsLocalization.IsInteger R ((b : R) • f i) := by
haveI := Classical.propDecidable
refine ⟨∏ i ∈ s, (sec M (f i)).2, fun i hi => ⟨?_, ?_⟩⟩
· exact (∏ j ∈ s.erase i, (sec M (f j)).2) * (sec M (f i)).1
rw [RingHom.map_mul, sec_spec', ← mul_assoc, ← (algebraMap R S).map_mul, ← Algebra.smul_def]
congr 2
refine _root_.trans ?_ (map_prod (Submonoid.subtype M) _ _).symm
rw [mul_comm,Submonoid.coe_finset_prod,
-- Porting note: explicitly supplied `f`
← Finset.prod_insert (f := fun i => ((sec M (f i)).snd : R)) (s.not_mem_erase i),
Finset.insert_erase hi]
rfl
/-- We can clear the denominators of a finite indexed family of fractions. -/
theorem exist_integer_multiples_of_finite {ι : Type*} [Finite ι] (f : ι → S) :
∃ b : M, ∀ i, IsLocalization.IsInteger R ((b : R) • f i) := by
cases nonempty_fintype ι
obtain ⟨b, hb⟩ := exist_integer_multiples M Finset.univ f
exact ⟨b, fun i => hb i (Finset.mem_univ _)⟩
/-- We can clear the denominators of a finite set of fractions. -/
theorem exist_integer_multiples_of_finset (s : Finset S) :
∃ b : M, ∀ a ∈ s, IsInteger R ((b : R) • a) :=
exist_integer_multiples M s id
/-- A choice of a common multiple of the denominators of a `Finset`-indexed family of fractions. -/
noncomputable def commonDenom {ι : Type*} (s : Finset ι) (f : ι → S) : M :=
(exist_integer_multiples M s f).choose
/-- The numerator of a fraction after clearing the denominators
of a `Finset`-indexed family of fractions. -/
noncomputable def integerMultiple {ι : Type*} (s : Finset ι) (f : ι → S) (i : s) : R :=
((exist_integer_multiples M s f).choose_spec i i.prop).choose
@[simp]
theorem map_integerMultiple {ι : Type*} (s : Finset ι) (f : ι → S) (i : s) :
algebraMap R S (integerMultiple M s f i) = commonDenom M s f • f i :=
((exist_integer_multiples M s f).choose_spec _ i.prop).choose_spec
/-- A choice of a common multiple of the denominators of a finite set of fractions. -/
noncomputable def commonDenomOfFinset (s : Finset S) : M :=
commonDenom M s id
/-- The finset of numerators after clearing the denominators of a finite set of fractions. -/
noncomputable def finsetIntegerMultiple [DecidableEq R] (s : Finset S) : Finset R :=
s.attach.image fun t => integerMultiple M s id t
open Pointwise
theorem finsetIntegerMultiple_image [DecidableEq R] (s : Finset S) :
algebraMap R S '' finsetIntegerMultiple M s = commonDenomOfFinset M s • (s : Set S) := by
delta finsetIntegerMultiple commonDenom
rw [Finset.coe_image]
ext
constructor
· rintro ⟨_, ⟨x, -, rfl⟩, rfl⟩
rw [map_integerMultiple]
exact Set.mem_image_of_mem _ x.prop
· rintro ⟨x, hx, rfl⟩
exact ⟨_, ⟨⟨x, hx⟩, s.mem_attach _, rfl⟩, map_integerMultiple M s id _⟩
end IsLocalization
|
RingTheory\Localization\Integral.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.Algebra.GroupWithZero.NonZeroDivisors
import Mathlib.Algebra.Polynomial.Lifts
import Mathlib.GroupTheory.MonoidLocalization.Basic
import Mathlib.RingTheory.Algebraic
import Mathlib.RingTheory.IntegralClosure.Algebra.Basic
import Mathlib.RingTheory.Localization.FractionRing
import Mathlib.RingTheory.Localization.Integer
/-!
# Integral and algebraic elements of a fraction field
## Implementation notes
See `RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
variable {R : Type*} [CommRing R] (M : Submonoid R) {S : Type*} [CommRing S]
variable [Algebra R S] {P : Type*} [CommRing P]
open Polynomial
namespace IsLocalization
section IntegerNormalization
open Polynomial
variable [IsLocalization M S]
open scoped Classical
/-- `coeffIntegerNormalization p` gives the coefficients of the polynomial
`integerNormalization p` -/
noncomputable def coeffIntegerNormalization (p : S[X]) (i : ℕ) : R :=
if hi : i ∈ p.support then
Classical.choose
(Classical.choose_spec (exist_integer_multiples_of_finset M (p.support.image p.coeff))
(p.coeff i) (Finset.mem_image.mpr ⟨i, hi, rfl⟩))
else 0
theorem coeffIntegerNormalization_of_not_mem_support (p : S[X]) (i : ℕ) (h : coeff p i = 0) :
coeffIntegerNormalization M p i = 0 := by
simp only [coeffIntegerNormalization, h, mem_support_iff, eq_self_iff_true, not_true, Ne,
dif_neg, not_false_iff]
theorem coeffIntegerNormalization_mem_support (p : S[X]) (i : ℕ)
(h : coeffIntegerNormalization M p i ≠ 0) : i ∈ p.support := by
contrapose h
rw [Ne, Classical.not_not, coeffIntegerNormalization, dif_neg h]
/-- `integerNormalization g` normalizes `g` to have integer coefficients
by clearing the denominators -/
noncomputable def integerNormalization (p : S[X]) : R[X] :=
∑ i ∈ p.support, monomial i (coeffIntegerNormalization M p i)
@[simp]
theorem integerNormalization_coeff (p : S[X]) (i : ℕ) :
(integerNormalization M p).coeff i = coeffIntegerNormalization M p i := by
simp (config := { contextual := true }) [integerNormalization, coeff_monomial,
coeffIntegerNormalization_of_not_mem_support]
theorem integerNormalization_spec (p : S[X]) :
∃ b : M, ∀ i, algebraMap R S ((integerNormalization M p).coeff i) = (b : R) • p.coeff i := by
use Classical.choose (exist_integer_multiples_of_finset M (p.support.image p.coeff))
intro i
rw [integerNormalization_coeff, coeffIntegerNormalization]
split_ifs with hi
· exact
Classical.choose_spec
(Classical.choose_spec (exist_integer_multiples_of_finset M (p.support.image p.coeff))
(p.coeff i) (Finset.mem_image.mpr ⟨i, hi, rfl⟩))
· rw [RingHom.map_zero, not_mem_support_iff.mp hi, smul_zero]
-- Porting note: was `convert (smul_zero _).symm, ...`
theorem integerNormalization_map_to_map (p : S[X]) :
∃ b : M, (integerNormalization M p).map (algebraMap R S) = (b : R) • p :=
let ⟨b, hb⟩ := integerNormalization_spec M p
⟨b,
Polynomial.ext fun i => by
rw [coeff_map, coeff_smul]
exact hb i⟩
variable {R' : Type*} [CommRing R']
theorem integerNormalization_eval₂_eq_zero (g : S →+* R') (p : S[X]) {x : R'}
(hx : eval₂ g x p = 0) : eval₂ (g.comp (algebraMap R S)) x (integerNormalization M p) = 0 :=
let ⟨b, hb⟩ := integerNormalization_map_to_map M p
_root_.trans (eval₂_map (algebraMap R S) g x).symm
(by rw [hb, ← IsScalarTower.algebraMap_smul S (b : R) p, eval₂_smul, hx, mul_zero])
theorem integerNormalization_aeval_eq_zero [Algebra R R'] [Algebra S R'] [IsScalarTower R S R']
(p : S[X]) {x : R'} (hx : aeval x p = 0) : aeval x (integerNormalization M p) = 0 := by
rw [aeval_def, IsScalarTower.algebraMap_eq R S R',
integerNormalization_eval₂_eq_zero _ (algebraMap _ _) _ hx]
end IntegerNormalization
end IsLocalization
namespace IsFractionRing
open IsLocalization
variable {A K C : Type*} [CommRing A] [IsDomain A] [Field K] [Algebra A K] [IsFractionRing A K]
variable [CommRing C]
theorem integerNormalization_eq_zero_iff {p : K[X]} :
integerNormalization (nonZeroDivisors A) p = 0 ↔ p = 0 := by
refine Polynomial.ext_iff.trans (Polynomial.ext_iff.trans ?_).symm
obtain ⟨⟨b, nonzero⟩, hb⟩ := integerNormalization_spec (nonZeroDivisors A) p
constructor <;> intro h i
· -- Porting note: avoided some defeq abuse
rw [coeff_zero, ← to_map_eq_zero_iff (K := K), hb i, h i, coeff_zero, smul_zero]
· have hi := h i
rw [Polynomial.coeff_zero, ← @to_map_eq_zero_iff A _ K, hb i, Algebra.smul_def] at hi
apply Or.resolve_left (eq_zero_or_eq_zero_of_mul_eq_zero hi)
intro h
apply mem_nonZeroDivisors_iff_ne_zero.mp nonzero
exact to_map_eq_zero_iff.mp h
variable (A K C)
/-- An element of a ring is algebraic over the ring `A` iff it is algebraic
over the field of fractions of `A`.
-/
theorem isAlgebraic_iff [Algebra A C] [Algebra K C] [IsScalarTower A K C] {x : C} :
IsAlgebraic A x ↔ IsAlgebraic K x := by
constructor <;> rintro ⟨p, hp, px⟩
· refine ⟨p.map (algebraMap A K), fun h => hp (Polynomial.ext fun i => ?_), ?_⟩
· have : algebraMap A K (p.coeff i) = 0 :=
_root_.trans (Polynomial.coeff_map _ _).symm (by simp [h])
exact to_map_eq_zero_iff.mp this
· exact (Polynomial.aeval_map_algebraMap K _ _).trans px
· exact
⟨integerNormalization _ p, mt integerNormalization_eq_zero_iff.mp hp,
integerNormalization_aeval_eq_zero _ p px⟩
variable {A K C}
/-- A ring is algebraic over the ring `A` iff it is algebraic over the field of fractions of `A`.
-/
theorem comap_isAlgebraic_iff [Algebra A C] [Algebra K C] [IsScalarTower A K C] :
Algebra.IsAlgebraic A C ↔ Algebra.IsAlgebraic K C :=
⟨fun h => ⟨fun x => (isAlgebraic_iff A K C).mp (h.isAlgebraic x)⟩,
fun h => ⟨fun x => (isAlgebraic_iff A K C).mpr (h.isAlgebraic x)⟩⟩
end IsFractionRing
open IsLocalization
section IsIntegral
variable {Rₘ Sₘ : Type*} [CommRing Rₘ] [CommRing Sₘ]
variable [Algebra R Rₘ] [IsLocalization M Rₘ]
variable [Algebra S Sₘ] [IsLocalization (Algebra.algebraMapSubmonoid S M) Sₘ]
variable {M}
open Polynomial
theorem RingHom.isIntegralElem_localization_at_leadingCoeff {R S : Type*} [CommRing R] [CommRing S]
(f : R →+* S) (x : S) (p : R[X]) (hf : p.eval₂ f x = 0) (M : Submonoid R)
(hM : p.leadingCoeff ∈ M) {Rₘ Sₘ : Type*} [CommRing Rₘ] [CommRing Sₘ] [Algebra R Rₘ]
[IsLocalization M Rₘ] [Algebra S Sₘ] [IsLocalization (M.map f : Submonoid S) Sₘ] :
(map Sₘ f M.le_comap_map : Rₘ →+* _).IsIntegralElem (algebraMap S Sₘ x) := by
by_cases triv : (1 : Rₘ) = 0
· exact ⟨0, ⟨_root_.trans leadingCoeff_zero triv.symm, eval₂_zero _ _⟩⟩
haveI : Nontrivial Rₘ := nontrivial_of_ne 1 0 triv
obtain ⟨b, hb⟩ := isUnit_iff_exists_inv.mp (map_units Rₘ ⟨p.leadingCoeff, hM⟩)
refine ⟨p.map (algebraMap R Rₘ) * C b, ⟨?_, ?_⟩⟩
· refine monic_mul_C_of_leadingCoeff_mul_eq_one ?_
rwa [leadingCoeff_map_of_leadingCoeff_ne_zero (algebraMap R Rₘ)]
refine fun hfp => zero_ne_one
(_root_.trans (zero_mul b).symm (hfp ▸ hb) : (0 : Rₘ) = 1)
· refine eval₂_mul_eq_zero_of_left _ _ _ ?_
erw [eval₂_map, IsLocalization.map_comp, ← hom_eval₂ _ f (algebraMap S Sₘ) x]
exact _root_.trans (congr_arg (algebraMap S Sₘ) hf) (RingHom.map_zero _)
/-- Given a particular witness to an element being algebraic over an algebra `R → S`,
We can localize to a submonoid containing the leading coefficient to make it integral.
Explicitly, the map between the localizations will be an integral ring morphism -/
theorem is_integral_localization_at_leadingCoeff {x : S} (p : R[X]) (hp : aeval x p = 0)
(hM : p.leadingCoeff ∈ M) :
(map Sₘ (algebraMap R S)
(show _ ≤ (Algebra.algebraMapSubmonoid S M).comap _ from M.le_comap_map) :
Rₘ →+* _).IsIntegralElem
(algebraMap S Sₘ x) :=
-- Porting note: added `haveI`
haveI : IsLocalization (Submonoid.map (algebraMap R S) M) Sₘ :=
inferInstanceAs (IsLocalization (Algebra.algebraMapSubmonoid S M) Sₘ)
(algebraMap R S).isIntegralElem_localization_at_leadingCoeff x p hp M hM
/-- If `R → S` is an integral extension, `M` is a submonoid of `R`,
`Rₘ` is the localization of `R` at `M`,
and `Sₘ` is the localization of `S` at the image of `M` under the extension map,
then the induced map `Rₘ → Sₘ` is also an integral extension -/
theorem isIntegral_localization [Algebra.IsIntegral R S] :
(map Sₘ (algebraMap R S)
(show _ ≤ (Algebra.algebraMapSubmonoid S M).comap _ from M.le_comap_map) :
Rₘ →+* _).IsIntegral := by
intro x
obtain ⟨⟨s, ⟨u, hu⟩⟩, hx⟩ := surj (Algebra.algebraMapSubmonoid S M) x
obtain ⟨v, hv⟩ := hu
obtain ⟨v', hv'⟩ := isUnit_iff_exists_inv'.1 (map_units Rₘ ⟨v, hv.1⟩)
refine @IsIntegral.of_mul_unit Rₘ _ _ _ (localizationAlgebra M S) x (algebraMap S Sₘ u) v' ?_ ?_
· replace hv' := congr_arg (@algebraMap Rₘ Sₘ _ _ (localizationAlgebra M S)) hv'
rw [RingHom.map_mul, RingHom.map_one, ← RingHom.comp_apply _ (algebraMap R Rₘ)] at hv'
-- Porting note: added argument
erw [IsLocalization.map_comp
(show _ ≤ (Algebra.algebraMapSubmonoid S M).comap _ from M.le_comap_map)] at hv'
exact hv.2 ▸ hv'
· obtain ⟨p, hp⟩ := Algebra.IsIntegral.isIntegral (R := R) s
exact hx.symm ▸ is_integral_localization_at_leadingCoeff p hp.2 (hp.1.symm ▸ M.one_mem)
@[nolint unusedHavesSuffices] -- It claims the `have : IsLocalization` line is unnecessary,
-- but remove it and the proof won't work.
theorem isIntegral_localization' {R S : Type*} [CommRing R] [CommRing S] {f : R →+* S}
(hf : f.IsIntegral) (M : Submonoid R) :
(map (Localization (M.map (f : R →* S))) f
(M.le_comap_map : _ ≤ Submonoid.comap (f : R →* S) _) :
Localization M →+* _).IsIntegral :=
-- Porting note: added
let _ := f.toAlgebra
have : Algebra.IsIntegral R S := ⟨hf⟩
have : IsLocalization (Algebra.algebraMapSubmonoid S M)
(Localization (Submonoid.map (f : R →* S) M)) := Localization.isLocalization
isIntegral_localization
variable (M)
theorem IsLocalization.scaleRoots_commonDenom_mem_lifts (p : Rₘ[X])
(hp : p.leadingCoeff ∈ (algebraMap R Rₘ).range) :
p.scaleRoots (algebraMap R Rₘ <| IsLocalization.commonDenom M p.support p.coeff) ∈
Polynomial.lifts (algebraMap R Rₘ) := by
rw [Polynomial.lifts_iff_coeff_lifts]
intro n
rw [Polynomial.coeff_scaleRoots]
by_cases h₁ : n ∈ p.support
on_goal 1 => by_cases h₂ : n = p.natDegree
· rwa [h₂, Polynomial.coeff_natDegree, tsub_self, pow_zero, _root_.mul_one]
· have : n + 1 ≤ p.natDegree := lt_of_le_of_ne (Polynomial.le_natDegree_of_mem_supp _ h₁) h₂
rw [← tsub_add_cancel_of_le (le_tsub_of_add_le_left this), pow_add, pow_one, mul_comm,
_root_.mul_assoc, ← map_pow]
change _ ∈ (algebraMap R Rₘ).range
apply mul_mem
· exact RingHom.mem_range_self _ _
· rw [← Algebra.smul_def]
exact ⟨_, IsLocalization.map_integerMultiple M p.support p.coeff ⟨n, h₁⟩⟩
· rw [Polynomial.not_mem_support_iff] at h₁
rw [h₁, zero_mul]
exact zero_mem (algebraMap R Rₘ).range
theorem IsIntegral.exists_multiple_integral_of_isLocalization [Algebra Rₘ S] [IsScalarTower R Rₘ S]
(x : S) (hx : IsIntegral Rₘ x) : ∃ m : M, IsIntegral R (m • x) := by
cases' subsingleton_or_nontrivial Rₘ with _ nontriv
· haveI := (_root_.algebraMap Rₘ S).codomain_trivial
exact ⟨1, Polynomial.X, Polynomial.monic_X, Subsingleton.elim _ _⟩
obtain ⟨p, hp₁, hp₂⟩ := hx
-- Porting note: obtain doesn't support side goals
have :=
lifts_and_natDegree_eq_and_monic (IsLocalization.scaleRoots_commonDenom_mem_lifts M p ?_) ?_
· obtain ⟨p', hp'₁, -, hp'₂⟩ := this
refine ⟨IsLocalization.commonDenom M p.support p.coeff, p', hp'₂, ?_⟩
rw [IsScalarTower.algebraMap_eq R Rₘ S, ← Polynomial.eval₂_map, hp'₁, Submonoid.smul_def,
Algebra.smul_def, IsScalarTower.algebraMap_apply R Rₘ S]
exact Polynomial.scaleRoots_eval₂_eq_zero _ hp₂
· rw [hp₁.leadingCoeff]
exact one_mem _
· rwa [Polynomial.monic_scaleRoots_iff]
end IsIntegral
variable {A K : Type*} [CommRing A] [IsDomain A]
namespace IsIntegralClosure
variable (A)
variable {L : Type*} [Field K] [Field L] [Algebra A K] [Algebra A L] [IsFractionRing A K]
variable (C : Type*) [CommRing C] [IsDomain C] [Algebra C L] [IsIntegralClosure C A L]
variable [Algebra A C] [IsScalarTower A C L]
open Algebra
/-- If the field `L` is an algebraic extension of the integral domain `A`,
the integral closure `C` of `A` in `L` has fraction field `L`. -/
theorem isFractionRing_of_algebraic [Algebra.IsAlgebraic A L]
(inj : ∀ x, algebraMap A L x = 0 → x = 0) : IsFractionRing C L :=
{ map_units' := fun ⟨y, hy⟩ =>
IsUnit.mk0 _
(show algebraMap C L y ≠ 0 from fun h =>
mem_nonZeroDivisors_iff_ne_zero.mp hy
((injective_iff_map_eq_zero (algebraMap C L)).mp (algebraMap_injective C A L) _ h))
surj' := fun z =>
let ⟨x, y, hy, hxy⟩ := exists_integral_multiple (Algebra.IsAlgebraic.isAlgebraic z) inj
⟨⟨mk' C (x : L) x.2, algebraMap _ _ y,
mem_nonZeroDivisors_iff_ne_zero.mpr fun h =>
hy (inj _ (by rw [IsScalarTower.algebraMap_apply A C L, h, RingHom.map_zero]))⟩,
by
simp only
rw [algebraMap_mk', ← IsScalarTower.algebraMap_apply A C L, hxy]⟩
exists_of_eq := fun {x y} h => ⟨1, by simpa using algebraMap_injective C A L h⟩ }
variable (K L)
/-- If the field `L` is a finite extension of the fraction field of the integral domain `A`,
the integral closure `C` of `A` in `L` has fraction field `L`. -/
theorem isFractionRing_of_finite_extension [Algebra K L] [IsScalarTower A K L]
[FiniteDimensional K L] : IsFractionRing C L :=
have : Algebra.IsAlgebraic A L := IsFractionRing.comap_isAlgebraic_iff.mpr
(inferInstanceAs (Algebra.IsAlgebraic K L))
isFractionRing_of_algebraic A C
fun _ hx =>
IsFractionRing.to_map_eq_zero_iff.mp
((_root_.map_eq_zero <|
algebraMap K L).mp <| (IsScalarTower.algebraMap_apply _ _ _ _).symm.trans hx)
end IsIntegralClosure
namespace integralClosure
variable {L : Type*} [Field K] [Field L] [Algebra A K] [IsFractionRing A K]
open Algebra
/-- If the field `L` is an algebraic extension of the integral domain `A`,
the integral closure of `A` in `L` has fraction field `L`. -/
theorem isFractionRing_of_algebraic [Algebra A L] [Algebra.IsAlgebraic A L]
(inj : ∀ x, algebraMap A L x = 0 → x = 0) : IsFractionRing (integralClosure A L) L :=
IsIntegralClosure.isFractionRing_of_algebraic A (integralClosure A L) inj
variable (K L)
/-- If the field `L` is a finite extension of the fraction field of the integral domain `A`,
the integral closure of `A` in `L` has fraction field `L`. -/
theorem isFractionRing_of_finite_extension [Algebra A L] [Algebra K L] [IsScalarTower A K L]
[FiniteDimensional K L] : IsFractionRing (integralClosure A L) L :=
IsIntegralClosure.isFractionRing_of_finite_extension A K L (integralClosure A L)
end integralClosure
namespace IsFractionRing
variable (R S K)
/-- `S` is algebraic over `R` iff a fraction ring of `S` is algebraic over `R` -/
theorem isAlgebraic_iff' [Field K] [IsDomain R] [IsDomain S] [Algebra R K] [Algebra S K]
[NoZeroSMulDivisors R K] [IsFractionRing S K] [IsScalarTower R S K] :
Algebra.IsAlgebraic R S ↔ Algebra.IsAlgebraic R K := by
simp only [Algebra.isAlgebraic_def]
constructor
· intro h x
letI := FractionRing.liftAlgebra R K
have := FractionRing.isScalarTower_liftAlgebra R K
rw [IsFractionRing.isAlgebraic_iff R (FractionRing R) K, isAlgebraic_iff_isIntegral]
obtain ⟨a : S, b, ha, rfl⟩ := @div_surjective S _ _ _ _ _ _ x
obtain ⟨f, hf₁, hf₂⟩ := h b
rw [div_eq_mul_inv]
refine IsIntegral.mul ?_ ?_
· rw [← isAlgebraic_iff_isIntegral]
refine .tower_top_of_injective
(NoZeroSMulDivisors.algebraMap_injective R (FractionRing R)) ?_
exact .algebraMap (h a)
· rw [← isAlgebraic_iff_isIntegral]
use (f.map (algebraMap R (FractionRing R))).reverse
constructor
· rwa [Ne, Polynomial.reverse_eq_zero, ← Polynomial.degree_eq_bot,
Polynomial.degree_map_eq_of_injective
(NoZeroSMulDivisors.algebraMap_injective R (FractionRing R)),
Polynomial.degree_eq_bot]
· have : Invertible (algebraMap S K b) :=
IsUnit.invertible
(isUnit_of_mem_nonZeroDivisors
(mem_nonZeroDivisors_iff_ne_zero.2 fun h =>
nonZeroDivisors.ne_zero ha
((injective_iff_map_eq_zero (algebraMap S K)).1
(NoZeroSMulDivisors.algebraMap_injective _ _) b h)))
rw [Polynomial.aeval_def, ← invOf_eq_inv, Polynomial.eval₂_reverse_eq_zero_iff,
Polynomial.eval₂_map, ← IsScalarTower.algebraMap_eq, ← Polynomial.aeval_def,
Polynomial.aeval_algebraMap_apply, hf₂, RingHom.map_zero]
· intro h x
obtain ⟨f, hf₁, hf₂⟩ := h (algebraMap S K x)
use f, hf₁
rw [Polynomial.aeval_algebraMap_apply] at hf₂
exact
(injective_iff_map_eq_zero (algebraMap S K)).1 (NoZeroSMulDivisors.algebraMap_injective _ _) _
hf₂
open nonZeroDivisors
variable {S K}
/-- If the `S`-multiples of `a` are contained in some `R`-span, then `Frac(S)`-multiples of `a`
are contained in the equivalent `Frac(R)`-span. -/
theorem ideal_span_singleton_map_subset {L : Type*} [IsDomain R] [IsDomain S] [Field K] [Field L]
[Algebra R K] [Algebra R L] [Algebra S L] [IsIntegralClosure S R L] [IsFractionRing S L]
[Algebra K L] [IsScalarTower R S L] [IsScalarTower R K L] {a : S} {b : Set S}
[Algebra.IsAlgebraic R L] (inj : Function.Injective (algebraMap R L))
(h : (Ideal.span ({a} : Set S) : Set S) ⊆ Submodule.span R b) :
(Ideal.span ({algebraMap S L a} : Set L) : Set L) ⊆ Submodule.span K (algebraMap S L '' b) := by
intro x hx
obtain ⟨x', rfl⟩ := Ideal.mem_span_singleton.mp hx
obtain ⟨y', z', rfl⟩ := IsLocalization.mk'_surjective S⁰ x'
obtain ⟨y, z, hz0, yz_eq⟩ :=
IsIntegralClosure.exists_smul_eq_mul inj y' (nonZeroDivisors.coe_ne_zero z')
have injRS : Function.Injective (algebraMap R S) := by
refine
Function.Injective.of_comp (show Function.Injective (algebraMap S L ∘ algebraMap R S) from ?_)
rwa [← RingHom.coe_comp, ← IsScalarTower.algebraMap_eq]
have hz0' : algebraMap R S z ∈ S⁰ :=
map_mem_nonZeroDivisors (algebraMap R S) injRS (mem_nonZeroDivisors_of_ne_zero hz0)
have mk_yz_eq : IsLocalization.mk' L y' z' = IsLocalization.mk' L y ⟨_, hz0'⟩ := by
rw [Algebra.smul_def, mul_comm _ y, mul_comm _ y'] at yz_eq
exact IsLocalization.mk'_eq_of_eq (by rw [mul_comm _ y, mul_comm _ y', yz_eq])
suffices hy : algebraMap S L (a * y) ∈ Submodule.span K ((algebraMap S L) '' b) by
rw [mk_yz_eq, IsFractionRing.mk'_eq_div, ← IsScalarTower.algebraMap_apply,
IsScalarTower.algebraMap_apply R K L, div_eq_mul_inv, ← mul_assoc, mul_comm, ← map_inv₀, ←
Algebra.smul_def, ← _root_.map_mul]
exact (Submodule.span K _).smul_mem _ hy
refine Submodule.span_subset_span R K _ ?_
rw [Submodule.span_algebraMap_image_of_tower]
-- Note: #8386 had to specify the value of `f` here:
exact Submodule.mem_map_of_mem (f := LinearMap.restrictScalars _ _)
(h (Ideal.mem_span_singleton.mpr ⟨y, rfl⟩))
end IsFractionRing
lemma isAlgebraic_of_isLocalization {R} [CommRing R] (M : Submonoid R) (S) [CommRing S]
[Nontrivial R] [Algebra R S] [IsLocalization M S] : Algebra.IsAlgebraic R S := by
constructor
intro x
obtain ⟨x, s, rfl⟩ := IsLocalization.mk'_surjective M x
by_cases hs : (s : R) = 0
· have := IsLocalization.mk'_spec S x s
rw [hs, map_zero, mul_zero] at this
exact ⟨X, X_ne_zero, by simp [IsLocalization.mk'_eq_mul_mk'_one x, ← this]⟩
refine ⟨s • X - C x, ?_, ?_⟩
· intro e; apply hs
simpa only [coeff_sub, coeff_smul, coeff_X_one, coeff_C_succ, sub_zero, coeff_zero,
← Algebra.algebraMap_eq_smul_one, Submonoid.smul_def,
Algebra.id.map_eq_id, RingHom.id_apply] using congr_arg (Polynomial.coeff · 1) e
· simp only [map_sub, Algebra.smul_def, Submonoid.smul_def,
map_mul, AlgHom.commutes, aeval_X, IsLocalization.mk'_spec', aeval_C, sub_self]
open nonZeroDivisors in
lemma isAlgebraic_of_isFractionRing {R S} (K L) [CommRing R] [CommRing S] [Field K] [CommRing L]
[Algebra R S] [Algebra R K] [Algebra R L] [Algebra S L] [Algebra K L] [IsScalarTower R S L]
[IsScalarTower R K L] [IsFractionRing S L]
[Algebra.IsIntegral R S] : Algebra.IsAlgebraic K L := by
constructor
intro x
obtain ⟨x, s, rfl⟩ := IsLocalization.mk'_surjective S⁰ x
apply IsIntegral.isAlgebraic
rw [IsLocalization.mk'_eq_mul_mk'_one]
apply RingHom.IsIntegralElem.mul
· apply IsIntegral.tower_top (R := R)
apply IsIntegral.map (IsScalarTower.toAlgHom R S L)
exact Algebra.IsIntegral.isIntegral x
· show IsIntegral _ _
rw [← isAlgebraic_iff_isIntegral, ← IsAlgebraic.invOf_iff, isAlgebraic_iff_isIntegral]
apply IsIntegral.tower_top (R := R)
apply IsIntegral.map (IsScalarTower.toAlgHom R S L)
exact Algebra.IsIntegral.isIntegral (s : S)
|
RingTheory\Localization\InvSubmonoid.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.GroupTheory.Submonoid.Inverses
import Mathlib.RingTheory.FiniteType
import Mathlib.RingTheory.Localization.Basic
/-!
# Submonoid of inverses
## Main definitions
* `IsLocalization.invSubmonoid M S` is the submonoid of `S = M⁻¹R` consisting of inverses of
each element `x ∈ M`
## Implementation notes
See `Mathlib/RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
variable {R : Type*} [CommRing R] (M : Submonoid R) (S : Type*) [CommRing S]
variable [Algebra R S] {P : Type*} [CommRing P]
open Function
namespace IsLocalization
section InvSubmonoid
/-- The submonoid of `S = M⁻¹R` consisting of `{ 1 / x | x ∈ M }`. -/
def invSubmonoid : Submonoid S :=
(M.map (algebraMap R S)).leftInv
variable [IsLocalization M S]
theorem submonoid_map_le_is_unit : M.map (algebraMap R S) ≤ IsUnit.submonoid S := by
rintro _ ⟨a, ha, rfl⟩
exact IsLocalization.map_units S ⟨_, ha⟩
/-- There is an equivalence of monoids between the image of `M` and `invSubmonoid`. -/
noncomputable abbrev equivInvSubmonoid : M.map (algebraMap R S) ≃* invSubmonoid M S :=
((M.map (algebraMap R S)).leftInvEquiv (submonoid_map_le_is_unit M S)).symm
/-- There is a canonical map from `M` to `invSubmonoid` sending `x` to `1 / x`. -/
noncomputable def toInvSubmonoid : M →* invSubmonoid M S :=
(equivInvSubmonoid M S).toMonoidHom.comp ((algebraMap R S : R →* S).submonoidMap M)
theorem toInvSubmonoid_surjective : Function.Surjective (toInvSubmonoid M S) :=
Function.Surjective.comp (β := M.map (algebraMap R S))
(Equiv.surjective (equivInvSubmonoid _ _).toEquiv) (MonoidHom.submonoidMap_surjective _ _)
@[simp]
theorem toInvSubmonoid_mul (m : M) : (toInvSubmonoid M S m : S) * algebraMap R S m = 1 :=
Submonoid.leftInvEquiv_symm_mul _ (submonoid_map_le_is_unit _ _) _
@[simp]
theorem mul_toInvSubmonoid (m : M) : algebraMap R S m * (toInvSubmonoid M S m : S) = 1 :=
Submonoid.mul_leftInvEquiv_symm _ (submonoid_map_le_is_unit _ _) ⟨_, _⟩
@[simp]
theorem smul_toInvSubmonoid (m : M) : m • (toInvSubmonoid M S m : S) = 1 := by
convert mul_toInvSubmonoid M S m
ext
rw [← Algebra.smul_def]
rfl
variable {S}
-- Porting note: `surj'` was taken, so use `surj''` instead
theorem surj'' (z : S) : ∃ (r : R) (m : M), z = r • (toInvSubmonoid M S m : S) := by
rcases IsLocalization.surj M z with ⟨⟨r, m⟩, e : z * _ = algebraMap R S r⟩
refine ⟨r, m, ?_⟩
rw [Algebra.smul_def, ← e, mul_assoc]
simp
theorem toInvSubmonoid_eq_mk' (x : M) : (toInvSubmonoid M S x : S) = mk' S 1 x := by
rw [← (IsLocalization.map_units S x).mul_left_inj]
simp
theorem mem_invSubmonoid_iff_exists_mk' (x : S) :
x ∈ invSubmonoid M S ↔ ∃ m : M, mk' S 1 m = x := by
simp_rw [← toInvSubmonoid_eq_mk']
exact ⟨fun h => ⟨_, congr_arg Subtype.val (toInvSubmonoid_surjective M S ⟨x, h⟩).choose_spec⟩,
fun h => h.choose_spec ▸ (toInvSubmonoid M S h.choose).prop⟩
variable (S)
theorem span_invSubmonoid : Submodule.span R (invSubmonoid M S : Set S) = ⊤ := by
rw [eq_top_iff]
rintro x -
rcases IsLocalization.surj'' M x with ⟨r, m, rfl⟩
exact Submodule.smul_mem _ _ (Submodule.subset_span (toInvSubmonoid M S m).prop)
theorem finiteType_of_monoid_fg [Monoid.FG M] : Algebra.FiniteType R S := by
have := Monoid.fg_of_surjective _ (toInvSubmonoid_surjective M S)
rw [Monoid.fg_iff_submonoid_fg] at this
rcases this with ⟨s, hs⟩
refine ⟨⟨s, ?_⟩⟩
rw [eq_top_iff]
rintro x -
change x ∈ (Subalgebra.toSubmodule (Algebra.adjoin R _ : Subalgebra R S) : Set S)
rw [Algebra.adjoin_eq_span, hs, span_invSubmonoid]
trivial
end InvSubmonoid
end IsLocalization
|
RingTheory\Localization\LocalizationLocalization.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.Localization.AtPrime
import Mathlib.RingTheory.Localization.Basic
import Mathlib.RingTheory.Localization.FractionRing
/-!
# Localizations of localizations
## Implementation notes
See `Mathlib/RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
open Function
namespace IsLocalization
section LocalizationLocalization
variable {R : Type*} [CommSemiring R] (M : Submonoid R) {S : Type*} [CommSemiring S]
variable [Algebra R S] {P : Type*} [CommSemiring P]
variable (N : Submonoid S) (T : Type*) [CommSemiring T] [Algebra R T]
section
variable [Algebra S T] [IsScalarTower R S T]
-- This should only be defined when `S` is the localization `M⁻¹R`, hence the nolint.
/-- Localizing wrt `M ⊆ R` and then wrt `N ⊆ S = M⁻¹R` is equal to the localization of `R` wrt this
module. See `localization_localization_isLocalization`.
-/
@[nolint unusedArguments]
def localizationLocalizationSubmodule : Submonoid R :=
(N ⊔ M.map (algebraMap R S)).comap (algebraMap R S)
variable {M N}
@[simp]
theorem mem_localizationLocalizationSubmodule {x : R} :
x ∈ localizationLocalizationSubmodule M N ↔
∃ (y : N) (z : M), algebraMap R S x = y * algebraMap R S z := by
rw [localizationLocalizationSubmodule, Submonoid.mem_comap, Submonoid.mem_sup]
constructor
· rintro ⟨y, hy, _, ⟨z, hz, rfl⟩, e⟩
exact ⟨⟨y, hy⟩, ⟨z, hz⟩, e.symm⟩
· rintro ⟨y, z, e⟩
exact ⟨y, y.prop, _, ⟨z, z.prop, rfl⟩, e.symm⟩
variable (M N)
variable [IsLocalization M S]
theorem localization_localization_map_units [IsLocalization N T]
(y : localizationLocalizationSubmodule M N) : IsUnit (algebraMap R T y) := by
obtain ⟨y', z, eq⟩ := mem_localizationLocalizationSubmodule.mp y.prop
rw [IsScalarTower.algebraMap_apply R S T, eq, RingHom.map_mul, IsUnit.mul_iff]
exact ⟨IsLocalization.map_units T y', (IsLocalization.map_units _ z).map (algebraMap S T)⟩
theorem localization_localization_surj [IsLocalization N T] (x : T) :
∃ y : R × localizationLocalizationSubmodule M N,
x * algebraMap R T y.2 = algebraMap R T y.1 := by
rcases IsLocalization.surj N x with ⟨⟨y, s⟩, eq₁⟩
-- x = y / s
rcases IsLocalization.surj M y with ⟨⟨z, t⟩, eq₂⟩
-- y = z / t
rcases IsLocalization.surj M (s : S) with ⟨⟨z', t'⟩, eq₃⟩
-- s = z' / t'
dsimp only at eq₁ eq₂ eq₃
refine ⟨⟨z * t', z' * t, ?_⟩, ?_⟩ -- x = y / s = (z * t') / (z' * t)
· rw [mem_localizationLocalizationSubmodule]
refine ⟨s, t * t', ?_⟩
rw [RingHom.map_mul, ← eq₃, mul_assoc, ← RingHom.map_mul, mul_comm t, Submonoid.coe_mul]
· simp only [Subtype.coe_mk, RingHom.map_mul, IsScalarTower.algebraMap_apply R S T, ← eq₃, ← eq₂,
← eq₁]
ring
theorem localization_localization_exists_of_eq [IsLocalization N T] (x y : R) :
algebraMap R T x = algebraMap R T y →
∃ c : localizationLocalizationSubmodule M N, ↑c * x = ↑c * y := by
rw [IsScalarTower.algebraMap_apply R S T, IsScalarTower.algebraMap_apply R S T,
IsLocalization.eq_iff_exists N T]
rintro ⟨z, eq₁⟩
rcases IsLocalization.surj M (z : S) with ⟨⟨z', s⟩, eq₂⟩
dsimp only at eq₂
suffices (algebraMap R S) (x * z' : R) = (algebraMap R S) (y * z') by
obtain ⟨c, eq₃ : ↑c * (x * z') = ↑c * (y * z')⟩ := (IsLocalization.eq_iff_exists M S).mp this
refine ⟨⟨c * z', ?_⟩, ?_⟩
· rw [mem_localizationLocalizationSubmodule]
refine ⟨z, c * s, ?_⟩
rw [map_mul, ← eq₂, Submonoid.coe_mul, map_mul, mul_left_comm]
· rwa [mul_comm _ z', mul_comm _ z', ← mul_assoc, ← mul_assoc] at eq₃
rw [map_mul, map_mul, ← eq₂, ← mul_assoc, ← mul_assoc, mul_comm _ (z : S), eq₁,
mul_comm _ (z : S)]
/-- Given submodules `M ⊆ R` and `N ⊆ S = M⁻¹R`, with `f : R →+* S` the localization map, we have
`N ⁻¹ S = T = (f⁻¹ (N • f(M))) ⁻¹ R`. I.e., the localization of a localization is a localization.
-/
theorem localization_localization_isLocalization [IsLocalization N T] :
IsLocalization (localizationLocalizationSubmodule M N) T :=
{ map_units' := localization_localization_map_units M N T
surj' := localization_localization_surj M N T
exists_of_eq := localization_localization_exists_of_eq M N T _ _ }
/-- Given submodules `M ⊆ R` and `N ⊆ S = M⁻¹R`, with `f : R →+* S` the localization map, if
`N` contains all the units of `S`, then `N ⁻¹ S = T = (f⁻¹ N) ⁻¹ R`. I.e., the localization of a
localization is a localization.
-/
theorem localization_localization_isLocalization_of_has_all_units [IsLocalization N T]
(H : ∀ x : S, IsUnit x → x ∈ N) : IsLocalization (N.comap (algebraMap R S)) T := by
convert localization_localization_isLocalization M N T using 1
dsimp [localizationLocalizationSubmodule]
congr
symm
rw [sup_eq_left]
rintro _ ⟨x, hx, rfl⟩
exact H _ (IsLocalization.map_units _ ⟨x, hx⟩)
/--
Given a submodule `M ⊆ R` and a prime ideal `p` of `S = M⁻¹R`, with `f : R →+* S` the localization
map, then `T = Sₚ` is the localization of `R` at `f⁻¹(p)`.
-/
theorem isLocalization_isLocalization_atPrime_isLocalization (p : Ideal S) [Hp : p.IsPrime]
[IsLocalization.AtPrime T p] : IsLocalization.AtPrime T (p.comap (algebraMap R S)) := by
apply localization_localization_isLocalization_of_has_all_units M p.primeCompl T
intro x hx hx'
exact (Hp.1 : ¬_) (p.eq_top_of_isUnit_mem hx' hx)
instance (p : Ideal (Localization M)) [p.IsPrime] : Algebra R (Localization.AtPrime p) :=
inferInstance
instance (p : Ideal (Localization M)) [p.IsPrime] :
IsScalarTower R (Localization M) (Localization.AtPrime p) :=
IsScalarTower.of_algebraMap_eq' rfl
instance localization_localization_atPrime_is_localization (p : Ideal (Localization M))
[p.IsPrime] : IsLocalization.AtPrime (Localization.AtPrime p) (p.comap (algebraMap R _)) :=
isLocalization_isLocalization_atPrime_isLocalization M _ _
/-- Given a submodule `M ⊆ R` and a prime ideal `p` of `M⁻¹R`, with `f : R →+* S` the localization
map, then `(M⁻¹R)ₚ` is isomorphic (as an `R`-algebra) to the localization of `R` at `f⁻¹(p)`.
-/
noncomputable def localizationLocalizationAtPrimeIsoLocalization (p : Ideal (Localization M))
[p.IsPrime] :
Localization.AtPrime (p.comap (algebraMap R (Localization M))) ≃ₐ[R] Localization.AtPrime p :=
IsLocalization.algEquiv (p.comap (algebraMap R (Localization M))).primeCompl _ _
end
variable (S)
/-- Given submonoids `M ≤ N` of `R`, this is the canonical algebra structure
of `M⁻¹S` acting on `N⁻¹S`. -/
noncomputable def localizationAlgebraOfSubmonoidLe (M N : Submonoid R) (h : M ≤ N)
[IsLocalization M S] [IsLocalization N T] : Algebra S T :=
(@IsLocalization.lift R _ M S _ _ T _ _ (algebraMap R T)
(fun y => map_units T ⟨↑y, h y.prop⟩)).toAlgebra
/-- If `M ≤ N` are submonoids of `R`, then the natural map `M⁻¹S →+* N⁻¹S` commutes with the
localization maps -/
theorem localization_isScalarTower_of_submonoid_le (M N : Submonoid R) (h : M ≤ N)
[IsLocalization M S] [IsLocalization N T] :
@IsScalarTower R S T _ (localizationAlgebraOfSubmonoidLe S T M N h).toSMul _ :=
letI := localizationAlgebraOfSubmonoidLe S T M N h
IsScalarTower.of_algebraMap_eq' (IsLocalization.lift_comp _).symm
noncomputable instance (x : Ideal R) [H : x.IsPrime] [IsDomain R] :
Algebra (Localization.AtPrime x) (Localization (nonZeroDivisors R)) :=
localizationAlgebraOfSubmonoidLe _ _ x.primeCompl (nonZeroDivisors R)
(by
intro a ha
rw [mem_nonZeroDivisors_iff_ne_zero]
exact fun h => ha (h.symm ▸ x.zero_mem))
/-- If `M ≤ N` are submonoids of `R`, then `N⁻¹S` is also the localization of `M⁻¹S` at `N`. -/
theorem isLocalization_of_submonoid_le (M N : Submonoid R) (h : M ≤ N) [IsLocalization M S]
[IsLocalization N T] [Algebra S T] [IsScalarTower R S T] :
IsLocalization (N.map (algebraMap R S)) T :=
{ map_units' := by
rintro ⟨_, ⟨y, hy, rfl⟩⟩
convert IsLocalization.map_units T ⟨y, hy⟩
exact (IsScalarTower.algebraMap_apply _ _ _ _).symm
surj' := fun y => by
obtain ⟨⟨x, s⟩, e⟩ := IsLocalization.surj N y
refine ⟨⟨algebraMap R S x, _, _, s.prop, rfl⟩, ?_⟩
simpa [← IsScalarTower.algebraMap_apply] using e
exists_of_eq := fun {x₁ x₂} => by
obtain ⟨⟨y₁, s₁⟩, e₁⟩ := IsLocalization.surj M x₁
obtain ⟨⟨y₂, s₂⟩, e₂⟩ := IsLocalization.surj M x₂
refine (Set.exists_image_iff (algebraMap R S) N fun c => c * x₁ = c * x₂).mpr.comp ?_
dsimp only at e₁ e₂ ⊢
suffices algebraMap R T (y₁ * s₂) = algebraMap R T (y₂ * s₁) →
∃ a : N, algebraMap R S (a * (y₁ * s₂)) = algebraMap R S (a * (y₂ * s₁)) by
have h₁ := @IsUnit.mul_left_inj T _ _ (algebraMap S T x₁) (algebraMap S T x₂)
(IsLocalization.map_units T ⟨(s₁ : R), h s₁.prop⟩)
have h₂ := @IsUnit.mul_left_inj T _ _ ((algebraMap S T x₁) * (algebraMap R T s₁))
((algebraMap S T x₂) * (algebraMap R T s₁))
(IsLocalization.map_units T ⟨(s₂ : R), h s₂.prop⟩)
simp only [IsScalarTower.algebraMap_apply R S T, Subtype.coe_mk] at h₁ h₂
simp only [IsScalarTower.algebraMap_apply R S T, map_mul, ← e₁, ← e₂, ← mul_assoc,
mul_right_comm _ (algebraMap R S s₂),
mul_right_comm _ (algebraMap S T (algebraMap R S s₂)),
(IsLocalization.map_units S s₁).mul_left_inj,
(IsLocalization.map_units S s₂).mul_left_inj] at this
rw [h₂, h₁] at this
simpa only [mul_comm] using this
simp_rw [IsLocalization.eq_iff_exists N T, IsLocalization.eq_iff_exists M S]
intro ⟨a, e⟩
exact ⟨a, 1, by convert e using 1 <;> simp⟩ }
/-- If `M ≤ N` are submonoids of `R` such that `∀ x : N, ∃ m : R, m * x ∈ M`, then the
localization at `N` is equal to the localizaton of `M`. -/
theorem isLocalization_of_is_exists_mul_mem (M N : Submonoid R) [IsLocalization M S] (h : M ≤ N)
(h' : ∀ x : N, ∃ m : R, m * x ∈ M) : IsLocalization N S :=
{ map_units' := fun y => by
obtain ⟨m, hm⟩ := h' y
have := IsLocalization.map_units S ⟨_, hm⟩
erw [map_mul] at this
exact (IsUnit.mul_iff.mp this).2
surj' := fun z => by
obtain ⟨⟨y, s⟩, e⟩ := IsLocalization.surj M z
exact ⟨⟨y, _, h s.prop⟩, e⟩
exists_of_eq := fun {_ _} => by
rw [IsLocalization.eq_iff_exists M]
exact fun ⟨x, hx⟩ => ⟨⟨_, h x.prop⟩, hx⟩ }
end LocalizationLocalization
end IsLocalization
namespace IsFractionRing
variable {R : Type*} [CommRing R] (M : Submonoid R) {S : Type*} [CommRing S]
open IsLocalization
theorem isFractionRing_of_isLocalization (S T : Type*) [CommRing S] [CommRing T] [Algebra R S]
[Algebra R T] [Algebra S T] [IsScalarTower R S T] [IsLocalization M S] [IsFractionRing R T]
(hM : M ≤ nonZeroDivisors R) : IsFractionRing S T := by
have := isLocalization_of_submonoid_le S T M (nonZeroDivisors R) hM
refine @isLocalization_of_is_exists_mul_mem _ _ _ _ _ _ _ this ?_ ?_
· exact map_nonZeroDivisors_le M S
· rintro ⟨x, hx⟩
obtain ⟨⟨y, s⟩, e⟩ := IsLocalization.surj M x
use algebraMap R S s
rw [mul_comm, Subtype.coe_mk, e]
refine Set.mem_image_of_mem (algebraMap R S) ?_
intro z hz
apply IsLocalization.injective S hM
rw [map_zero]
apply hx
rw [← (map_units S s).mul_left_inj, mul_assoc, e, ← map_mul, hz, map_zero,
zero_mul]
theorem isFractionRing_of_isDomain_of_isLocalization [IsDomain R] (S T : Type*) [CommRing S]
[CommRing T] [Algebra R S] [Algebra R T] [Algebra S T] [IsScalarTower R S T]
[IsLocalization M S] [IsFractionRing R T] : IsFractionRing S T := by
haveI := IsFractionRing.nontrivial R T
haveI := (algebraMap S T).domain_nontrivial
apply isFractionRing_of_isLocalization M S T
intro x hx
rw [mem_nonZeroDivisors_iff_ne_zero]
intro hx'
apply @zero_ne_one S
rw [← (algebraMap R S).map_one, ← @mk'_one R _ M, @comm _ Eq, mk'_eq_zero_iff]
exact ⟨⟨x, hx⟩, by simp [hx']⟩
end IsFractionRing
|
RingTheory\Localization\Module.lean | /-
Copyright (c) 2022 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Junyan Xu, Anne Baanen
-/
import Mathlib.LinearAlgebra.Basis
import Mathlib.Algebra.Module.LocalizedModule
import Mathlib.RingTheory.Localization.FractionRing
import Mathlib.RingTheory.Localization.Integer
/-!
# Modules / vector spaces over localizations / fraction fields
This file contains some results about vector spaces over the field of fractions of a ring.
## Main results
* `LinearIndependent.localization`: `b` is linear independent over a localization of `R`
if it is linear independent over `R` itself
* `Basis.ofIsLocalizedModule` / `Basis.localizationLocalization`: promote an `R`-basis `b` of `A`
to an `Rₛ`-basis of `Aₛ`, where `Rₛ` and `Aₛ` are localizations of `R` and `A` at `s`
respectively
* `LinearIndependent.iff_fractionRing`: `b` is linear independent over `R` iff it is
linear independent over `Frac(R)`
-/
open nonZeroDivisors
section Localization
variable {R : Type*} (Rₛ : Type*) [CommSemiring R] (S : Submonoid R)
section IsLocalizedModule
section AddCommMonoid
open Submodule
variable [CommSemiring Rₛ] [Algebra R Rₛ] [hT : IsLocalization S Rₛ]
variable {M M' : Type*} [AddCommMonoid M] [Module R M] [Module Rₛ M] [IsScalarTower R Rₛ M]
[AddCommMonoid M'] [Module R M'] [Module Rₛ M'] [IsScalarTower R Rₛ M'] (f : M →ₗ[R] M')
[IsLocalizedModule S f]
theorem span_eq_top_of_isLocalizedModule {v : Set M} (hv : span R v = ⊤) :
span Rₛ (f '' v) = ⊤ := top_unique fun x _ ↦ by
obtain ⟨⟨m, s⟩, h⟩ := IsLocalizedModule.surj S f x
rw [Submonoid.smul_def, ← algebraMap_smul Rₛ, ← Units.smul_isUnit (IsLocalization.map_units Rₛ s),
eq_comm, ← inv_smul_eq_iff] at h
refine h ▸ smul_mem _ _ (span_subset_span R Rₛ _ ?_)
rw [← LinearMap.coe_restrictScalars R, ← LinearMap.map_span, hv]
exact mem_map_of_mem mem_top
theorem LinearIndependent.of_isLocalizedModule {ι : Type*} {v : ι → M}
(hv : LinearIndependent R v) : LinearIndependent Rₛ (f ∘ v) := by
rw [linearIndependent_iff'] at hv ⊢
intro t g hg i hi
choose! a g' hg' using IsLocalization.exist_integer_multiples S t g
have h0 : f (∑ i ∈ t, g' i • v i) = 0 := by
apply_fun ((a : R) • ·) at hg
rw [smul_zero, Finset.smul_sum] at hg
rw [map_sum, ← hg]
refine Finset.sum_congr rfl fun i hi => ?_
rw [← smul_assoc, ← hg' i hi, map_smul, Function.comp_apply, algebraMap_smul]
obtain ⟨s, hs⟩ := (IsLocalizedModule.eq_zero_iff S f).mp h0
simp_rw [Finset.smul_sum, Submonoid.smul_def, smul_smul] at hs
specialize hv t _ hs i hi
rw [← (IsLocalization.map_units Rₛ a).mul_right_eq_zero, ← Algebra.smul_def, ← hg' i hi]
exact (IsLocalization.map_eq_zero_iff S _ _).2 ⟨s, hv⟩
theorem LinearIndependent.localization {ι : Type*} {b : ι → M} (hli : LinearIndependent R b) :
LinearIndependent Rₛ b := by
have := isLocalizedModule_id S M Rₛ
exact hli.of_isLocalizedModule Rₛ S .id
end AddCommMonoid
section Basis
variable [CommRing Rₛ] [Algebra R Rₛ] [hT : IsLocalization S Rₛ]
open Submodule
variable {M Mₛ : Type*} [AddCommGroup M] [AddCommGroup Mₛ] [Module R M] [Module R Mₛ] [Module R Mₛ]
[Module Rₛ Mₛ] (f : M →ₗ[R] Mₛ) [IsLocalizedModule S f] [IsScalarTower R Rₛ Mₛ]
{ι : Type*} (b : Basis ι R M)
/-- If `M` has an `R`-basis, then localizing `M` at `S` has a basis over `R` localized at `S`. -/
noncomputable def Basis.ofIsLocalizedModule : Basis ι Rₛ Mₛ :=
.mk (b.linearIndependent.of_isLocalizedModule Rₛ S f) <| by
rw [Set.range_comp, span_eq_top_of_isLocalizedModule Rₛ S _ b.span_eq]
@[simp]
theorem Basis.ofIsLocalizedModule_apply (i : ι) : b.ofIsLocalizedModule Rₛ S f i = f (b i) := by
rw [ofIsLocalizedModule, coe_mk, Function.comp_apply]
@[simp]
theorem Basis.ofIsLocalizedModule_repr_apply (m : M) (i : ι) :
((b.ofIsLocalizedModule Rₛ S f).repr (f m)) i = algebraMap R Rₛ (b.repr m i) := by
suffices ((b.ofIsLocalizedModule Rₛ S f).repr.toLinearMap.restrictScalars R) ∘ₗ f =
Finsupp.mapRange.linearMap (Algebra.linearMap R Rₛ) ∘ₗ b.repr.toLinearMap by
exact DFunLike.congr_fun (LinearMap.congr_fun this m) i
refine Basis.ext b fun i ↦ ?_
rw [LinearMap.coe_comp, Function.comp_apply, LinearMap.coe_restrictScalars,
LinearEquiv.coe_coe, ← b.ofIsLocalizedModule_apply Rₛ S f, repr_self, LinearMap.coe_comp,
Function.comp_apply, LinearEquiv.coe_coe, repr_self, Finsupp.mapRange.linearMap_apply,
Finsupp.mapRange_single, Algebra.linearMap_apply, map_one]
theorem Basis.ofIsLocalizedModule_span :
span R (Set.range (b.ofIsLocalizedModule Rₛ S f)) = LinearMap.range f := by
calc span R (Set.range (b.ofIsLocalizedModule Rₛ S f))
_ = span R (f '' (Set.range b)) := by congr; ext; simp
_ = map f (span R (Set.range b)) := by rw [Submodule.map_span]
_ = LinearMap.range f := by rw [b.span_eq, Submodule.map_top]
end Basis
end IsLocalizedModule
section LocalizationLocalization
variable {R : Type*} (Rₛ : Type*) [CommSemiring R] [CommRing Rₛ] [Algebra R Rₛ]
variable (S : Submonoid R) [hT : IsLocalization S Rₛ]
variable {A : Type*} [CommRing A] [Algebra R A]
variable (Aₛ : Type*) [CommRing Aₛ] [Algebra A Aₛ]
variable [Algebra Rₛ Aₛ] [Algebra R Aₛ] [IsScalarTower R Rₛ Aₛ] [IsScalarTower R A Aₛ]
variable [hA : IsLocalization (Algebra.algebraMapSubmonoid A S) Aₛ]
open Submodule
theorem LinearIndependent.localization_localization {ι : Type*} {v : ι → A}
(hv : LinearIndependent R v) : LinearIndependent Rₛ ((algebraMap A Aₛ) ∘ v) :=
hv.of_isLocalizedModule Rₛ S (IsScalarTower.toAlgHom R A Aₛ).toLinearMap
theorem span_eq_top_localization_localization {v : Set A} (hv : span R v = ⊤) :
span Rₛ (algebraMap A Aₛ '' v) = ⊤ :=
span_eq_top_of_isLocalizedModule Rₛ S (IsScalarTower.toAlgHom R A Aₛ).toLinearMap hv
/-- If `A` has an `R`-basis, then localizing `A` at `S` has a basis over `R` localized at `S`.
A suitable instance for `[Algebra A Aₛ]` is `localizationAlgebra`.
-/
noncomputable def Basis.localizationLocalization {ι : Type*} (b : Basis ι R A) : Basis ι Rₛ Aₛ :=
b.ofIsLocalizedModule Rₛ S (IsScalarTower.toAlgHom R A Aₛ).toLinearMap
@[simp]
theorem Basis.localizationLocalization_apply {ι : Type*} (b : Basis ι R A) (i) :
b.localizationLocalization Rₛ S Aₛ i = algebraMap A Aₛ (b i) :=
b.ofIsLocalizedModule_apply Rₛ S _ i
@[simp]
theorem Basis.localizationLocalization_repr_algebraMap {ι : Type*} (b : Basis ι R A) (x i) :
(b.localizationLocalization Rₛ S Aₛ).repr (algebraMap A Aₛ x) i =
algebraMap R Rₛ (b.repr x i) := b.ofIsLocalizedModule_repr_apply Rₛ S _ _ i
theorem Basis.localizationLocalization_span {ι : Type*} (b : Basis ι R A) :
Submodule.span R (Set.range (b.localizationLocalization Rₛ S Aₛ)) =
LinearMap.range (IsScalarTower.toAlgHom R A Aₛ) := b.ofIsLocalizedModule_span Rₛ S _
end LocalizationLocalization
section FractionRing
variable (R K : Type*) [CommRing R] [Field K] [Algebra R K] [IsFractionRing R K]
variable {V : Type*} [AddCommGroup V] [Module R V] [Module K V] [IsScalarTower R K V]
theorem LinearIndependent.iff_fractionRing {ι : Type*} {b : ι → V} :
LinearIndependent R b ↔ LinearIndependent K b :=
⟨LinearIndependent.localization K R⁰,
LinearIndependent.restrict_scalars (smul_left_injective R one_ne_zero)⟩
end FractionRing
section
variable {R : Type*} [CommSemiring R] (S : Submonoid R)
variable (A : Type*) [CommSemiring A] [Algebra R A] [IsLocalization S A]
variable {M N : Type*}
[AddCommMonoid M] [Module R M] [Module A M] [IsScalarTower R A M]
[AddCommMonoid N] [Module R N] [Module A N] [IsScalarTower R A N]
open IsLocalization
/-- An `R`-linear map between two `S⁻¹R`-modules is actually `S⁻¹R`-linear. -/
def LinearMap.extendScalarsOfIsLocalization (f : M →ₗ[R] N) : M →ₗ[A] N where
toFun := f
map_add' := f.map_add
map_smul' := by
intro r m
simp only [RingHom.id_apply]
rcases mk'_surjective S r with ⟨r, s, rfl⟩
calc f (mk' A r s • m)
= ((s : R) • mk' A 1 s) • f (mk' A r s • m) := by simp
_ = (mk' A 1 s) • (s : R) • f (mk' A r s • m) := by rw [smul_comm, smul_assoc]
_ = (mk' A 1 s) • f ((s : R) • mk' A r s • m) := by simp
_ = (mk' A 1 s) • f (r • m) := by rw [← smul_assoc, smul_mk'_self, algebraMap_smul]
_ = (mk' A 1 s) • r • f m := by simp
_ = mk' A r s • f m := by rw [smul_comm, ← smul_assoc, smul_mk'_one]
@[simp] lemma LinearMap.restrictScalars_extendScalarsOfIsLocalization (f : M →ₗ[R] N) :
(f.extendScalarsOfIsLocalization S A).restrictScalars R = f := rfl
@[simp] lemma LinearMap.extendScalarsOfIsLocalization_apply (f : M →ₗ[A] N) :
f.extendScalarsOfIsLocalization S A = f := rfl
@[simp] lemma LinearMap.extendScalarsOfIsLocalization_apply' (f : M →ₗ[R] N) (x : M) :
(f.extendScalarsOfIsLocalization S A) x = f x := rfl
end
end Localization
|
RingTheory\Localization\NormTrace.lean | /-
Copyright (c) 2023 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen
-/
import Mathlib.RingTheory.Localization.Module
import Mathlib.RingTheory.Norm.Basic
import Mathlib.RingTheory.Discriminant
/-!
# Field/algebra norm / trace and localization
This file contains results on the combination of `IsLocalization` and `Algebra.norm`,
`Algebra.trace` and `Algebra.discr`.
## Main results
* `Algebra.norm_localization`: let `S` be an extension of `R` and `Rₘ Sₘ` be localizations at `M`
of `R S` respectively. Then the norm of `a : Sₘ` over `Rₘ` is the norm of `a : S` over `R`
if `S` is free as `R`-module.
* `Algebra.trace_localization`: let `S` be an extension of `R` and `Rₘ Sₘ` be localizations at `M`
of `R S` respectively. Then the trace of `a : Sₘ` over `Rₘ` is the trace of `a : S` over `R`
if `S` is free as `R`-module.
* `Algebra.discr_localizationLocalization`: let `S` be an extension of `R` and `Rₘ Sₘ` be
localizations at `M` of `R S` respectively. Let `b` be a `R`-basis of `S`. Then discriminant of
the `Rₘ`-basis of `Sₘ` induced by `b` is the discriminant of `b`.
## Tags
field norm, algebra norm, localization
-/
open scoped nonZeroDivisors
variable (R : Type*) {S : Type*} [CommRing R] [CommRing S] [Algebra R S]
variable {Rₘ Sₘ : Type*} [CommRing Rₘ] [Algebra R Rₘ] [CommRing Sₘ] [Algebra S Sₘ]
variable (M : Submonoid R)
variable [IsLocalization M Rₘ] [IsLocalization (Algebra.algebraMapSubmonoid S M) Sₘ]
variable [Algebra Rₘ Sₘ] [Algebra R Sₘ] [IsScalarTower R Rₘ Sₘ] [IsScalarTower R S Sₘ]
open Algebra
theorem Algebra.map_leftMulMatrix_localization {ι : Type*} [Fintype ι] [DecidableEq ι]
(b : Basis ι R S) (a : S) :
(algebraMap R Rₘ).mapMatrix (leftMulMatrix b a) =
leftMulMatrix (b.localizationLocalization Rₘ M Sₘ) (algebraMap S Sₘ a) := by
ext i j
simp only [Matrix.map_apply, RingHom.mapMatrix_apply, leftMulMatrix_eq_repr_mul, ← map_mul,
Basis.localizationLocalization_apply, Basis.localizationLocalization_repr_algebraMap]
/-- Let `S` be an extension of `R` and `Rₘ Sₘ` be localizations at `M` of `R S` respectively.
Then the norm of `a : Sₘ` over `Rₘ` is the norm of `a : S` over `R` if `S` is free as `R`-module.
-/
theorem Algebra.norm_localization [Module.Free R S] [Module.Finite R S] (a : S) :
Algebra.norm Rₘ (algebraMap S Sₘ a) = algebraMap R Rₘ (Algebra.norm R a) := by
cases subsingleton_or_nontrivial R
· haveI : Subsingleton Rₘ := Module.subsingleton R Rₘ
simp [eq_iff_true_of_subsingleton]
let b := Module.Free.chooseBasis R S
letI := Classical.decEq (Module.Free.ChooseBasisIndex R S)
rw [Algebra.norm_eq_matrix_det (b.localizationLocalization Rₘ M Sₘ),
Algebra.norm_eq_matrix_det b, RingHom.map_det, ← Algebra.map_leftMulMatrix_localization]
variable {M} in
/-- The norm of `a : S` in `R` can be computed in `Sₘ`. -/
lemma Algebra.norm_eq_iff [Module.Free R S] [Module.Finite R S] {a : S} {b : R}
(hM : M ≤ nonZeroDivisors R) : Algebra.norm R a = b ↔
(Algebra.norm Rₘ) ((algebraMap S Sₘ) a) = algebraMap R Rₘ b :=
⟨fun h ↦ h.symm ▸ Algebra.norm_localization _ M _, fun h ↦
IsLocalization.injective Rₘ hM <| h.symm ▸ (Algebra.norm_localization R M a).symm⟩
/-- Let `S` be an extension of `R` and `Rₘ Sₘ` be localizations at `M` of `R S` respectively.
Then the trace of `a : Sₘ` over `Rₘ` is the trace of `a : S` over `R` if `S` is free as `R`-module.
-/
theorem Algebra.trace_localization [Module.Free R S] [Module.Finite R S] (a : S) :
Algebra.trace Rₘ Sₘ (algebraMap S Sₘ a) = algebraMap R Rₘ (Algebra.trace R S a) := by
cases subsingleton_or_nontrivial R
· haveI : Subsingleton Rₘ := Module.subsingleton R Rₘ
simp [eq_iff_true_of_subsingleton]
let b := Module.Free.chooseBasis R S
letI := Classical.decEq (Module.Free.ChooseBasisIndex R S)
rw [Algebra.trace_eq_matrix_trace (b.localizationLocalization Rₘ M Sₘ),
Algebra.trace_eq_matrix_trace b, ← Algebra.map_leftMulMatrix_localization]
exact (AddMonoidHom.map_trace (algebraMap R Rₘ).toAddMonoidHom _).symm
section LocalizationLocalization
variable (Sₘ : Type*) [CommRing Sₘ] [Algebra S Sₘ] [Algebra Rₘ Sₘ] [Algebra R Sₘ]
variable [IsScalarTower R Rₘ Sₘ] [IsScalarTower R S Sₘ]
variable [IsLocalization (Algebra.algebraMapSubmonoid S M) Sₘ]
variable {ι : Type*} [Fintype ι] [DecidableEq ι]
theorem Algebra.traceMatrix_localizationLocalization (b : Basis ι R S) :
Algebra.traceMatrix Rₘ (b.localizationLocalization Rₘ M Sₘ) =
(algebraMap R Rₘ).mapMatrix (Algebra.traceMatrix R b) := by
have : Module.Finite R S := Module.Finite.of_basis b
have : Module.Free R S := Module.Free.of_basis b
ext i j : 2
simp_rw [RingHom.mapMatrix_apply, Matrix.map_apply, traceMatrix_apply, traceForm_apply,
Basis.localizationLocalization_apply, ← map_mul]
exact Algebra.trace_localization R M _
/-- Let `S` be an extension of `R` and `Rₘ Sₘ` be localizations at `M` of `R S` respectively. Let
`b` be a `R`-basis of `S`. Then discriminant of the `Rₘ`-basis of `Sₘ` induced by `b` is the
discriminant of `b`.
-/
theorem Algebra.discr_localizationLocalization (b : Basis ι R S) :
Algebra.discr Rₘ (b.localizationLocalization Rₘ M Sₘ) =
algebraMap R Rₘ (Algebra.discr R b) := by
rw [Algebra.discr_def, Algebra.discr_def, RingHom.map_det,
Algebra.traceMatrix_localizationLocalization]
end LocalizationLocalization
|
RingTheory\Localization\NumDen.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.Localization.FractionRing
import Mathlib.RingTheory.Localization.Integer
import Mathlib.RingTheory.UniqueFactorizationDomain
/-!
# Numerator and denominator in a localization
## Implementation notes
See `Mathlib/RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
variable {R : Type*} [CommRing R] (M : Submonoid R) {S : Type*} [CommRing S]
variable [Algebra R S] {P : Type*} [CommRing P]
namespace IsFractionRing
open IsLocalization
section NumDen
variable (A : Type*) [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A]
variable {K : Type*} [Field K] [Algebra A K] [IsFractionRing A K]
theorem exists_reduced_fraction (x : K) :
∃ (a : A) (b : nonZeroDivisors A), IsRelPrime a b ∧ mk' K a b = x := by
obtain ⟨⟨b, b_nonzero⟩, a, hab⟩ := exists_integer_multiple (nonZeroDivisors A) x
obtain ⟨a', b', c', no_factor, rfl, rfl⟩ :=
UniqueFactorizationMonoid.exists_reduced_factors' a b
(mem_nonZeroDivisors_iff_ne_zero.mp b_nonzero)
obtain ⟨_, b'_nonzero⟩ := mul_mem_nonZeroDivisors.mp b_nonzero
refine ⟨a', ⟨b', b'_nonzero⟩, no_factor, ?_⟩
refine mul_left_cancel₀ (IsFractionRing.to_map_ne_zero_of_mem_nonZeroDivisors b_nonzero) ?_
simp only [Subtype.coe_mk, RingHom.map_mul, Algebra.smul_def] at *
erw [← hab, mul_assoc, mk'_spec' _ a' ⟨b', b'_nonzero⟩]
/-- `f.num x` is the numerator of `x : f.codomain` as a reduced fraction. -/
noncomputable def num (x : K) : A :=
Classical.choose (exists_reduced_fraction A x)
/-- `f.den x` is the denominator of `x : f.codomain` as a reduced fraction. -/
noncomputable def den (x : K) : nonZeroDivisors A :=
Classical.choose (Classical.choose_spec (exists_reduced_fraction A x))
theorem num_den_reduced (x : K) : IsRelPrime (num A x) (den A x) :=
(Classical.choose_spec (Classical.choose_spec (exists_reduced_fraction A x))).1
-- @[simp] -- Porting note: LHS reduces to give the simp lemma below
theorem mk'_num_den (x : K) : mk' K (num A x) (den A x) = x :=
(Classical.choose_spec (Classical.choose_spec (exists_reduced_fraction A x))).2
@[simp]
theorem mk'_num_den' (x : K) : algebraMap A K (num A x) / algebraMap A K (den A x) = x := by
rw [← mk'_eq_div]
apply mk'_num_den
variable {A}
theorem num_mul_den_eq_num_iff_eq {x y : K} :
x * algebraMap A K (den A y) = algebraMap A K (num A y) ↔ x = y :=
⟨fun h => by simpa only [mk'_num_den] using eq_mk'_iff_mul_eq.mpr h, fun h ↦
eq_mk'_iff_mul_eq.mp (by rw [h, mk'_num_den])⟩
theorem num_mul_den_eq_num_iff_eq' {x y : K} :
y * algebraMap A K (den A x) = algebraMap A K (num A x) ↔ x = y :=
⟨fun h ↦ by simpa only [eq_comm, mk'_num_den] using eq_mk'_iff_mul_eq.mpr h, fun h ↦
eq_mk'_iff_mul_eq.mp (by rw [h, mk'_num_den])⟩
theorem num_mul_den_eq_num_mul_den_iff_eq {x y : K} :
num A y * den A x = num A x * den A y ↔ x = y :=
⟨fun h ↦ by simpa only [mk'_num_den] using mk'_eq_of_eq' (S := K) h, fun h ↦ by rw [h]⟩
theorem eq_zero_of_num_eq_zero {x : K} (h : num A x = 0) : x = 0 :=
num_mul_den_eq_num_iff_eq'.mp (by rw [zero_mul, h, RingHom.map_zero])
@[simp]
lemma num_zero : IsFractionRing.num A (0 : K) = 0 := by
have := mk'_num_den' A (0 : K)
simp only [div_eq_zero_iff] at this
rcases this with h | h
· exact NoZeroSMulDivisors.algebraMap_injective A K (by convert h; simp)
· replace h : algebraMap A K (den A (0 : K)) = algebraMap A K 0 := by convert h; simp
absurd NoZeroSMulDivisors.algebraMap_injective A K h
apply nonZeroDivisors.coe_ne_zero
@[simp]
lemma num_eq_zero (x : K) : IsFractionRing.num A x = 0 ↔ x = 0 :=
⟨eq_zero_of_num_eq_zero, fun h ↦ h ▸ num_zero⟩
theorem isInteger_of_isUnit_den {x : K} (h : IsUnit (den A x : A)) : IsInteger A x := by
cases' h with d hd
have d_ne_zero : algebraMap A K (den A x) ≠ 0 :=
IsFractionRing.to_map_ne_zero_of_mem_nonZeroDivisors (den A x).2
use ↑d⁻¹ * num A x
refine _root_.trans ?_ (mk'_num_den A x)
rw [map_mul, map_units_inv, hd]
apply mul_left_cancel₀ d_ne_zero
rw [← mul_assoc, mul_inv_cancel d_ne_zero, one_mul, mk'_spec']
theorem isUnit_den_iff (x : K) : IsUnit (den A x : A) ↔ IsLocalization.IsInteger A x where
mp h := by
obtain ⟨den, hd⟩ := IsUnit.exists_right_inv h
use (num A x) * den
conv => rhs; rw [← mk'_num_den' A x]
rw [map_mul, div_eq_mul_inv]
congr 1
apply eq_inv_of_mul_eq_one_right
rw [← map_mul]
norm_cast
simp only [hd, OneMemClass.coe_one, map_one]
mpr h := by
have ⟨v, h⟩ := h
apply IsRelPrime.isUnit_of_dvd (num_den_reduced A x).symm
use v
apply_fun algebraMap A K
· simp only [map_mul, h]
rw [mul_comm, ← div_eq_iff]
· simp only [mk'_num_den']
intro h
replace h : algebraMap A K (den A x : A) = algebraMap A K 0 := by convert h; simp
exact nonZeroDivisors.coe_ne_zero _ <| NoZeroSMulDivisors.algebraMap_injective A K h
exact NoZeroSMulDivisors.algebraMap_injective A K
theorem isUnit_den_zero : IsUnit (den A (0 : K) : A) := by
simp [isUnit_den_iff, IsLocalization.isInteger_zero]
@[deprecated isUnit_den_zero (since := "2024-07-11")]
theorem isUnit_den_of_num_eq_zero {x : K} (h : num A x = 0) : IsUnit (den A x : A) :=
eq_zero_of_num_eq_zero h ▸ isUnit_den_zero
lemma associated_den_num_inv (x : K) (hx : x ≠ 0) : Associated (den A x : A) (num A x⁻¹) :=
associated_of_dvd_dvd
(IsRelPrime.dvd_of_dvd_mul_right (IsFractionRing.num_den_reduced A x).symm <|
dvd_of_mul_left_dvd (a := (den A x⁻¹ : A)) <| dvd_of_eq <|
NoZeroSMulDivisors.algebraMap_injective A K <| Eq.symm <| eq_of_div_eq_one
(by simp [mul_div_mul_comm, hx]))
(IsRelPrime.dvd_of_dvd_mul_right (IsFractionRing.num_den_reduced A x⁻¹) <|
dvd_of_mul_left_dvd (a := (num A x : A)) <| dvd_of_eq <|
NoZeroSMulDivisors.algebraMap_injective A K <| eq_of_div_eq_one
(by simp [mul_div_mul_comm, hx]))
lemma associated_num_den_inv (x : K) (hx : x ≠ 0) : Associated (num A x : A) (den A x⁻¹) := by
have : Associated (num A x⁻¹⁻¹ : A) (den A x⁻¹) :=
(associated_den_num_inv x⁻¹ (inv_ne_zero hx)).symm
rw [inv_inv] at this
exact this
end NumDen
variable (S)
end IsFractionRing
|
RingTheory\Localization\Submodule.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.Localization.FractionRing
import Mathlib.RingTheory.Localization.Ideal
import Mathlib.RingTheory.Noetherian
/-!
# Submodules in localizations of commutative rings
## Implementation notes
See `RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
variable {R : Type*} [CommRing R] (M : Submonoid R) (S : Type*) [CommRing S]
variable [Algebra R S] {P : Type*} [CommRing P]
namespace IsLocalization
-- This was previously a `hasCoe` instance, but if `S = R` then this will loop.
-- It could be a `hasCoeT` instance, but we keep it explicit here to avoid slowing down
-- the rest of the library.
/-- Map from ideals of `R` to submodules of `S` induced by `f`. -/
def coeSubmodule (I : Ideal R) : Submodule R S :=
Submodule.map (Algebra.linearMap R S) I
theorem mem_coeSubmodule (I : Ideal R) {x : S} :
x ∈ coeSubmodule S I ↔ ∃ y : R, y ∈ I ∧ algebraMap R S y = x :=
Iff.rfl
theorem coeSubmodule_mono {I J : Ideal R} (h : I ≤ J) : coeSubmodule S I ≤ coeSubmodule S J :=
Submodule.map_mono h
@[simp]
theorem coeSubmodule_bot : coeSubmodule S (⊥ : Ideal R) = ⊥ := by
rw [coeSubmodule, Submodule.map_bot]
@[simp]
theorem coeSubmodule_top : coeSubmodule S (⊤ : Ideal R) = 1 := by
rw [coeSubmodule, Submodule.map_top, Submodule.one_eq_range]
@[simp]
theorem coeSubmodule_sup (I J : Ideal R) :
coeSubmodule S (I ⊔ J) = coeSubmodule S I ⊔ coeSubmodule S J :=
Submodule.map_sup _ _ _
@[simp]
theorem coeSubmodule_mul (I J : Ideal R) :
coeSubmodule S (I * J) = coeSubmodule S I * coeSubmodule S J :=
Submodule.map_mul _ _ (Algebra.ofId R S)
theorem coeSubmodule_fg (hS : Function.Injective (algebraMap R S)) (I : Ideal R) :
Submodule.FG (coeSubmodule S I) ↔ Submodule.FG I :=
⟨Submodule.fg_of_fg_map _ (LinearMap.ker_eq_bot.mpr hS), Submodule.FG.map _⟩
@[simp]
theorem coeSubmodule_span (s : Set R) :
coeSubmodule S (Ideal.span s) = Submodule.span R (algebraMap R S '' s) := by
rw [IsLocalization.coeSubmodule, Ideal.span, Submodule.map_span]
rfl
-- @[simp] -- Porting note (#10618): simp can prove this
theorem coeSubmodule_span_singleton (x : R) :
coeSubmodule S (Ideal.span {x}) = Submodule.span R {(algebraMap R S) x} := by
rw [coeSubmodule_span, Set.image_singleton]
variable {g : R →+* P}
variable {T : Submonoid P} (hy : M ≤ T.comap g) {Q : Type*} [CommRing Q]
variable [Algebra P Q] [IsLocalization T Q]
variable [IsLocalization M S]
section
theorem isNoetherianRing (h : IsNoetherianRing R) : IsNoetherianRing S := by
rw [isNoetherianRing_iff, isNoetherian_iff_wellFounded] at h ⊢
exact OrderEmbedding.wellFounded (IsLocalization.orderEmbedding M S).dual h
end
variable {S M}
@[mono]
theorem coeSubmodule_le_coeSubmodule (h : M ≤ nonZeroDivisors R) {I J : Ideal R} :
coeSubmodule S I ≤ coeSubmodule S J ↔ I ≤ J :=
-- Note: #8386 had to specify the value of `f` here:
Submodule.map_le_map_iff_of_injective (f := Algebra.linearMap R S) (IsLocalization.injective _ h)
_ _
@[mono]
theorem coeSubmodule_strictMono (h : M ≤ nonZeroDivisors R) :
StrictMono (coeSubmodule S : Ideal R → Submodule R S) :=
strictMono_of_le_iff_le fun _ _ => (coeSubmodule_le_coeSubmodule h).symm
variable (S)
theorem coeSubmodule_injective (h : M ≤ nonZeroDivisors R) :
Function.Injective (coeSubmodule S : Ideal R → Submodule R S) :=
injective_of_le_imp_le _ fun hl => (coeSubmodule_le_coeSubmodule h).mp hl
theorem coeSubmodule_isPrincipal {I : Ideal R} (h : M ≤ nonZeroDivisors R) :
(coeSubmodule S I).IsPrincipal ↔ I.IsPrincipal := by
constructor <;> rintro ⟨⟨x, hx⟩⟩
· have x_mem : x ∈ coeSubmodule S I := hx.symm ▸ Submodule.mem_span_singleton_self x
obtain ⟨x, _, rfl⟩ := (mem_coeSubmodule _ _).mp x_mem
refine ⟨⟨x, coeSubmodule_injective S h ?_⟩⟩
rw [Ideal.submodule_span_eq, hx, coeSubmodule_span_singleton]
· refine ⟨⟨algebraMap R S x, ?_⟩⟩
rw [hx, Ideal.submodule_span_eq, coeSubmodule_span_singleton]
variable {S} (M)
theorem mem_span_iff {N : Type*} [AddCommGroup N] [Module R N] [Module S N] [IsScalarTower R S N]
{x : N} {a : Set N} :
x ∈ Submodule.span S a ↔ ∃ y ∈ Submodule.span R a, ∃ z : M, x = mk' S 1 z • y := by
constructor
· intro h
refine Submodule.span_induction h ?_ ?_ ?_ ?_
· rintro x hx
exact ⟨x, Submodule.subset_span hx, 1, by rw [mk'_one, _root_.map_one, one_smul]⟩
· exact ⟨0, Submodule.zero_mem _, 1, by rw [mk'_one, _root_.map_one, one_smul]⟩
· rintro _ _ ⟨y, hy, z, rfl⟩ ⟨y', hy', z', rfl⟩
refine
⟨(z' : R) • y + (z : R) • y',
Submodule.add_mem _ (Submodule.smul_mem _ _ hy) (Submodule.smul_mem _ _ hy'), z * z', ?_⟩
rw [smul_add, ← IsScalarTower.algebraMap_smul S (z : R), ←
IsScalarTower.algebraMap_smul S (z' : R), smul_smul, smul_smul]
congr 1
· rw [← mul_one (1 : R), mk'_mul, mul_assoc, mk'_spec, _root_.map_one, mul_one, mul_one]
· rw [← mul_one (1 : R), mk'_mul, mul_right_comm, mk'_spec, _root_.map_one, mul_one, one_mul]
· rintro a _ ⟨y, hy, z, rfl⟩
obtain ⟨y', z', rfl⟩ := mk'_surjective M a
refine ⟨y' • y, Submodule.smul_mem _ _ hy, z' * z, ?_⟩
rw [← IsScalarTower.algebraMap_smul S y', smul_smul, ← mk'_mul, smul_smul,
mul_comm (mk' S _ _), mul_mk'_eq_mk'_of_mul]
· rintro ⟨y, hy, z, rfl⟩
exact Submodule.smul_mem _ _ (Submodule.span_subset_span R S _ hy)
theorem mem_span_map {x : S} {a : Set R} :
x ∈ Ideal.span (algebraMap R S '' a) ↔ ∃ y ∈ Ideal.span a, ∃ z : M, x = mk' S y z := by
refine (mem_span_iff M).trans ?_
constructor
· rw [← coeSubmodule_span]
rintro ⟨_, ⟨y, hy, rfl⟩, z, hz⟩
refine ⟨y, hy, z, ?_⟩
rw [hz, Algebra.linearMap_apply, smul_eq_mul, mul_comm, mul_mk'_eq_mk'_of_mul, mul_one]
· rintro ⟨y, hy, z, hz⟩
refine ⟨algebraMap R S y, Submodule.map_mem_span_algebraMap_image _ _ hy, z, ?_⟩
rw [hz, smul_eq_mul, mul_comm, mul_mk'_eq_mk'_of_mul, mul_one]
end IsLocalization
namespace IsFractionRing
open IsLocalization
variable {A K : Type*} [CommRing A]
section CommRing
variable [CommRing K] [Algebra R K] [IsFractionRing R K] [Algebra A K] [IsFractionRing A K]
@[simp, mono]
theorem coeSubmodule_le_coeSubmodule {I J : Ideal R} :
coeSubmodule K I ≤ coeSubmodule K J ↔ I ≤ J :=
IsLocalization.coeSubmodule_le_coeSubmodule le_rfl
@[mono]
theorem coeSubmodule_strictMono : StrictMono (coeSubmodule K : Ideal R → Submodule R K) :=
strictMono_of_le_iff_le fun _ _ => coeSubmodule_le_coeSubmodule.symm
variable (R K)
theorem coeSubmodule_injective : Function.Injective (coeSubmodule K : Ideal R → Submodule R K) :=
injective_of_le_imp_le _ fun hl => coeSubmodule_le_coeSubmodule.mp hl
@[simp]
theorem coeSubmodule_isPrincipal {I : Ideal R} : (coeSubmodule K I).IsPrincipal ↔ I.IsPrincipal :=
IsLocalization.coeSubmodule_isPrincipal _ le_rfl
end CommRing
end IsFractionRing
|
RingTheory\Localization\Away\AdjoinRoot.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.AdjoinRoot
import Mathlib.RingTheory.Localization.Away.Basic
/-!
The `R`-`AlgEquiv` between the localization of `R` away from `r` and
`R` with an inverse of `r` adjoined.
-/
open Polynomial AdjoinRoot Localization
variable {R : Type*} [CommRing R]
-- Porting note: removed `IsLocalization.algHom_subsingleton` due to
-- `cannot find synthesization order for instance`
attribute [local instance] AdjoinRoot.algHom_subsingleton
/-- The `R`-`AlgEquiv` between the localization of `R` away from `r` and
`R` with an inverse of `r` adjoined. -/
noncomputable def Localization.awayEquivAdjoin (r : R) : Away r ≃ₐ[R] AdjoinRoot (C r * X - 1) :=
AlgEquiv.ofAlgHom
{ awayLift _ r
-- Porting note: This argument used to be found automatically, i.e. `_`
(isUnit_of_mul_eq_one ((algebraMap R (AdjoinRoot (C r * X - 1))) r) (root (C r * X - 1))
(root_isInv r)) with
commutes' :=
IsLocalization.Away.AwayMap.lift_eq r (isUnit_of_mul_eq_one _ _ <| root_isInv r) }
(liftHom _ (IsLocalization.Away.invSelf r) <| by
simp only [map_sub, map_mul, aeval_C, aeval_X, IsLocalization.Away.mul_invSelf, aeval_one,
sub_self])
(Subsingleton.elim _ _)
-- Porting note: fix since `IsLocalization.algHom_subsingleton` is no local instance anymore
(Subsingleton.elim (h := IsLocalization.algHom_subsingleton (Submonoid.powers r)) _ _)
theorem IsLocalization.adjoin_inv (r : R) : IsLocalization.Away r (AdjoinRoot <| C r * X - 1) :=
IsLocalization.isLocalization_of_algEquiv _ (Localization.awayEquivAdjoin r)
theorem IsLocalization.Away.finitePresentation (r : R) {S} [CommRing S] [Algebra R S]
[IsLocalization.Away r S] : Algebra.FinitePresentation R S :=
(AdjoinRoot.finitePresentation _).equiv <|
(Localization.awayEquivAdjoin r).symm.trans <| IsLocalization.algEquiv (Submonoid.powers r) _ _
|
RingTheory\Localization\Away\Basic.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Mario Carneiro, Johan Commelin, Amelia Livingston, Anne Baanen
-/
import Mathlib.RingTheory.UniqueFactorizationDomain
import Mathlib.RingTheory.Localization.Basic
/-!
# Localizations away from an element
## Main definitions
* `IsLocalization.Away (x : R) S` expresses that `S` is a localization away from `x`, as an
abbreviation of `IsLocalization (Submonoid.powers x) S`.
* `exists_reduced_fraction' (hb : b ≠ 0)` produces a reduced fraction of the form `b = a * x^n` for
some `n : ℤ` and some `a : R` that is not divisible by `x`.
## Implementation notes
See `Mathlib/RingTheory/Localization/Basic.lean` for a design overview.
## Tags
localization, ring localization, commutative ring localization, characteristic predicate,
commutative ring, field of fractions
-/
section CommSemiring
variable {R : Type*} [CommSemiring R] (M : Submonoid R) {S : Type*} [CommSemiring S]
variable [Algebra R S] {P : Type*} [CommSemiring P]
namespace IsLocalization
section Away
variable (x : R)
/-- Given `x : R`, the typeclass `IsLocalization.Away x S` states that `S` is
isomorphic to the localization of `R` at the submonoid generated by `x`. -/
abbrev Away (S : Type*) [CommSemiring S] [Algebra R S] :=
IsLocalization (Submonoid.powers x) S
namespace Away
variable [IsLocalization.Away x S]
/-- Given `x : R` and a localization map `F : R →+* S` away from `x`, `invSelf` is `(F x)⁻¹`. -/
noncomputable def invSelf : S :=
mk' S (1 : R) ⟨x, Submonoid.mem_powers _⟩
@[simp]
theorem mul_invSelf : algebraMap R S x * invSelf x = 1 := by
convert IsLocalization.mk'_mul_mk'_eq_one (M := Submonoid.powers x) (S := S) _ 1
symm
apply IsLocalization.mk'_one
/-- For `s : S` with `S` being the localization of `R` away from `x`,
this is a choice of `(r, n) : R × ℕ` such that `s * algebraMap R S (x ^ n) = algebraMap R S r`. -/
noncomputable def sec (s : S) : R × ℕ :=
⟨(IsLocalization.sec (Submonoid.powers x) s).1,
(IsLocalization.sec (Submonoid.powers x) s).2.property.choose⟩
lemma sec_spec (s : S) : s * (algebraMap R S) (x ^ (IsLocalization.Away.sec x s).2) =
algebraMap R S (IsLocalization.Away.sec x s).1 := by
simp only [IsLocalization.Away.sec, ← IsLocalization.sec_spec]
congr
exact (IsLocalization.sec (Submonoid.powers x) s).2.property.choose_spec
variable {g : R →+* P}
/-- Given `x : R`, a localization map `F : R →+* S` away from `x`, and a map of `CommSemiring`s
`g : R →+* P` such that `g x` is invertible, the homomorphism induced from `S` to `P` sending
`z : S` to `g y * (g x)⁻ⁿ`, where `y : R, n : ℕ` are such that `z = F y * (F x)⁻ⁿ`. -/
noncomputable def lift (hg : IsUnit (g x)) : S →+* P :=
IsLocalization.lift fun y : Submonoid.powers x =>
show IsUnit (g y.1) by
obtain ⟨n, hn⟩ := y.2
rw [← hn, g.map_pow]
exact IsUnit.map (powMonoidHom n : P →* P) hg
@[simp]
theorem AwayMap.lift_eq (hg : IsUnit (g x)) (a : R) : lift x hg ((algebraMap R S) a) = g a :=
IsLocalization.lift_eq _ _
@[simp]
theorem AwayMap.lift_comp (hg : IsUnit (g x)) : (lift x hg).comp (algebraMap R S) = g :=
IsLocalization.lift_comp _
/-- Given `x y : R` and localizations `S`, `P` away from `x` and `x * y`
respectively, the homomorphism induced from `S` to `P`. -/
noncomputable def awayToAwayRight (y : R) [Algebra R P] [IsLocalization.Away (x * y) P] : S →+* P :=
lift x <|
show IsUnit ((algebraMap R P) x) from
isUnit_of_mul_eq_one ((algebraMap R P) x) (mk' P y ⟨x * y, Submonoid.mem_powers _⟩) <| by
rw [mul_mk'_eq_mk'_of_mul, mk'_self]
variable (S) (Q : Type*) [CommSemiring Q] [Algebra P Q]
/-- Given a map `f : R →+* S` and an element `r : R`, we may construct a map `Rᵣ →+* Sᵣ`. -/
noncomputable def map (f : R →+* P) (r : R) [IsLocalization.Away r S]
[IsLocalization.Away (f r) Q] : S →+* Q :=
IsLocalization.map Q f
(show Submonoid.powers r ≤ (Submonoid.powers (f r)).comap f by
rintro x ⟨n, rfl⟩
use n
simp)
section Algebra
variable {A : Type*} [CommRing A] [Algebra R A]
variable {B : Type*} [CommRing B] [Algebra R B]
variable (Aₚ : Type*) [CommRing Aₚ] [Algebra A Aₚ] [Algebra R Aₚ] [IsScalarTower R A Aₚ]
variable (Bₚ : Type*) [CommRing Bₚ] [Algebra B Bₚ] [Algebra R Bₚ] [IsScalarTower R B Bₚ]
instance {f : A →+* B} (a : A) [Away (f a) Bₚ] : IsLocalization (.map f (.powers a)) Bₚ := by
simpa
/-- Given a algebra map `f : A →ₐ[R] B` and an element `a : A`, we may construct a map
`Aₐ →ₐ[R] Bₐ`. -/
noncomputable def mapₐ (f : A →ₐ[R] B) (a : A) [Away a Aₚ] [Away (f a) Bₚ] : Aₚ →ₐ[R] Bₚ :=
⟨map Aₚ Bₚ f.toRingHom a, fun r ↦ by
dsimp only [AlgHom.toRingHom_eq_coe, map, RingHom.coe_coe, OneHom.toFun_eq_coe]
rw [IsScalarTower.algebraMap_apply R A Aₚ, IsScalarTower.algebraMap_eq R B Bₚ]
erw [IsLocalization.map_eq]
simp⟩
@[simp]
lemma mapₐ_apply (f : A →ₐ[R] B) (a : A) [Away a Aₚ] [Away (f a) Bₚ] (x : Aₚ) :
mapₐ Aₚ Bₚ f a x = map Aₚ Bₚ f.toRingHom a x :=
rfl
variable {Aₚ} {Bₚ}
lemma mapₐ_injective_of_injective {f : A →ₐ[R] B} (a : A) [Away a Aₚ] [Away (f a) Bₚ]
(hf : Function.Injective f) : Function.Injective (mapₐ Aₚ Bₚ f a) :=
IsLocalization.map_injective_of_injective _ _ _ hf
lemma mapₐ_surjective_of_surjective {f : A →ₐ[R] B} (a : A) [Away a Aₚ] [Away (f a) Bₚ]
(hf : Function.Surjective f) : Function.Surjective (mapₐ Aₚ Bₚ f a) :=
have : IsLocalization (Submonoid.map f.toRingHom (Submonoid.powers a)) Bₚ := by
simp only [AlgHom.toRingHom_eq_coe, Submonoid.map_powers, RingHom.coe_coe]
infer_instance
IsLocalization.map_surjective_of_surjective _ _ _ hf
end Algebra
end Away
end Away
variable [IsLocalization M S]
section AtUnits
variable (R) (S)
/-- The localization away from a unit is isomorphic to the ring. -/
noncomputable def atUnit (x : R) (e : IsUnit x) [IsLocalization.Away x S] : R ≃ₐ[R] S :=
atUnits R (Submonoid.powers x)
(by rwa [Submonoid.powers_eq_closure, Submonoid.closure_le, Set.singleton_subset_iff])
/-- The localization at one is isomorphic to the ring. -/
noncomputable def atOne [IsLocalization.Away (1 : R) S] : R ≃ₐ[R] S :=
@atUnit R _ S _ _ (1 : R) isUnit_one _
theorem away_of_isUnit_of_bijective {R : Type*} (S : Type*) [CommRing R] [CommRing S]
[Algebra R S] {r : R} (hr : IsUnit r) (H : Function.Bijective (algebraMap R S)) :
IsLocalization.Away r S :=
{ map_units' := by
rintro ⟨_, n, rfl⟩
exact (algebraMap R S).isUnit_map (hr.pow _)
surj' := fun z => by
obtain ⟨z', rfl⟩ := H.2 z
exact ⟨⟨z', 1⟩, by simp⟩
exists_of_eq := fun {x y} => by
erw [H.1.eq_iff]
rintro rfl
exact ⟨1, rfl⟩ }
end AtUnits
section Prod
lemma away_of_isIdempotentElem {R S} [CommRing R] [CommRing S] [Algebra R S]
{e : R} (he : IsIdempotentElem e)
(H : RingHom.ker (algebraMap R S) = Ideal.span {1 - e})
(H' : Function.Surjective (algebraMap R S)) :
IsLocalization.Away e S where
map_units' r := by
have : algebraMap R S e = 1 := by
rw [← (algebraMap R S).map_one, eq_comm, ← sub_eq_zero, ← map_sub, ← RingHom.mem_ker,
H, Ideal.mem_span_singleton]
obtain ⟨r, n, rfl⟩ := r
simp [this]
surj' z := by
obtain ⟨z, rfl⟩ := H' z
exact ⟨⟨z, 1⟩, by simp⟩
exists_of_eq {x y} h := by
rw [← sub_eq_zero, ← map_sub, ← RingHom.mem_ker, H, Ideal.mem_span_singleton] at h
obtain ⟨k, hk⟩ := h
refine ⟨⟨e, Submonoid.mem_powers e⟩, ?_⟩
rw [← sub_eq_zero, ← mul_sub, hk, ← mul_assoc, mul_sub, mul_one, he.eq, sub_self, zero_mul]
instance away_fst {R S} [CommRing R] [CommRing S] :
letI := (RingHom.fst R S).toAlgebra
IsLocalization.Away (R := R × S) (1, 0) R := by
letI := (RingHom.fst R S).toAlgebra
apply away_of_isIdempotentElem
· ext <;> simp
· ext x
simp only [RingHom.algebraMap_toAlgebra, RingHom.mem_ker, RingHom.coe_fst,
Ideal.mem_span_singleton, Prod.one_eq_mk, Prod.mk_sub_mk, sub_self, sub_zero]
constructor
· intro e; use x; ext <;> simp [e]
· rintro ⟨⟨i, j⟩, rfl⟩; simp
· exact Prod.fst_surjective
instance away_snd {R S} [CommRing R] [CommRing S] :
letI := (RingHom.snd R S).toAlgebra
IsLocalization.Away (R := R × S) (0, 1) S := by
letI := (RingHom.snd R S).toAlgebra
apply away_of_isIdempotentElem
· ext <;> simp
· ext x
simp only [RingHom.algebraMap_toAlgebra, RingHom.mem_ker, RingHom.coe_snd,
Ideal.mem_span_singleton, Prod.one_eq_mk, Prod.mk_sub_mk, sub_self, sub_zero]
constructor
· intro e; use x; ext <;> simp [e]
· rintro ⟨⟨i, j⟩, rfl⟩; simp
· exact Prod.snd_surjective
end Prod
end IsLocalization
namespace Localization
open IsLocalization
variable {M}
/-- Given a map `f : R →+* S` and an element `r : R`, such that `f r` is invertible,
we may construct a map `Rᵣ →+* S`. -/
noncomputable abbrev awayLift (f : R →+* P) (r : R) (hr : IsUnit (f r)) :
Localization.Away r →+* P :=
IsLocalization.Away.lift r hr
/-- Given a map `f : R →+* S` and an element `r : R`, we may construct a map `Rᵣ →+* Sᵣ`. -/
noncomputable abbrev awayMap (f : R →+* P) (r : R) :
Localization.Away r →+* Localization.Away (f r) :=
IsLocalization.Away.map _ _ f r
variable {A : Type*} [CommRing A] [Algebra R A]
variable {B : Type*} [CommRing B] [Algebra R B]
/-- Given a map `f : A →ₐ[R] B` and an element `a : A`, we may construct a map `Aₐ →ₐ[R] Bₐ`. -/
noncomputable abbrev awayMapₐ (f : A →ₐ[R] B) (a : A) :
Localization.Away a →ₐ[R] Localization.Away (f a) :=
IsLocalization.Away.mapₐ _ _ f a
end Localization
end CommSemiring
open Localization
variable {R : Type*} [CommRing R]
section NumDen
open IsLocalization
variable (x : R)
variable (B : Type*) [CommRing B] [Algebra R B] [IsLocalization.Away x B]
/-- `selfZPow x (m : ℤ)` is `x ^ m` as an element of the localization away from `x`. -/
noncomputable def selfZPow (m : ℤ) : B :=
if _ : 0 ≤ m then algebraMap _ _ x ^ m.natAbs else mk' _ (1 : R) (Submonoid.pow x m.natAbs)
theorem selfZPow_of_nonneg {n : ℤ} (hn : 0 ≤ n) : selfZPow x B n = algebraMap R B x ^ n.natAbs :=
dif_pos hn
@[simp]
theorem selfZPow_natCast (d : ℕ) : selfZPow x B d = algebraMap R B x ^ d :=
selfZPow_of_nonneg _ _ (Int.natCast_nonneg d)
@[simp]
theorem selfZPow_zero : selfZPow x B 0 = 1 := by
simp [selfZPow_of_nonneg _ _ le_rfl]
theorem selfZPow_of_neg {n : ℤ} (hn : n < 0) :
selfZPow x B n = mk' _ (1 : R) (Submonoid.pow x n.natAbs) :=
dif_neg hn.not_le
theorem selfZPow_of_nonpos {n : ℤ} (hn : n ≤ 0) :
selfZPow x B n = mk' _ (1 : R) (Submonoid.pow x n.natAbs) := by
by_cases hn0 : n = 0
· simp [hn0, selfZPow_zero, Submonoid.pow_apply]
· simp [selfZPow_of_neg _ _ (lt_of_le_of_ne hn hn0)]
@[simp]
theorem selfZPow_neg_natCast (d : ℕ) : selfZPow x B (-d) = mk' _ (1 : R) (Submonoid.pow x d) := by
simp [selfZPow_of_nonpos _ _ (neg_nonpos.mpr (Int.natCast_nonneg d))]
@[simp]
theorem selfZPow_sub_natCast {n m : ℕ} :
selfZPow x B (n - m) = mk' _ (x ^ n) (Submonoid.pow x m) := by
by_cases h : m ≤ n
· rw [IsLocalization.eq_mk'_iff_mul_eq, Submonoid.pow_apply, Subtype.coe_mk, ← Int.ofNat_sub h,
selfZPow_natCast, ← map_pow, ← map_mul, ← pow_add, Nat.sub_add_cancel h]
· rw [← neg_sub, ← Int.ofNat_sub (le_of_not_le h), selfZPow_neg_natCast,
IsLocalization.mk'_eq_iff_eq]
simp [Submonoid.pow_apply, ← pow_add, Nat.sub_add_cancel (le_of_not_le h)]
@[deprecated (since := "2024-04-05")] alias selfZPow_coe_nat := selfZPow_natCast
@[deprecated (since := "2024-04-05")] alias selfZPow_neg_coe_nat := selfZPow_neg_natCast
@[deprecated (since := "2024-04-05")] alias selfZPow_sub_cast_nat := selfZPow_sub_natCast
@[simp]
theorem selfZPow_add {n m : ℤ} : selfZPow x B (n + m) = selfZPow x B n * selfZPow x B m := by
rcases le_or_lt 0 n with hn | hn <;> rcases le_or_lt 0 m with hm | hm
· rw [selfZPow_of_nonneg _ _ hn, selfZPow_of_nonneg _ _ hm,
selfZPow_of_nonneg _ _ (add_nonneg hn hm), Int.natAbs_add_of_nonneg hn hm, pow_add]
· have : n + m = n.natAbs - m.natAbs := by
rw [Int.natAbs_of_nonneg hn, Int.ofNat_natAbs_of_nonpos hm.le, sub_neg_eq_add]
rw [selfZPow_of_nonneg _ _ hn, selfZPow_of_neg _ _ hm, this, selfZPow_sub_natCast,
IsLocalization.mk'_eq_mul_mk'_one, map_pow]
· have : n + m = m.natAbs - n.natAbs := by
rw [Int.natAbs_of_nonneg hm, Int.ofNat_natAbs_of_nonpos hn.le, sub_neg_eq_add, add_comm]
rw [selfZPow_of_nonneg _ _ hm, selfZPow_of_neg _ _ hn, this, selfZPow_sub_natCast,
IsLocalization.mk'_eq_mul_mk'_one, map_pow, mul_comm]
· rw [selfZPow_of_neg _ _ hn, selfZPow_of_neg _ _ hm, selfZPow_of_neg _ _ (add_neg hn hm),
Int.natAbs_add_of_nonpos hn.le hm.le, ← mk'_mul, one_mul]
congr
ext
simp [pow_add]
theorem selfZPow_mul_neg (d : ℤ) : selfZPow x B d * selfZPow x B (-d) = 1 := by
by_cases hd : d ≤ 0
· erw [selfZPow_of_nonpos x B hd, selfZPow_of_nonneg, ← map_pow, Int.natAbs_neg,
IsLocalization.mk'_spec, map_one]
apply nonneg_of_neg_nonpos
rwa [neg_neg]
· erw [selfZPow_of_nonneg x B (le_of_not_le hd), selfZPow_of_nonpos, ← map_pow, Int.natAbs_neg,
@IsLocalization.mk'_spec' R _ (Submonoid.powers x) B _ _ _ 1 (Submonoid.pow x d.natAbs),
map_one]
refine nonpos_of_neg_nonneg (le_of_lt ?_)
rwa [neg_neg, ← not_le]
theorem selfZPow_neg_mul (d : ℤ) : selfZPow x B (-d) * selfZPow x B d = 1 := by
rw [mul_comm, selfZPow_mul_neg x B d]
theorem selfZPow_pow_sub (a : R) (b : B) (m d : ℤ) :
selfZPow x B (m - d) * mk' B a (1 : Submonoid.powers x) = b ↔
selfZPow x B m * mk' B a (1 : Submonoid.powers x) = selfZPow x B d * b := by
rw [sub_eq_add_neg, selfZPow_add, mul_assoc, mul_comm _ (mk' B a 1), ← mul_assoc]
constructor
· intro h
have := congr_arg (fun s : B => s * selfZPow x B d) h
simp only at this
rwa [mul_assoc, mul_assoc, selfZPow_neg_mul, mul_one, mul_comm b _] at this
· intro h
have := congr_arg (fun s : B => s * selfZPow x B (-d)) h
simp only at this
rwa [mul_comm _ b, mul_assoc b _ _, selfZPow_mul_neg, mul_one] at this
variable [IsDomain R] [WfDvdMonoid R]
theorem exists_reduced_fraction' {b : B} (hb : b ≠ 0) (hx : Irreducible x) :
∃ (a : R) (n : ℤ), ¬x ∣ a ∧ selfZPow x B n * algebraMap R B a = b := by
obtain ⟨⟨a₀, y⟩, H⟩ := surj (Submonoid.powers x) b
obtain ⟨d, hy⟩ := (Submonoid.mem_powers_iff y.1 x).mp y.2
have ha₀ : a₀ ≠ 0 := by
haveI :=
@isDomain_of_le_nonZeroDivisors B _ R _ _ _ (Submonoid.powers x) _
(powers_le_nonZeroDivisors_of_noZeroDivisors hx.ne_zero)
simp only [map_zero, ← hy, map_pow] at H
apply ((injective_iff_map_eq_zero' (algebraMap R B)).mp _ a₀).mpr.mt
· rw [← H]
apply mul_ne_zero hb (pow_ne_zero _ _)
exact
IsLocalization.to_map_ne_zero_of_mem_nonZeroDivisors B
(powers_le_nonZeroDivisors_of_noZeroDivisors hx.ne_zero)
(mem_nonZeroDivisors_iff_ne_zero.mpr hx.ne_zero)
· exact IsLocalization.injective B (powers_le_nonZeroDivisors_of_noZeroDivisors hx.ne_zero)
simp only [← hy] at H
obtain ⟨m, a, hyp1, hyp2⟩ := WfDvdMonoid.max_power_factor ha₀ hx
refine ⟨a, m - d, ?_⟩
rw [← mk'_one (M := Submonoid.powers x) B, selfZPow_pow_sub, selfZPow_natCast, selfZPow_natCast,
← map_pow _ _ d, mul_comm _ b, H, hyp2, map_mul, map_pow _ _ m]
exact ⟨hyp1, congr_arg _ (IsLocalization.mk'_one _ _)⟩
end NumDen
|
RingTheory\LocalRing\Basic.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.RingTheory.LocalRing.Defs
import Mathlib.RingTheory.Ideal.Basic
/-!
# Local rings
We prove basic properties of local rings.
-/
variable {R S : Type*}
section CommSemiring
variable [CommSemiring R]
namespace LocalRing
theorem of_isUnit_or_isUnit_of_isUnit_add [Nontrivial R]
(h : ∀ a b : R, IsUnit (a + b) → IsUnit a ∨ IsUnit b) : LocalRing R :=
⟨fun {a b} hab => h a b <| hab.symm ▸ isUnit_one⟩
/-- A semiring is local if it is nontrivial and the set of nonunits is closed under the addition. -/
theorem of_nonunits_add [Nontrivial R]
(h : ∀ a b : R, a ∈ nonunits R → b ∈ nonunits R → a + b ∈ nonunits R) : LocalRing R :=
⟨fun {a b} hab => or_iff_not_and_not.2 fun H => h a b H.1 H.2 <| hab.symm ▸ isUnit_one⟩
/-- A semiring is local if it has a unique maximal ideal. -/
theorem of_unique_max_ideal (h : ∃! I : Ideal R, I.IsMaximal) : LocalRing R :=
@of_nonunits_add _ _
(nontrivial_of_ne (0 : R) 1 <|
let ⟨I, Imax, _⟩ := h
fun H : 0 = 1 => Imax.1.1 <| I.eq_top_iff_one.2 <| H ▸ I.zero_mem)
fun x y hx hy H =>
let ⟨I, Imax, Iuniq⟩ := h
let ⟨Ix, Ixmax, Hx⟩ := exists_max_ideal_of_mem_nonunits hx
let ⟨Iy, Iymax, Hy⟩ := exists_max_ideal_of_mem_nonunits hy
have xmemI : x ∈ I := Iuniq Ix Ixmax ▸ Hx
have ymemI : y ∈ I := Iuniq Iy Iymax ▸ Hy
Imax.1.1 <| I.eq_top_of_isUnit_mem (I.add_mem xmemI ymemI) H
theorem of_unique_nonzero_prime (h : ∃! P : Ideal R, P ≠ ⊥ ∧ Ideal.IsPrime P) : LocalRing R :=
of_unique_max_ideal
(by
rcases h with ⟨P, ⟨hPnonzero, hPnot_top, _⟩, hPunique⟩
refine ⟨P, ⟨⟨hPnot_top, ?_⟩⟩, fun M hM => hPunique _ ⟨?_, Ideal.IsMaximal.isPrime hM⟩⟩
· refine Ideal.maximal_of_no_maximal fun M hPM hM => ne_of_lt hPM ?_
exact (hPunique _ ⟨ne_bot_of_gt hPM, Ideal.IsMaximal.isPrime hM⟩).symm
· rintro rfl
exact hPnot_top (hM.1.2 P (bot_lt_iff_ne_bot.2 hPnonzero)))
variable [LocalRing R]
theorem isUnit_or_isUnit_of_isUnit_add {a b : R} (h : IsUnit (a + b)) : IsUnit a ∨ IsUnit b := by
rcases h with ⟨u, hu⟩
rw [← Units.inv_mul_eq_one, mul_add] at hu
apply Or.imp _ _ (isUnit_or_isUnit_of_add_one hu) <;> exact isUnit_of_mul_isUnit_right
theorem nonunits_add {a b : R} (ha : a ∈ nonunits R) (hb : b ∈ nonunits R) : a + b ∈ nonunits R :=
fun H => not_or_of_not ha hb (isUnit_or_isUnit_of_isUnit_add H)
end LocalRing
end CommSemiring
namespace LocalRing
variable [CommRing R]
theorem of_isUnit_or_isUnit_one_sub_self [Nontrivial R] (h : ∀ a : R, IsUnit a ∨ IsUnit (1 - a)) :
LocalRing R :=
⟨fun {a b} hab => add_sub_cancel_left a b ▸ hab.symm ▸ h a⟩
variable [LocalRing R]
theorem isUnit_or_isUnit_one_sub_self (a : R) : IsUnit a ∨ IsUnit (1 - a) :=
isUnit_or_isUnit_of_isUnit_add <| (add_sub_cancel a 1).symm ▸ isUnit_one
theorem isUnit_of_mem_nonunits_one_sub_self (a : R) (h : 1 - a ∈ nonunits R) : IsUnit a :=
or_iff_not_imp_right.1 (isUnit_or_isUnit_one_sub_self a) h
theorem isUnit_one_sub_self_of_mem_nonunits (a : R) (h : a ∈ nonunits R) : IsUnit (1 - a) :=
or_iff_not_imp_left.1 (isUnit_or_isUnit_one_sub_self a) h
theorem of_surjective' [CommRing S] [Nontrivial S] (f : R →+* S) (hf : Function.Surjective f) :
LocalRing S :=
of_isUnit_or_isUnit_one_sub_self (by
intro b
obtain ⟨a, rfl⟩ := hf b
apply (isUnit_or_isUnit_one_sub_self a).imp <| RingHom.isUnit_map _
rw [← f.map_one, ← f.map_sub]
apply f.isUnit_map)
end LocalRing
namespace Field
variable (K : Type*) [Field K]
-- see Note [lower instance priority]
instance (priority := 100) : LocalRing K := by
classical exact LocalRing.of_isUnit_or_isUnit_one_sub_self fun a =>
if h : a = 0 then Or.inr (by rw [h, sub_zero]; exact isUnit_one)
else Or.inl <| IsUnit.mk0 a h
end Field
|
RingTheory\LocalRing\Defs.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.Algebra.Group.Units
import Mathlib.Algebra.Ring.Defs
/-!
# Local rings
Define local rings as commutative rings having a unique maximal ideal.
## Main definitions
* `LocalRing`: A predicate on semirings, stating that for any pair of elements that
adds up to `1`, one of them is a unit. In the commutative case this is shown to be equivalent
to the condition that there exists a unique maximal ideal, see
`LocalRing.of_unique_max_ideal` and `LocalRing.maximal_ideal_unique`.
-/
/-- A semiring is local if it is nontrivial and `a` or `b` is a unit whenever `a + b = 1`.
Note that `LocalRing` is a predicate. -/
class LocalRing (R : Type*) [Semiring R] extends Nontrivial R : Prop where
of_is_unit_or_is_unit_of_add_one ::
/-- in a local ring `R`, if `a + b = 1`, then either `a` is a unit or `b` is a unit. In another
word, for every `a : R`, either `a` is a unit or `1 - a` is a unit. -/
isUnit_or_isUnit_of_add_one {a b : R} (h : a + b = 1) : IsUnit a ∨ IsUnit b
|
RingTheory\LocalRing\Module.lean | /-
Copyright (c) 2024 Andrew Yang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Andrew Yang
-/
import Mathlib.RingTheory.FiniteType
import Mathlib.RingTheory.Nakayama
import Mathlib.Algebra.Module.FinitePresentation
import Mathlib.LinearAlgebra.FiniteDimensional
import Mathlib.LinearAlgebra.TensorProduct.RightExactness
import Mathlib.RingTheory.Flat.Basic
import Mathlib.RingTheory.LocalRing.ResidueField.Basic
/-!
# Finite modules over local rings
This file gathers various results about finite modules over a local ring `(R, 𝔪, k)`.
## Main results
- `LocalRing.subsingleton_tensorProduct`: If `M` is finitely generated, `k ⊗ M = 0 ↔ M = 0`.
- `Module.free_of_maximalIdeal_rTensor_injective`:
If `M` is a finitely presented module such that `m ⊗ M → M` is injective
(for example when `M` is flat), then `M` is free.
- `Module.free_of_lTensor_residueField_injective`: If `N → M → P → 0` is a presentation of `P` with
`N` finite and `M` finite free, then injectivity of `k ⊗ N → k ⊗ M` implies that `P` is free.
- `LocalRing.split_injective_iff_lTensor_residueField_injective`:
Given an `R`-linear map `l : M → N` with `M` finite and `N` finite free,
`l` is a split injection if and only if `k ⊗ l` is a (split) injection.
-/
variable {R S} [CommRing R] [CommRing S] [Algebra R S]
section
variable {M N} [AddCommGroup M] [AddCommGroup N] [Module R M] [Module R N]
open Function (Injective Surjective Exact)
open LocalRing TensorProduct
local notation "k" => ResidueField R
local notation "𝔪" => maximalIdeal R
variable {P} [AddCommGroup P] [Module R P] (f : M →ₗ[R] N) (g : N →ₗ[R] P)
namespace LocalRing
variable [LocalRing R]
theorem map_mkQ_eq {N₁ N₂ : Submodule R M} (h : N₁ ≤ N₂) (h' : N₂.FG) :
N₁.map (Submodule.mkQ (𝔪 • N₂)) = N₂.map (Submodule.mkQ (𝔪 • N₂)) ↔ N₁ = N₂ := by
constructor
· intro hN
have : N₂ ≤ 𝔪 • N₂ ⊔ N₁ := by
simpa using Submodule.comap_mono (f := Submodule.mkQ (𝔪 • N₂)) hN.ge
rw [sup_comm] at this
exact h.antisymm (Submodule.le_of_le_smul_of_le_jacobson_bot h'
(by rw [jacobson_eq_maximalIdeal]; exact bot_ne_top) this)
· rintro rfl; simp
theorem map_mkQ_eq_top {N : Submodule R M} [Module.Finite R M] :
N.map (Submodule.mkQ (𝔪 • ⊤)) = ⊤ ↔ N = ⊤ := by
rw [← map_mkQ_eq (N₁ := N) le_top Module.Finite.out, Submodule.map_top, Submodule.range_mkQ]
theorem map_tensorProduct_mk_eq_top {N : Submodule R M} [Module.Finite R M] :
N.map (TensorProduct.mk R k M 1) = ⊤ ↔ N = ⊤ := by
constructor
· intro hN
letI : Module k (M ⧸ (𝔪 • ⊤ : Submodule R M)) :=
inferInstanceAs (Module (R ⧸ 𝔪) (M ⧸ 𝔪 • (⊤ : Submodule R M)))
letI : IsScalarTower R k (M ⧸ (𝔪 • ⊤ : Submodule R M)) :=
inferInstanceAs (IsScalarTower R (R ⧸ 𝔪) (M ⧸ 𝔪 • (⊤ : Submodule R M)))
let f := AlgebraTensorModule.lift (((LinearMap.ringLmapEquivSelf k k _).symm
(Submodule.mkQ (𝔪 • ⊤ : Submodule R M))).restrictScalars R)
have : f.comp (TensorProduct.mk R k M 1) = Submodule.mkQ (𝔪 • ⊤) := by ext; simp [f]
have hf : Function.Surjective f := by
intro x; obtain ⟨x, rfl⟩ := Submodule.mkQ_surjective _ x
rw [← this, LinearMap.comp_apply]; exact ⟨_, rfl⟩
apply_fun Submodule.map f at hN
rwa [← Submodule.map_comp, this, Submodule.map_top, LinearMap.range_eq_top.mpr hf,
map_mkQ_eq_top] at hN
· rintro rfl; rw [Submodule.map_top, LinearMap.range_eq_top]
exact TensorProduct.mk_surjective R M k Ideal.Quotient.mk_surjective
theorem subsingleton_tensorProduct [Module.Finite R M] :
Subsingleton (k ⊗[R] M) ↔ Subsingleton M := by
rw [← Submodule.subsingleton_iff R, ← subsingleton_iff_bot_eq_top,
← Submodule.subsingleton_iff R, ← subsingleton_iff_bot_eq_top,
← map_tensorProduct_mk_eq_top (M := M), Submodule.map_bot]
theorem span_eq_top_of_tmul_eq_basis [Module.Finite R M] {ι}
(f : ι → M) (b : Basis ι k (k ⊗[R] M))
(hb : ∀ i, 1 ⊗ₜ f i = b i) : Submodule.span R (Set.range f) = ⊤ := by
rw [← map_tensorProduct_mk_eq_top, Submodule.map_span, ← Submodule.restrictScalars_span R k
Ideal.Quotient.mk_surjective, Submodule.restrictScalars_eq_top_iff,
← b.span_eq, ← Set.range_comp]
simp only [Function.comp, mk_apply, hb, Basis.span_eq]
end LocalRing
open Function in
/--
Given `M₁ → M₂ → M₃ → 0` and `N₁ → N₂ → N₃ → 0`,
if `M₁ ⊗ N₃ → M₂ ⊗ N₃` and `M₂ ⊗ N₁ → M₂ ⊗ N₂` are both injective,
then `M₃ ⊗ N₁ → M₃ ⊗ N₂` is also injective.
-/
theorem lTensor_injective_of_exact_of_exact_of_rTensor_injective
{M₁ M₂ M₃ N₁ N₂ N₃}
[AddCommGroup M₁] [Module R M₁] [AddCommGroup M₂] [Module R M₂] [AddCommGroup M₃] [Module R M₃]
[AddCommGroup N₁] [Module R N₁] [AddCommGroup N₂] [Module R N₂] [AddCommGroup N₃] [Module R N₃]
{f₁ : M₁ →ₗ[R] M₂} {f₂ : M₂ →ₗ[R] M₃} {g₁ : N₁ →ₗ[R] N₂} {g₂ : N₂ →ₗ[R] N₃}
(hfexact : Exact f₁ f₂) (hfsurj : Surjective f₂)
(hgexact : Exact g₁ g₂) (hgsurj : Surjective g₂)
(hfinj : Injective (f₁.rTensor N₃)) (hginj : Injective (g₁.lTensor M₂)) :
Injective (g₁.lTensor M₃) := by
rw [injective_iff_map_eq_zero]
intro x hx
obtain ⟨x, rfl⟩ := f₂.rTensor_surjective N₁ hfsurj x
have : f₂.rTensor N₂ (g₁.lTensor M₂ x) = 0 := by
rw [← hx, ← LinearMap.comp_apply, ← LinearMap.comp_apply, LinearMap.rTensor_comp_lTensor,
LinearMap.lTensor_comp_rTensor]
obtain ⟨y, hy⟩ := (rTensor_exact N₂ hfexact hfsurj _).mp this
have : g₂.lTensor M₁ y = 0 := by
apply hfinj
trans g₂.lTensor M₂ (g₁.lTensor M₂ x)
· rw [← hy, ← LinearMap.comp_apply, ← LinearMap.comp_apply, LinearMap.rTensor_comp_lTensor,
LinearMap.lTensor_comp_rTensor]
rw [← LinearMap.comp_apply, ← LinearMap.lTensor_comp, hgexact.linearMap_comp_eq_zero]
simp
obtain ⟨z, rfl⟩ := (lTensor_exact _ hgexact hgsurj _).mp this
obtain rfl : f₁.rTensor N₁ z = x := by
apply hginj
simp only [← hy, ← LinearMap.comp_apply, ← LinearMap.comp_apply, LinearMap.lTensor_comp_rTensor,
LinearMap.rTensor_comp_lTensor]
rw [← LinearMap.comp_apply, ← LinearMap.rTensor_comp, hfexact.linearMap_comp_eq_zero]
simp
namespace Module
variable [LocalRing R]
/--
If `M` is a finitely presented module over a local ring `(R, 𝔪)` such that `m ⊗ M → M` is
injective, then `M` is free.
-/
theorem free_of_maximalIdeal_rTensor_injective [Module.FinitePresentation R M]
(H : Function.Injective ((𝔪).subtype.rTensor M)) :
Module.Free R M := by
let I := Module.Free.ChooseBasisIndex k (k ⊗[R] M)
-- Let `b : I → k ⊗ M` be a `k`-basis.
let b : Basis I k _ := Module.Free.chooseBasis k (k ⊗[R] M)
letI : IsNoetherian k (k ⊗[R] (I →₀ R)) :=
isNoetherian_of_isNoetherianRing_of_finite k (k ⊗[R] (I →₀ R))
choose f hf using TensorProduct.mk_surjective R M k Ideal.Quotient.mk_surjective
-- By choosing an abitrary lift of `b` to `I → M`, we get a surjection `i : Rᴵ → M`.
let i := Finsupp.total I M R (f ∘ b)
have hi : Surjective i := by
rw [← LinearMap.range_eq_top, Finsupp.range_total]
exact LocalRing.span_eq_top_of_tmul_eq_basis (R := R) (f := f ∘ b) b (fun _ ↦ hf _)
have : Module.Finite R (LinearMap.ker i) := by
constructor
exact (Submodule.fg_top _).mpr (Module.FinitePresentation.fg_ker i hi)
-- We claim that `i` is actually a bijection,
-- hence an isomorphism exhibing `M` as the free module `Rᴵ`
refine Module.Free.of_equiv (LinearEquiv.ofBijective i ⟨?_, hi⟩)
-- By Nakayama's lemma, it suffices to show that `k ⊗ ker(i) = 0`.
rw [← LinearMap.ker_eq_bot, ← Submodule.subsingleton_iff_eq_bot,
← LocalRing.subsingleton_tensorProduct (R := R), subsingleton_iff_forall_eq 0]
have : Function.Surjective (i.baseChange k) := i.lTensor_surjective _ hi
-- By construction, `k ⊗ i : kᴵ → k ⊗ M` is bijective.
have hi' : Function.Bijective (i.baseChange k) := by
refine ⟨?_, this⟩
rw [← LinearMap.ker_eq_bot (M := k ⊗[R] (I →₀ R)) (f := i.baseChange k),
← Submodule.finrank_eq_zero (R := k) (M := k ⊗[R] (I →₀ R)),
← Nat.add_right_inj (n := FiniteDimensional.finrank k (LinearMap.range <| i.baseChange k)),
LinearMap.finrank_range_add_finrank_ker (V := k ⊗[R] (I →₀ R)),
LinearMap.range_eq_top.mpr this, finrank_top]
simp only [FiniteDimensional.finrank_tensorProduct, FiniteDimensional.finrank_self,
FiniteDimensional.finrank_finsupp_self, one_mul, add_zero]
rw [FiniteDimensional.finrank_eq_card_chooseBasisIndex]
-- On the other hand, `m ⊗ M → M` injective => `Tor₁(k, M) = 0` => `k ⊗ ker(i) → kᴵ` injective.
have := @lTensor_injective_of_exact_of_exact_of_rTensor_injective
(N₁ := LinearMap.ker i) (N₂ := I →₀ R) (N₃ := M)
(f₁ := (𝔪).subtype) (f₂ := Submodule.mkQ 𝔪) inferInstance inferInstance inferInstance
inferInstance inferInstance inferInstance
intro x
apply @this (LinearMap.ker i).subtype i (LinearMap.exact_subtype_mkQ 𝔪)
(Submodule.mkQ_surjective _) (LinearMap.exact_subtype_ker_map i) hi H
(Module.Flat.lTensor_preserves_injective_linearMap _ Subtype.val_injective)
apply hi'.injective
rw [LinearMap.baseChange_eq_ltensor]
erw [← LinearMap.comp_apply (i.lTensor k), ← LinearMap.lTensor_comp]
rw [(LinearMap.exact_subtype_ker_map i).linearMap_comp_eq_zero]
simp only [LinearMap.lTensor_zero, LinearMap.zero_apply, map_zero]
-- TODO: Generalise this to finite free modules.
theorem free_of_flat_of_localRing [Module.FinitePresentation R P] [Module.Flat R P] :
Module.Free R P :=
free_of_maximalIdeal_rTensor_injective
(Module.Flat.rTensor_preserves_injective_linearMap _ Subtype.val_injective)
/--
If `M → N → P → 0` is a presentation of `P` over a local ring `(R, 𝔪, k)` with
`M` finite and `N` finite free, then injectivity of `k ⊗ M → k ⊗ N` implies that `P` is free.
-/
theorem free_of_lTensor_residueField_injective (hg : Surjective g) (h : Exact f g)
[Module.Finite R M] [Module.Finite R N] [Module.Free R N]
(hf : Function.Injective (f.lTensor k)) :
Module.Free R P := by
have := Module.finitePresentation_of_free_of_surjective g hg
(by rw [h.linearMap_ker_eq, LinearMap.range_eq_map]; exact (Module.Finite.out).map f)
apply free_of_maximalIdeal_rTensor_injective
rw [← LinearMap.lTensor_inj_iff_rTensor_inj]
apply lTensor_injective_of_exact_of_exact_of_rTensor_injective
h hg (LinearMap.exact_subtype_mkQ 𝔪) (Submodule.mkQ_surjective _)
((LinearMap.lTensor_inj_iff_rTensor_inj _ _).mp hf)
(Module.Flat.lTensor_preserves_injective_linearMap _ Subtype.val_injective)
end Module
/--
Given a linear map `l : M → N` over a local ring `(R, 𝔪, k)`
with `M` finite and `N` finite free,
`l` is a split injection if and only if `k ⊗ l` is a (split) injection.
-/
theorem LocalRing.split_injective_iff_lTensor_residueField_injective [LocalRing R]
[Module.Finite R M] [Module.Finite R N] [Module.Free R N] (l : M →ₗ[R] N) :
(∃ l', l' ∘ₗ l = LinearMap.id) ↔ Function.Injective (l.lTensor (ResidueField R)) := by
constructor
· intro ⟨l', hl⟩
have : l'.lTensor (ResidueField R) ∘ₗ l.lTensor (ResidueField R) = .id := by
rw [← LinearMap.lTensor_comp, hl, LinearMap.lTensor_id]
exact Function.HasLeftInverse.injective ⟨_, LinearMap.congr_fun this⟩
· intro h
-- By `Module.free_of_lTensor_residueField_injective`, `k ⊗ l` injective => `N ⧸ l(M)` free.
have := Module.free_of_lTensor_residueField_injective l (LinearMap.range l).mkQ
(Submodule.mkQ_surjective _) l.exact_map_mkQ_range h
-- Hence `l(M)` is projective because `0 → l(M) → N → N ⧸ l(M) → 0` splits.
have : Module.Projective R (LinearMap.range l) := by
have := (Exact.split_tfae (LinearMap.exact_subtype_mkQ (LinearMap.range l))
Subtype.val_injective (Submodule.mkQ_surjective _)).out 0 1
obtain ⟨l', hl'⟩ := this.mp
(Module.projective_lifting_property _ _ (Submodule.mkQ_surjective _))
exact Module.Projective.of_split _ _ hl'
-- Then `0 → ker l → M → l(M) → 0` splits.
obtain ⟨l', hl'⟩ : ∃ l', l' ∘ₗ (LinearMap.ker l).subtype = LinearMap.id := by
have : Function.Exact (LinearMap.ker l).subtype
(l.codRestrict (LinearMap.range l) (LinearMap.mem_range_self l)) := by
rw [LinearMap.exact_iff, LinearMap.ker_rangeRestrict, Submodule.range_subtype]
have := (Exact.split_tfae this
Subtype.val_injective (fun ⟨x, y, e⟩ ↦ ⟨y, Subtype.ext e⟩)).out 0 1
exact this.mp (Module.projective_lifting_property _ _ (fun ⟨x, y, e⟩ ↦ ⟨y, Subtype.ext e⟩))
have : Module.Finite R (LinearMap.ker l) := by
refine Module.Finite.of_surjective l' ?_
exact Function.HasRightInverse.surjective ⟨_, DFunLike.congr_fun hl'⟩
-- And tensoring with `k` preserves the injectivity of the first arrow.
-- That is, `k ⊗ ker l → k ⊗ M` is also injective.
have H : Function.Injective ((LinearMap.ker l).subtype.lTensor k) := by
apply_fun (LinearMap.lTensor k) at hl'
rw [LinearMap.lTensor_comp, LinearMap.lTensor_id] at hl'
exact Function.HasLeftInverse.injective ⟨l'.lTensor k, DFunLike.congr_fun hl'⟩
-- But by assumption `k ⊗ M → k ⊗ l(M)` is already injective, so `k ⊗ ker l = 0`.
have : Subsingleton (k ⊗[R] LinearMap.ker l) := by
refine (subsingleton_iff_forall_eq 0).mpr fun y ↦ H (h ?_)
rw [map_zero, map_zero, ← LinearMap.comp_apply, ← LinearMap.lTensor_comp,
l.exact_subtype_ker_map.linearMap_comp_eq_zero, LinearMap.lTensor_zero,
LinearMap.zero_apply]
-- By Nakayama's lemma, `l` is injective.
have : Function.Injective l := by
rwa [← LinearMap.ker_eq_bot, ← Submodule.subsingleton_iff_eq_bot,
← LocalRing.subsingleton_tensorProduct (R := R)]
-- Whence `M ≃ l(M)` is projective and the result follows.
have := (Exact.split_tfae l.exact_map_mkQ_range this (Submodule.mkQ_surjective _)).out 0 1
rw [← this]
exact Module.projective_lifting_property _ _ (Submodule.mkQ_surjective _)
end
|
RingTheory\LocalRing\MaximalIdeal\Basic.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.RingTheory.LocalRing.MaximalIdeal.Defs
import Mathlib.RingTheory.JacobsonIdeal
/-!
# Maximal ideal of local rings
We prove basic properties of the maximal ideal of a local ring.
-/
variable {R S K : Type*}
section CommSemiring
variable [CommSemiring R]
namespace LocalRing
variable [LocalRing R]
variable (R)
instance maximalIdeal.isMaximal : (maximalIdeal R).IsMaximal := by
rw [Ideal.isMaximal_iff]
constructor
· intro h
apply h
exact isUnit_one
· intro I x _ hx H
erw [Classical.not_not] at hx
rcases hx with ⟨u, rfl⟩
simpa using I.mul_mem_left (↑u⁻¹) H
theorem maximal_ideal_unique : ∃! I : Ideal R, I.IsMaximal :=
⟨maximalIdeal R, maximalIdeal.isMaximal R, fun I hI =>
hI.eq_of_le (maximalIdeal.isMaximal R).1.1 fun _ hx => hI.1.1 ∘ I.eq_top_of_isUnit_mem hx⟩
variable {R}
theorem eq_maximalIdeal {I : Ideal R} (hI : I.IsMaximal) : I = maximalIdeal R :=
ExistsUnique.unique (maximal_ideal_unique R) hI <| maximalIdeal.isMaximal R
theorem le_maximalIdeal {J : Ideal R} (hJ : J ≠ ⊤) : J ≤ maximalIdeal R := by
rcases Ideal.exists_le_maximal J hJ with ⟨M, hM1, hM2⟩
rwa [← eq_maximalIdeal hM1]
@[simp]
theorem mem_maximalIdeal (x) : x ∈ maximalIdeal R ↔ x ∈ nonunits R :=
Iff.rfl
theorem isField_iff_maximalIdeal_eq : IsField R ↔ maximalIdeal R = ⊥ :=
not_iff_not.mp
⟨Ring.ne_bot_of_isMaximal_of_not_isField inferInstance, fun h =>
Ring.not_isField_iff_exists_prime.mpr ⟨_, h, Ideal.IsMaximal.isPrime' _⟩⟩
end LocalRing
end CommSemiring
section CommRing
variable [CommRing R]
namespace LocalRing
variable [LocalRing R]
theorem maximalIdeal_le_jacobson (I : Ideal R) :
LocalRing.maximalIdeal R ≤ I.jacobson :=
le_sInf fun _ ⟨_, h⟩ => le_of_eq (LocalRing.eq_maximalIdeal h).symm
theorem jacobson_eq_maximalIdeal (I : Ideal R) (h : I ≠ ⊤) :
I.jacobson = LocalRing.maximalIdeal R :=
le_antisymm (sInf_le ⟨le_maximalIdeal h, maximalIdeal.isMaximal R⟩)
(maximalIdeal_le_jacobson I)
end LocalRing
end CommRing
namespace LocalRing
section
variable [CommRing R] [LocalRing R] [CommRing S] [LocalRing S]
theorem ker_eq_maximalIdeal [Field K] (φ : R →+* K) (hφ : Function.Surjective φ) :
RingHom.ker φ = maximalIdeal R :=
LocalRing.eq_maximalIdeal <| (RingHom.ker_isMaximal_of_surjective φ) hφ
end
end LocalRing
theorem LocalRing.maximalIdeal_eq_bot {R : Type*} [Field R] : LocalRing.maximalIdeal R = ⊥ :=
LocalRing.isField_iff_maximalIdeal_eq.mp (Field.toIsField R)
|
RingTheory\LocalRing\MaximalIdeal\Defs.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.RingTheory.LocalRing.Basic
/-!
# Maximal ideal of local rings
We define the maximal ideal of a local ring as the ideal of all non units.
## Main definitions
* `LocalRing.maximalIdeal`: The unique maximal ideal for a local rings. Its carrier set is the
set of non units.
-/
namespace LocalRing
variable (R : Type*) [CommSemiring R] [LocalRing R]
/-- The ideal of elements that are not units. -/
def maximalIdeal : Ideal R where
carrier := nonunits R
zero_mem' := zero_mem_nonunits.2 <| zero_ne_one
add_mem' {_ _} hx hy := nonunits_add hx hy
smul_mem' _ _ := mul_mem_nonunits_right
end LocalRing
|
RingTheory\LocalRing\ResidueField\Basic.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.RingTheory.LocalRing.ResidueField.Defs
import Mathlib.RingTheory.LocalRing.RingHom.Basic
import Mathlib.RingTheory.LocalRing.MaximalIdeal.Basic
/-!
# Residue Field of local rings
We prove basic properties of the residue field of a local ring.
-/
variable {R S T : Type*}
namespace LocalRing
section
variable [CommRing R] [LocalRing R] [CommRing S] [LocalRing S] [CommRing T] [LocalRing T]
lemma ker_residue : RingHom.ker (residue R) = maximalIdeal R :=
Ideal.mk_ker
@[simp]
lemma residue_eq_zero_iff (x : R) : residue R x = 0 ↔ x ∈ maximalIdeal R := by
rw [← RingHom.mem_ker, ker_residue]
lemma residue_ne_zero_iff_isUnit (x : R) : residue R x ≠ 0 ↔ IsUnit x := by
simp
variable (R)
instance ResidueField.algebra : Algebra R (ResidueField R) :=
Ideal.Quotient.algebra _
theorem ResidueField.algebraMap_eq : algebraMap R (ResidueField R) = residue R :=
rfl
instance : IsLocalRingHom (LocalRing.residue R) :=
⟨fun _ ha =>
Classical.not_not.mp (Ideal.Quotient.eq_zero_iff_mem.not.mp (isUnit_iff_ne_zero.mp ha))⟩
variable {R}
namespace ResidueField
/-- A local ring homomorphism into a field can be descended onto the residue field. -/
def lift {R S : Type*} [CommRing R] [LocalRing R] [Field S] (f : R →+* S) [IsLocalRingHom f] :
LocalRing.ResidueField R →+* S :=
Ideal.Quotient.lift _ f fun a ha =>
by_contradiction fun h => ha (isUnit_of_map_unit f a (isUnit_iff_ne_zero.mpr h))
theorem lift_comp_residue {R S : Type*} [CommRing R] [LocalRing R] [Field S] (f : R →+* S)
[IsLocalRingHom f] : (lift f).comp (residue R) = f :=
RingHom.ext fun _ => rfl
@[simp]
theorem lift_residue_apply {R S : Type*} [CommRing R] [LocalRing R] [Field S] (f : R →+* S)
[IsLocalRingHom f] (x) : lift f (residue R x) = f x :=
rfl
/-- The map on residue fields induced by a local homomorphism between local rings -/
def map (f : R →+* S) [IsLocalRingHom f] : ResidueField R →+* ResidueField S :=
Ideal.Quotient.lift (maximalIdeal R) ((Ideal.Quotient.mk _).comp f) fun a ha => by
erw [Ideal.Quotient.eq_zero_iff_mem]
exact map_nonunit f a ha
/-- Applying `LocalRing.ResidueField.map` to the identity ring homomorphism gives the identity
ring homomorphism. -/
@[simp]
theorem map_id :
LocalRing.ResidueField.map (RingHom.id R) = RingHom.id (LocalRing.ResidueField R) :=
Ideal.Quotient.ringHom_ext <| RingHom.ext fun _ => rfl
/-- The composite of two `LocalRing.ResidueField.map`s is the `LocalRing.ResidueField.map` of the
composite. -/
theorem map_comp (f : T →+* R) (g : R →+* S) [IsLocalRingHom f] [IsLocalRingHom g] :
LocalRing.ResidueField.map (g.comp f) =
(LocalRing.ResidueField.map g).comp (LocalRing.ResidueField.map f) :=
Ideal.Quotient.ringHom_ext <| RingHom.ext fun _ => rfl
theorem map_comp_residue (f : R →+* S) [IsLocalRingHom f] :
(ResidueField.map f).comp (residue R) = (residue S).comp f :=
rfl
theorem map_residue (f : R →+* S) [IsLocalRingHom f] (r : R) :
ResidueField.map f (residue R r) = residue S (f r) :=
rfl
theorem map_id_apply (x : ResidueField R) : map (RingHom.id R) x = x :=
DFunLike.congr_fun map_id x
@[simp]
theorem map_map (f : R →+* S) (g : S →+* T) (x : ResidueField R) [IsLocalRingHom f]
[IsLocalRingHom g] : map g (map f x) = map (g.comp f) x :=
DFunLike.congr_fun (map_comp f g).symm x
/-- A ring isomorphism defines an isomorphism of residue fields. -/
@[simps apply]
def mapEquiv (f : R ≃+* S) : LocalRing.ResidueField R ≃+* LocalRing.ResidueField S where
toFun := map (f : R →+* S)
invFun := map (f.symm : S →+* R)
left_inv x := by simp only [map_map, RingEquiv.symm_comp, map_id, RingHom.id_apply]
right_inv x := by simp only [map_map, RingEquiv.comp_symm, map_id, RingHom.id_apply]
map_mul' := RingHom.map_mul _
map_add' := RingHom.map_add _
@[simp]
theorem mapEquiv.symm (f : R ≃+* S) : (mapEquiv f).symm = mapEquiv f.symm :=
rfl
@[simp]
theorem mapEquiv_trans (e₁ : R ≃+* S) (e₂ : S ≃+* T) :
mapEquiv (e₁.trans e₂) = (mapEquiv e₁).trans (mapEquiv e₂) :=
RingEquiv.toRingHom_injective <| map_comp (e₁ : R →+* S) (e₂ : S →+* T)
@[simp]
theorem mapEquiv_refl : mapEquiv (RingEquiv.refl R) = RingEquiv.refl _ :=
RingEquiv.toRingHom_injective map_id
/-- The group homomorphism from `RingAut R` to `RingAut k` where `k`
is the residue field of `R`. -/
@[simps]
def mapAut : RingAut R →* RingAut (LocalRing.ResidueField R) where
toFun := mapEquiv
map_mul' e₁ e₂ := mapEquiv_trans e₂ e₁
map_one' := mapEquiv_refl
section MulSemiringAction
variable (G : Type*) [Group G] [MulSemiringAction G R]
/-- If `G` acts on `R` as a `MulSemiringAction`, then it also acts on `LocalRing.ResidueField R`. -/
instance : MulSemiringAction G (LocalRing.ResidueField R) :=
MulSemiringAction.compHom _ <| mapAut.comp (MulSemiringAction.toRingAut G R)
@[simp]
theorem residue_smul (g : G) (r : R) : residue R (g • r) = g • residue R r :=
rfl
end MulSemiringAction
end ResidueField
theorem isLocalRingHom_residue : IsLocalRingHom (LocalRing.residue R) := by
constructor
intro a ha
by_contra h
erw [Ideal.Quotient.eq_zero_iff_mem.mpr ((LocalRing.mem_maximalIdeal _).mpr h)] at ha
exact ha.ne_zero rfl
end
end LocalRing
|
RingTheory\LocalRing\ResidueField\Defs.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.RingTheory.LocalRing.MaximalIdeal.Basic
import Mathlib.RingTheory.Ideal.Quotient
/-!
# Residue Field of local rings
## Main definitions
* `LocalRing.ResidueField`: The quotient of a local ring by its maximal ideal.
* `LocalRing.residue`: The quotient map from a local ring to its residue field.
-/
namespace LocalRing
variable (R : Type*) [CommRing R] [LocalRing R]
/-- The residue field of a local ring is the quotient of the ring by its maximal ideal. -/
def ResidueField :=
R ⧸ maximalIdeal R
-- Porting note: failed at `deriving` instances automatically
instance ResidueFieldCommRing : CommRing (ResidueField R) :=
show CommRing (R ⧸ maximalIdeal R) from inferInstance
instance ResidueFieldInhabited : Inhabited (ResidueField R) :=
show Inhabited (R ⧸ maximalIdeal R) from inferInstance
noncomputable instance ResidueField.field : Field (ResidueField R) :=
Ideal.Quotient.field (maximalIdeal R)
/-- The quotient map from a local ring to its residue field. -/
def residue : R →+* ResidueField R :=
Ideal.Quotient.mk _
end LocalRing
|
RingTheory\LocalRing\RingHom\Basic.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.RingTheory.LocalRing.RingHom.Defs
import Mathlib.RingTheory.LocalRing.MaximalIdeal.Defs
import Mathlib.RingTheory.Ideal.Maps
import Mathlib.Logic.Equiv.TransferInstance
/-!
# Local rings homomorphisms
We prove basic properties of local rings homomorphisms.
-/
variable {R S T : Type*}
section
variable [Semiring R] [Semiring S] [Semiring T]
instance isLocalRingHom_id (R : Type*) [Semiring R] : IsLocalRingHom (RingHom.id R) where
map_nonunit _ := id
@[simp]
theorem isUnit_map_iff (f : R →+* S) [IsLocalRingHom f] (a) : IsUnit (f a) ↔ IsUnit a :=
⟨IsLocalRingHom.map_nonunit a, f.isUnit_map⟩
-- Porting note: as this can be proved by other `simp` lemmas, this is marked as high priority.
@[simp (high)]
theorem map_mem_nonunits_iff (f : R →+* S) [IsLocalRingHom f] (a) :
f a ∈ nonunits S ↔ a ∈ nonunits R :=
⟨fun h ha => h <| (isUnit_map_iff f a).mpr ha, fun h ha => h <| (isUnit_map_iff f a).mp ha⟩
instance isLocalRingHom_comp (g : S →+* T) (f : R →+* S) [IsLocalRingHom g] [IsLocalRingHom f] :
IsLocalRingHom (g.comp f) where
map_nonunit a := IsLocalRingHom.map_nonunit a ∘ IsLocalRingHom.map_nonunit (f a)
instance isLocalRingHom_equiv (f : R ≃+* S) : IsLocalRingHom (f : R →+* S) where
map_nonunit a ha := by
convert RingHom.isUnit_map (f.symm : S →+* R) ha
exact (RingEquiv.symm_apply_apply f a).symm
@[simp]
theorem isUnit_of_map_unit (f : R →+* S) [IsLocalRingHom f] (a) (h : IsUnit (f a)) : IsUnit a :=
IsLocalRingHom.map_nonunit a h
theorem of_irreducible_map (f : R →+* S) [h : IsLocalRingHom f] {x} (hfx : Irreducible (f x)) :
Irreducible x :=
⟨fun h => hfx.not_unit <| IsUnit.map f h, fun p q hx =>
let ⟨H⟩ := h
Or.imp (H p) (H q) <| hfx.isUnit_or_isUnit <| f.map_mul p q ▸ congr_arg f hx⟩
theorem isLocalRingHom_of_comp (f : R →+* S) (g : S →+* T) [IsLocalRingHom (g.comp f)] :
IsLocalRingHom f :=
⟨fun _ ha => (isUnit_map_iff (g.comp f) _).mp (g.isUnit_map ha)⟩
/-- If `f : R →+* S` is a local ring hom, then `R` is a local ring if `S` is. -/
theorem RingHom.domain_localRing {R S : Type*} [CommSemiring R] [CommSemiring S] [H : LocalRing S]
(f : R →+* S) [IsLocalRingHom f] : LocalRing R := by
haveI : Nontrivial R := pullback_nonzero f f.map_zero f.map_one
apply LocalRing.of_nonunits_add
intro a b
simp_rw [← map_mem_nonunits_iff f, f.map_add]
exact LocalRing.nonunits_add
end
section
open LocalRing
variable [CommSemiring R] [LocalRing R] [CommSemiring S] [LocalRing S]
/--
The image of the maximal ideal of the source is contained within the maximal ideal of the target.
-/
theorem map_nonunit (f : R →+* S) [IsLocalRingHom f] (a : R) (h : a ∈ maximalIdeal R) :
f a ∈ maximalIdeal S := fun H => h <| isUnit_of_map_unit f a H
end
namespace LocalRing
section
variable [CommSemiring R] [LocalRing R] [CommSemiring S] [LocalRing S]
/-- A ring homomorphism between local rings is a local ring hom iff it reflects units,
i.e. any preimage of a unit is still a unit. https://stacks.math.columbia.edu/tag/07BJ
-/
theorem local_hom_TFAE (f : R →+* S) :
List.TFAE
[IsLocalRingHom f, f '' (maximalIdeal R).1 ⊆ maximalIdeal S,
(maximalIdeal R).map f ≤ maximalIdeal S, maximalIdeal R ≤ (maximalIdeal S).comap f,
(maximalIdeal S).comap f = maximalIdeal R] := by
tfae_have 1 → 2
· rintro _ _ ⟨a, ha, rfl⟩
exact map_nonunit f a ha
tfae_have 2 → 4
· exact Set.image_subset_iff.1
tfae_have 3 ↔ 4
· exact Ideal.map_le_iff_le_comap
tfae_have 4 → 1
· intro h
constructor
exact fun x => not_imp_not.1 (@h x)
tfae_have 1 → 5
· intro
ext
exact not_iff_not.2 (isUnit_map_iff f _)
tfae_have 5 → 4
· exact fun h => le_of_eq h.symm
tfae_finish
end
theorem of_surjective [CommSemiring R] [LocalRing R] [CommSemiring S] [Nontrivial S] (f : R →+* S)
[IsLocalRingHom f] (hf : Function.Surjective f) : LocalRing S :=
of_isUnit_or_isUnit_of_isUnit_add (by
intro a b hab
obtain ⟨a, rfl⟩ := hf a
obtain ⟨b, rfl⟩ := hf b
rw [← map_add] at hab
exact
(isUnit_or_isUnit_of_isUnit_add <| IsLocalRingHom.map_nonunit _ hab).imp f.isUnit_map
f.isUnit_map)
/-- If `f : R →+* S` is a surjective local ring hom, then the induced units map is surjective. -/
theorem surjective_units_map_of_local_ringHom [CommRing R] [CommRing S] (f : R →+* S)
(hf : Function.Surjective f) (h : IsLocalRingHom f) :
Function.Surjective (Units.map <| f.toMonoidHom) := by
intro a
obtain ⟨b, hb⟩ := hf (a : S)
use (isUnit_of_map_unit f b (by rw [hb]; exact Units.isUnit _)).unit
ext
exact hb
-- see Note [lower instance priority]
/-- Every ring hom `f : K →+* R` from a division ring `K` to a nontrivial ring `R` is a
local ring hom. -/
instance (priority := 100) {K R} [DivisionRing K] [CommRing R] [Nontrivial R]
(f : K →+* R) : IsLocalRingHom f where
map_nonunit r hr := by simpa only [isUnit_iff_ne_zero, ne_eq, map_eq_zero] using hr.ne_zero
end LocalRing
namespace RingEquiv
protected theorem localRing {A B : Type*} [CommSemiring A] [LocalRing A] [CommSemiring B]
(e : A ≃+* B) : LocalRing B :=
haveI := e.symm.toEquiv.nontrivial
LocalRing.of_surjective (e : A →+* B) e.surjective
end RingEquiv
|
RingTheory\LocalRing\RingHom\Defs.lean | /-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Chris Hughes, Mario Carneiro
-/
import Mathlib.Algebra.Group.Units
import Mathlib.Algebra.Ring.Hom.Defs
/-!
# Local rings homomorphisms
We Define local ring homomorhpisms.
## Main definitions
* `IsLocalRingHom`: A predicate on semiring homomorphisms, requiring that it maps nonunits
to nonunits. For local rings, this means that the image of the unique maximal ideal is again
contained in the unique maximal ideal.
-/
/-- A local ring homomorphism is a homomorphism `f` between local rings such that `a` in the domain
is a unit if `f a` is a unit for any `a`. See `LocalRing.local_hom_TFAE` for other equivalent
definitions. -/
class IsLocalRingHom {R S : Type*} [Semiring R] [Semiring S] (f : R →+* S) : Prop where
/-- A local ring homomorphism `f : R ⟶ S` will send nonunits of `R` to nonunits of `S`. -/
map_nonunit : ∀ a, IsUnit (f a) → IsUnit a
|
RingTheory\MvPolynomial\Basic.lean | /-
Copyright (c) 2019 Johannes Hölzl. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johannes Hölzl
-/
import Mathlib.Algebra.CharP.Defs
import Mathlib.Algebra.MvPolynomial.Degrees
import Mathlib.Algebra.Polynomial.AlgebraMap
import Mathlib.LinearAlgebra.FinsuppVectorSpace
import Mathlib.LinearAlgebra.FreeModule.Finite.Basic
/-!
# Multivariate polynomials over commutative rings
This file contains basic facts about multivariate polynomials over commutative rings, for example
that the monomials form a basis.
## Main definitions
* `restrictTotalDegree σ R m`: the subspace of multivariate polynomials indexed by `σ` over the
commutative ring `R` of total degree at most `m`.
* `restrictDegree σ R m`: the subspace of multivariate polynomials indexed by `σ` over the
commutative ring `R` such that the degree in each individual variable is at most `m`.
## Main statements
* The multivariate polynomial ring over a commutative semiring of characteristic `p` has
characteristic `p`, and similarly for `CharZero`.
* `basisMonomials`: shows that the monomials form a basis of the vector space of multivariate
polynomials.
## TODO
Generalise to noncommutative (semi)rings
-/
noncomputable section
open Set LinearMap Submodule
open Polynomial
universe u v
variable (σ : Type u) (R : Type v) [CommSemiring R] (p m : ℕ)
namespace MvPolynomial
section CharP
instance [CharP R p] : CharP (MvPolynomial σ R) p where
cast_eq_zero_iff' n := by rw [← C_eq_coe_nat, ← C_0, C_inj, CharP.cast_eq_zero_iff R p]
end CharP
section CharZero
instance [CharZero R] : CharZero (MvPolynomial σ R) where
cast_injective x y hxy := by rwa [← C_eq_coe_nat, ← C_eq_coe_nat, C_inj, Nat.cast_inj] at hxy
end CharZero
section Homomorphism
theorem mapRange_eq_map {R S : Type*} [CommSemiring R] [CommSemiring S] (p : MvPolynomial σ R)
(f : R →+* S) : Finsupp.mapRange f f.map_zero p = map f p := by
rw [p.as_sum, Finsupp.mapRange_finset_sum, map_sum (map f)]
refine Finset.sum_congr rfl fun n _ => ?_
rw [map_monomial, ← single_eq_monomial, Finsupp.mapRange_single, single_eq_monomial]
end Homomorphism
section Degree
variable {σ}
/-- The submodule of polynomials that are sum of monomials in the set `s`. -/
def restrictSupport (s : Set (σ →₀ ℕ)) : Submodule R (MvPolynomial σ R) :=
Finsupp.supported _ _ s
/-- `restrictSupport R s` has a canonical `R`-basis indexed by `s`. -/
def basisRestrictSupport (s : Set (σ →₀ ℕ)) : Basis s R (restrictSupport R s) where
repr := Finsupp.supportedEquivFinsupp s
theorem restrictSupport_mono {s t : Set (σ →₀ ℕ)} (h : s ⊆ t) :
restrictSupport R s ≤ restrictSupport R t := Finsupp.supported_mono h
variable (σ)
/-- The submodule of polynomials of total degree less than or equal to `m`. -/
def restrictTotalDegree (m : ℕ) : Submodule R (MvPolynomial σ R) :=
restrictSupport R { n | (n.sum fun _ e => e) ≤ m }
/-- The submodule of polynomials such that the degree with respect to each individual variable is
less than or equal to `m`. -/
def restrictDegree (m : ℕ) : Submodule R (MvPolynomial σ R) :=
restrictSupport R { n | ∀ i, n i ≤ m }
variable {R}
theorem mem_restrictTotalDegree (p : MvPolynomial σ R) :
p ∈ restrictTotalDegree σ R m ↔ p.totalDegree ≤ m := by
rw [totalDegree, Finset.sup_le_iff]
rfl
theorem mem_restrictDegree (p : MvPolynomial σ R) (n : ℕ) :
p ∈ restrictDegree σ R n ↔ ∀ s ∈ p.support, ∀ i, (s : σ →₀ ℕ) i ≤ n := by
rw [restrictDegree, restrictSupport, Finsupp.mem_supported]
rfl
theorem mem_restrictDegree_iff_sup [DecidableEq σ] (p : MvPolynomial σ R) (n : ℕ) :
p ∈ restrictDegree σ R n ↔ ∀ i, p.degrees.count i ≤ n := by
simp only [mem_restrictDegree, degrees_def, Multiset.count_finset_sup, Finsupp.count_toMultiset,
Finset.sup_le_iff]
exact ⟨fun h n s hs => h s hs n, fun h s hs n => h n s hs⟩
variable (R)
theorem restrictTotalDegree_le_restrictDegree (m : ℕ) :
restrictTotalDegree σ R m ≤ restrictDegree σ R m :=
fun p hp ↦ (mem_restrictDegree _ _ _).mpr fun s hs i ↦ (degreeOf_le_iff.mp
(degreeOf_le_totalDegree p i) s hs).trans ((mem_restrictTotalDegree _ _ _).mp hp)
/-- The monomials form a basis on `MvPolynomial σ R`. -/
def basisMonomials : Basis (σ →₀ ℕ) R (MvPolynomial σ R) :=
Finsupp.basisSingleOne
@[simp]
theorem coe_basisMonomials :
(basisMonomials σ R : (σ →₀ ℕ) → MvPolynomial σ R) = fun s => monomial s 1 :=
rfl
/-- The `R`-module `MvPolynomial σ R` is free. -/
instance : Module.Free R (MvPolynomial σ R) :=
Module.Free.of_basis (MvPolynomial.basisMonomials σ R)
theorem linearIndependent_X : LinearIndependent R (X : σ → MvPolynomial σ R) :=
(basisMonomials σ R).linearIndependent.comp (fun s : σ => Finsupp.single s 1)
(Finsupp.single_left_injective one_ne_zero)
private lemma finite_setOf_bounded (α) [Finite α] (n : ℕ) : Finite {f : α →₀ ℕ | ∀ a, f a ≤ n} :=
((Set.Finite.pi' fun _ ↦ Set.finite_le_nat _).preimage DFunLike.coe_injective.injOn).to_subtype
instance [Finite σ] (N : ℕ) : Module.Finite R (restrictDegree σ R N) :=
have := finite_setOf_bounded σ N
Module.Finite.of_basis (basisRestrictSupport R _)
instance [Finite σ] (N : ℕ) : Module.Finite R (restrictTotalDegree σ R N) :=
have := finite_setOf_bounded σ N
have : Finite {s : σ →₀ ℕ | s.sum (fun _ e ↦ e) ≤ N} := by
rw [Set.finite_coe_iff] at this ⊢
exact this.subset fun n hn i ↦ (eq_or_ne (n i) 0).elim
(fun h ↦ h.trans_le N.zero_le) fun h ↦
(Finset.single_le_sum (fun _ _ ↦ Nat.zero_le _) <| Finsupp.mem_support_iff.mpr h).trans hn
Module.Finite.of_basis (basisRestrictSupport R _)
end Degree
section Algebra
variable {R S σ : Type*} [CommSemiring R] [CommSemiring S] [Algebra R S]
/--
If `S` is an `R`-algebra, then `MvPolynomial σ S` is a `MvPolynomial σ R` algebra.
Warning: This produces a diamond for
`Algebra (MvPolynomial σ R) (MvPolynomial σ (MvPolynomial σ S))`. That's why it is not a
global instance.
-/
noncomputable def algebraMvPolynomial : Algebra (MvPolynomial σ R) (MvPolynomial σ S) :=
(MvPolynomial.map (algebraMap R S)).toAlgebra
attribute [local instance] algebraMvPolynomial
@[simp]
lemma algebraMap_def :
algebraMap (MvPolynomial σ R) (MvPolynomial σ S) = MvPolynomial.map (algebraMap R S) :=
rfl
instance : IsScalarTower R (MvPolynomial σ R) (MvPolynomial σ S) :=
IsScalarTower.of_algebraMap_eq' (by ext; simp)
end Algebra
end MvPolynomial
-- this is here to avoid import cycle issues
namespace Polynomial
/-- The monomials form a basis on `R[X]`. -/
noncomputable def basisMonomials : Basis ℕ R R[X] :=
Basis.ofRepr (toFinsuppIsoAlg R).toLinearEquiv
@[simp]
theorem coe_basisMonomials : (basisMonomials R : ℕ → R[X]) = fun s => monomial s 1 :=
funext fun _ => ofFinsupp_single _ _
end Polynomial
|
RingTheory\MvPolynomial\Homogeneous.lean | /-
Copyright (c) 2020 Johan Commelin. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johan Commelin, Eric Wieser
-/
import Mathlib.Algebra.DirectSum.Internal
import Mathlib.Algebra.GradedMonoid
import Mathlib.Algebra.MvPolynomial.CommRing
import Mathlib.Algebra.MvPolynomial.Equiv
import Mathlib.Algebra.MvPolynomial.Variables
import Mathlib.RingTheory.MvPolynomial.WeightedHomogeneous
import Mathlib.Algebra.Polynomial.Roots
/-!
# Homogeneous polynomials
A multivariate polynomial `φ` is homogeneous of degree `n`
if all monomials occurring in `φ` have degree `n`.
## Main definitions/lemmas
* `IsHomogeneous φ n`: a predicate that asserts that `φ` is homogeneous of degree `n`.
* `homogeneousSubmodule σ R n`: the submodule of homogeneous polynomials of degree `n`.
* `homogeneousComponent n`: the additive morphism that projects polynomials onto
their summand that is homogeneous of degree `n`.
* `sum_homogeneousComponent`: every polynomial is the sum of its homogeneous components.
-/
namespace MvPolynomial
variable {σ : Type*} {τ : Type*} {R : Type*} {S : Type*}
/-
TODO
* show that `MvPolynomial σ R ≃ₐ[R] ⨁ i, homogeneousSubmodule σ R i`
-/
open Finsupp
/-- A multivariate polynomial `φ` is homogeneous of degree `n`
if all monomials occurring in `φ` have degree `n`. -/
def IsHomogeneous [CommSemiring R] (φ : MvPolynomial σ R) (n : ℕ) :=
IsWeightedHomogeneous 1 φ n
variable [CommSemiring R]
theorem weightedTotalDegree_one (φ : MvPolynomial σ R) :
weightedTotalDegree (1 : σ → ℕ) φ = φ.totalDegree := by
simp only [totalDegree, weightedTotalDegree, weight, LinearMap.toAddMonoidHom_coe,
Finsupp.total, Pi.one_apply, Finsupp.coe_lsum, LinearMap.coe_smulRight, LinearMap.id_coe,
id, Algebra.id.smul_eq_mul, mul_one]
variable (σ R)
/-- The submodule of homogeneous `MvPolynomial`s of degree `n`. -/
def homogeneousSubmodule (n : ℕ) : Submodule R (MvPolynomial σ R) where
carrier := { x | x.IsHomogeneous n }
smul_mem' r a ha c hc := by
rw [coeff_smul] at hc
apply ha
intro h
apply hc
rw [h]
exact smul_zero r
zero_mem' d hd := False.elim (hd <| coeff_zero _)
add_mem' {a b} ha hb c hc := by
rw [coeff_add] at hc
obtain h | h : coeff c a ≠ 0 ∨ coeff c b ≠ 0 := by
contrapose! hc
simp only [hc, add_zero]
· exact ha h
· exact hb h
@[simp]
lemma weightedHomogeneousSubmodule_one (n : ℕ) :
weightedHomogeneousSubmodule R 1 n = homogeneousSubmodule σ R n := rfl
variable {σ R}
@[simp]
theorem mem_homogeneousSubmodule (n : ℕ) (p : MvPolynomial σ R) :
p ∈ homogeneousSubmodule σ R n ↔ p.IsHomogeneous n := Iff.rfl
variable (σ R)
/-- While equal, the former has a convenient definitional reduction. -/
theorem homogeneousSubmodule_eq_finsupp_supported (n : ℕ) :
homogeneousSubmodule σ R n = Finsupp.supported _ R { d | d.degree = n } := by
simp_rw [degree_eq_weight_one]
exact weightedHomogeneousSubmodule_eq_finsupp_supported R 1 n
variable {σ R}
theorem homogeneousSubmodule_mul (m n : ℕ) :
homogeneousSubmodule σ R m * homogeneousSubmodule σ R n ≤ homogeneousSubmodule σ R (m + n) :=
weightedHomogeneousSubmodule_mul 1 m n
section
theorem isHomogeneous_monomial {d : σ →₀ ℕ} (r : R) {n : ℕ} (hn : d.degree = n) :
IsHomogeneous (monomial d r) n := by
rw [degree_eq_weight_one] at hn
exact isWeightedHomogeneous_monomial 1 d r hn
variable (σ)
theorem totalDegree_eq_zero_iff (p : MvPolynomial σ R) :
p.totalDegree = 0 ↔ ∀ (m : σ →₀ ℕ) (_ : m ∈ p.support) (x : σ), m x = 0 := by
rw [← weightedTotalDegree_one, weightedTotalDegree_eq_zero_iff _ p]
exact nonTorsionWeight_of (Function.const σ one_ne_zero)
theorem totalDegree_zero_iff_isHomogeneous {p : MvPolynomial σ R} :
p.totalDegree = 0 ↔ IsHomogeneous p 0 := by
rw [← weightedTotalDegree_one,
← isWeightedHomogeneous_zero_iff_weightedTotalDegree_eq_zero, IsHomogeneous]
alias ⟨isHomogeneous_of_totalDegree_zero, _⟩ := totalDegree_zero_iff_isHomogeneous
theorem isHomogeneous_C (r : R) : IsHomogeneous (C r : MvPolynomial σ R) 0 := by
apply isHomogeneous_monomial
simp only [Finsupp.degree, Finsupp.zero_apply, Finset.sum_const_zero]
variable (R)
theorem isHomogeneous_zero (n : ℕ) : IsHomogeneous (0 : MvPolynomial σ R) n :=
(homogeneousSubmodule σ R n).zero_mem
theorem isHomogeneous_one : IsHomogeneous (1 : MvPolynomial σ R) 0 :=
isHomogeneous_C _ _
variable {σ}
theorem isHomogeneous_X (i : σ) : IsHomogeneous (X i : MvPolynomial σ R) 1 := by
apply isHomogeneous_monomial
rw [Finsupp.degree, Finsupp.support_single_ne_zero _ one_ne_zero, Finset.sum_singleton]
exact Finsupp.single_eq_same
end
namespace IsHomogeneous
variable [CommSemiring S] {φ ψ : MvPolynomial σ R} {m n : ℕ}
theorem coeff_eq_zero (hφ : IsHomogeneous φ n) {d : σ →₀ ℕ} (hd : d.degree ≠ n) :
coeff d φ = 0 := by
rw [degree_eq_weight_one] at hd
exact IsWeightedHomogeneous.coeff_eq_zero hφ d hd
theorem inj_right (hm : IsHomogeneous φ m) (hn : IsHomogeneous φ n) (hφ : φ ≠ 0) : m = n := by
obtain ⟨d, hd⟩ : ∃ d, coeff d φ ≠ 0 := exists_coeff_ne_zero hφ
rw [← hm hd, ← hn hd]
theorem add (hφ : IsHomogeneous φ n) (hψ : IsHomogeneous ψ n) : IsHomogeneous (φ + ψ) n :=
(homogeneousSubmodule σ R n).add_mem hφ hψ
theorem sum {ι : Type*} (s : Finset ι) (φ : ι → MvPolynomial σ R) (n : ℕ)
(h : ∀ i ∈ s, IsHomogeneous (φ i) n) : IsHomogeneous (∑ i ∈ s, φ i) n :=
(homogeneousSubmodule σ R n).sum_mem h
theorem mul (hφ : IsHomogeneous φ m) (hψ : IsHomogeneous ψ n) : IsHomogeneous (φ * ψ) (m + n) :=
homogeneousSubmodule_mul m n <| Submodule.mul_mem_mul hφ hψ
theorem prod {ι : Type*} (s : Finset ι) (φ : ι → MvPolynomial σ R) (n : ι → ℕ)
(h : ∀ i ∈ s, IsHomogeneous (φ i) (n i)) : IsHomogeneous (∏ i ∈ s, φ i) (∑ i ∈ s, n i) := by
classical
revert h
refine Finset.induction_on s ?_ ?_
· intro
simp only [isHomogeneous_one, Finset.sum_empty, Finset.prod_empty]
· intro i s his IH h
simp only [his, Finset.prod_insert, Finset.sum_insert, not_false_iff]
apply (h i (Finset.mem_insert_self _ _)).mul (IH _)
intro j hjs
exact h j (Finset.mem_insert_of_mem hjs)
lemma C_mul (hφ : φ.IsHomogeneous m) (r : R) :
(C r * φ).IsHomogeneous m := by
simpa only [zero_add] using (isHomogeneous_C _ _).mul hφ
lemma _root_.MvPolynomial.isHomogeneous_C_mul_X (r : R) (i : σ) :
(C r * X i).IsHomogeneous 1 :=
(isHomogeneous_X _ _).C_mul _
@[deprecated (since := "2024-03-21")]
alias _root_.MvPolynomial.C_mul_X := _root_.MvPolynomial.isHomogeneous_C_mul_X
lemma pow (hφ : φ.IsHomogeneous m) (n : ℕ) : (φ ^ n).IsHomogeneous (m * n) := by
rw [show φ ^ n = ∏ _i ∈ Finset.range n, φ by simp]
rw [show m * n = ∑ _i ∈ Finset.range n, m by simp [mul_comm]]
apply IsHomogeneous.prod _ _ _ (fun _ _ ↦ hφ)
lemma _root_.MvPolynomial.isHomogeneous_X_pow (i : σ) (n : ℕ) :
(X (R := R) i ^ n).IsHomogeneous n := by
simpa only [one_mul] using (isHomogeneous_X _ _).pow n
lemma _root_.MvPolynomial.isHomogeneous_C_mul_X_pow (r : R) (i : σ) (n : ℕ) :
(C r * X i ^ n).IsHomogeneous n :=
(isHomogeneous_X_pow _ _).C_mul _
lemma eval₂ (hφ : φ.IsHomogeneous m) (f : R →+* MvPolynomial τ S) (g : σ → MvPolynomial τ S)
(hf : ∀ r, (f r).IsHomogeneous 0) (hg : ∀ i, (g i).IsHomogeneous n) :
(eval₂ f g φ).IsHomogeneous (n * m) := by
apply IsHomogeneous.sum
intro i hi
rw [← zero_add (n * m)]
apply IsHomogeneous.mul (hf _) _
convert IsHomogeneous.prod _ _ (fun k ↦ n * i k) _
· rw [Finsupp.mem_support_iff] at hi
rw [← Finset.mul_sum, ← hφ hi, weight_apply]
simp_rw [smul_eq_mul, Finsupp.sum, Pi.one_apply, mul_one]
· rintro k -
apply (hg k).pow
lemma map (hφ : φ.IsHomogeneous n) (f : R →+* S) : (map f φ).IsHomogeneous n := by
simpa only [one_mul] using hφ.eval₂ _ _ (fun r ↦ isHomogeneous_C _ (f r)) (isHomogeneous_X _)
lemma aeval [Algebra R S] (hφ : φ.IsHomogeneous m)
(g : σ → MvPolynomial τ S) (hg : ∀ i, (g i).IsHomogeneous n) :
(aeval g φ).IsHomogeneous (n * m) :=
hφ.eval₂ _ _ (fun _ ↦ isHomogeneous_C _ _) hg
section CommRing
-- In this section we shadow the semiring `R` with a ring `R`.
variable {R σ : Type*} [CommRing R] {φ ψ : MvPolynomial σ R} {n : ℕ}
theorem neg (hφ : IsHomogeneous φ n) : IsHomogeneous (-φ) n :=
(homogeneousSubmodule σ R n).neg_mem hφ
theorem sub (hφ : IsHomogeneous φ n) (hψ : IsHomogeneous ψ n) : IsHomogeneous (φ - ψ) n :=
(homogeneousSubmodule σ R n).sub_mem hφ hψ
end CommRing
/-- The homogeneous degree bounds the total degree.
See also `MvPolynomial.IsHomogeneous.totalDegree` when `φ` is non-zero. -/
lemma totalDegree_le (hφ : IsHomogeneous φ n) : φ.totalDegree ≤ n := by
apply Finset.sup_le
intro d hd
rw [mem_support_iff] at hd
simp_rw [Finsupp.sum, ← hφ hd, weight_apply, Pi.one_apply, smul_eq_mul, mul_one, Finsupp.sum,
le_rfl]
theorem totalDegree (hφ : IsHomogeneous φ n) (h : φ ≠ 0) : totalDegree φ = n := by
apply le_antisymm hφ.totalDegree_le
obtain ⟨d, hd⟩ : ∃ d, coeff d φ ≠ 0 := exists_coeff_ne_zero h
simp only [← hφ hd, MvPolynomial.totalDegree, Finsupp.sum]
replace hd := Finsupp.mem_support_iff.mpr hd
simp only [weight_apply, Pi.one_apply, smul_eq_mul, mul_one]
-- Porting note: Original proof did not define `f`
exact Finset.le_sup (f := fun s ↦ ∑ x ∈ s.support, s x) hd
theorem rename_isHomogeneous {f : σ → τ} (h : φ.IsHomogeneous n) :
(rename f φ).IsHomogeneous n := by
rw [← φ.support_sum_monomial_coeff, map_sum]; simp_rw [rename_monomial]
apply IsHomogeneous.sum _ _ _ fun d hd ↦ isHomogeneous_monomial _ _
intro d hd
apply (Finsupp.sum_mapDomain_index_addMonoidHom fun _ ↦ .id ℕ).trans
convert h (mem_support_iff.mp hd)
simp only [weight_apply, AddMonoidHom.id_apply, Pi.one_apply, smul_eq_mul, mul_one]
theorem rename_isHomogeneous_iff {f : σ → τ} (hf : f.Injective) :
(rename f φ).IsHomogeneous n ↔ φ.IsHomogeneous n := by
refine ⟨fun h d hd ↦ ?_, rename_isHomogeneous⟩
convert ← @h (d.mapDomain f) _
· simp only [weight_apply, Pi.one_apply, smul_eq_mul, mul_one]
exact Finsupp.sum_mapDomain_index_inj (h := fun _ ↦ id) hf
· rwa [coeff_rename_mapDomain f hf]
lemma finSuccEquiv_coeff_isHomogeneous {N : ℕ} {φ : MvPolynomial (Fin (N+1)) R} {n : ℕ}
(hφ : φ.IsHomogeneous n) (i j : ℕ) (h : i + j = n) :
((finSuccEquiv _ _ φ).coeff i).IsHomogeneous j := by
intro d hd
rw [finSuccEquiv_coeff_coeff] at hd
have h' : (weight 1) (Finsupp.cons i d) = i + j := by
simpa [Finset.sum_subset_zero_on_sdiff (g := d.cons i)
(d.cons_support (y := i)) (by simp) (fun _ _ ↦ rfl), ← h] using hφ hd
simp only [weight_apply, Pi.one_apply, smul_eq_mul, mul_one, Finsupp.sum_cons,
add_right_inj] at h' ⊢
exact h'
-- TODO: develop API for `optionEquivLeft` and get rid of the `[Fintype σ]` assumption
lemma coeff_isHomogeneous_of_optionEquivLeft_symm
[hσ : Finite σ] {p : Polynomial (MvPolynomial σ R)}
(hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n) (i j : ℕ) (h : i + j = n) :
(p.coeff i).IsHomogeneous j := by
obtain ⟨k, ⟨e⟩⟩ := Finite.exists_equiv_fin σ
let e' := e.optionCongr.trans (_root_.finSuccEquiv _).symm
let F := renameEquiv R e
let F' := renameEquiv R e'
let φ := F' ((optionEquivLeft R σ).symm p)
have hφ : φ.IsHomogeneous n := hp.rename_isHomogeneous
suffices IsHomogeneous (F (p.coeff i)) j by
rwa [← (IsHomogeneous.rename_isHomogeneous_iff e.injective)]
convert hφ.finSuccEquiv_coeff_isHomogeneous i j h using 1
dsimp only [φ, F', F, renameEquiv_apply]
rw [finSuccEquiv_rename_finSuccEquiv, AlgEquiv.apply_symm_apply]
simp
open Polynomial in
private
lemma exists_eval_ne_zero_of_coeff_finSuccEquiv_ne_zero_aux
{N : ℕ} {F : MvPolynomial (Fin (Nat.succ N)) R} {n : ℕ} (hF : IsHomogeneous F n)
(hFn : ((finSuccEquiv R N) F).coeff n ≠ 0) :
∃ r, eval r F ≠ 0 := by
have hF₀ : F ≠ 0 := by contrapose! hFn; simp [hFn]
have hdeg : natDegree (finSuccEquiv R N F) < n + 1 := by
linarith [natDegree_finSuccEquiv F, degreeOf_le_totalDegree F 0, hF.totalDegree hF₀]
use Fin.cons 1 0
have aux : ∀ i ∈ Finset.range n, constantCoeff ((finSuccEquiv R N F).coeff i) = 0 := by
intro i hi
rw [Finset.mem_range] at hi
apply (hF.finSuccEquiv_coeff_isHomogeneous i (n-i) (by omega)).coeff_eq_zero
simp only [Finsupp.degree_zero]
rw [← Nat.sub_ne_zero_iff_lt] at hi
exact hi.symm
simp_rw [eval_eq_eval_mv_eval', eval_one_map, Polynomial.eval_eq_sum_range' hdeg,
eval_zero, one_pow, mul_one, map_sum, Finset.sum_range_succ, Finset.sum_eq_zero aux, zero_add]
contrapose! hFn
ext d
rw [coeff_zero]
obtain rfl | hd := eq_or_ne d 0
· apply hFn
· contrapose! hd
ext i
rw [Finsupp.coe_zero, Pi.zero_apply]
by_cases hi : i ∈ d.support
· have := hF.finSuccEquiv_coeff_isHomogeneous n 0 (add_zero _) hd
simp only [weight_apply, Pi.one_apply, smul_eq_mul, mul_one, Finsupp.sum] at this
rw [Finset.sum_eq_zero_iff_of_nonneg (fun _ _ ↦ zero_le')] at this
exact this i hi
· simpa using hi
section IsDomain
-- In this section we shadow the semiring `R` with a domain `R`.
variable {R σ : Type*} [CommRing R] [IsDomain R] {F G : MvPolynomial σ R} {n : ℕ}
open Cardinal Polynomial
private
lemma exists_eval_ne_zero_of_totalDegree_le_card_aux {N : ℕ} {F : MvPolynomial (Fin N) R} {n : ℕ}
(hF : F.IsHomogeneous n) (hF₀ : F ≠ 0) (hnR : n ≤ #R) :
∃ r, eval r F ≠ 0 := by
induction N generalizing n with
| zero =>
use 0
contrapose! hF₀
ext d
simpa only [Subsingleton.elim d 0, eval_zero, coeff_zero] using hF₀
| succ N IH =>
have hdeg : natDegree (finSuccEquiv R N F) < n + 1 := by
linarith [natDegree_finSuccEquiv F, degreeOf_le_totalDegree F 0, hF.totalDegree hF₀]
obtain ⟨i, hi⟩ : ∃ i : ℕ, (finSuccEquiv R N F).coeff i ≠ 0 := by
contrapose! hF₀
exact (finSuccEquiv _ _).injective <| Polynomial.ext <| by simpa using hF₀
have hin : i ≤ n := by
contrapose! hi
exact coeff_eq_zero_of_natDegree_lt <| (Nat.le_of_lt_succ hdeg).trans_lt hi
obtain hFn | hFn := ne_or_eq ((finSuccEquiv R N F).coeff n) 0
· exact hF.exists_eval_ne_zero_of_coeff_finSuccEquiv_ne_zero_aux hFn
have hin : i < n := hin.lt_or_eq.elim id <| by aesop
obtain ⟨j, hj⟩ : ∃ j, i + (j + 1) = n := (Nat.exists_eq_add_of_lt hin).imp <| by intros; omega
obtain ⟨r, hr⟩ : ∃ r, (eval r) (Polynomial.coeff ((finSuccEquiv R N) F) i) ≠ 0 :=
IH (hF.finSuccEquiv_coeff_isHomogeneous _ _ hj) hi (.trans (by norm_cast; omega) hnR)
set φ : R[X] := Polynomial.map (eval r) (finSuccEquiv _ _ F) with hφ
have hφ₀ : φ ≠ 0 := fun hφ₀ ↦ hr <| by
rw [← coeff_eval_eq_eval_coeff, ← hφ, hφ₀, Polynomial.coeff_zero]
have hφR : φ.natDegree < #R := by
refine lt_of_lt_of_le ?_ hnR
norm_cast
refine lt_of_le_of_lt (natDegree_map_le _ _) ?_
suffices (finSuccEquiv _ _ F).natDegree ≠ n by omega
rintro rfl
refine leadingCoeff_ne_zero.mpr ?_ hFn
simpa using (finSuccEquiv R N).injective.ne hF₀
obtain ⟨r₀, hr₀⟩ : ∃ r₀, Polynomial.eval r₀ φ ≠ 0 :=
φ.exists_eval_ne_zero_of_natDegree_lt_card hφ₀ hφR
use Fin.cons r₀ r
rwa [eval_eq_eval_mv_eval']
/-- See `MvPolynomial.IsHomogeneous.eq_zero_of_forall_eval_eq_zero`
for a version that assumes `Infinite R`. -/
lemma eq_zero_of_forall_eval_eq_zero_of_le_card
(hF : F.IsHomogeneous n) (h : ∀ r : σ → R, eval r F = 0) (hnR : n ≤ #R) :
F = 0 := by
contrapose! h
-- reduce to the case where σ is finite
obtain ⟨k, f, hf, F, rfl⟩ := exists_fin_rename F
have hF₀ : F ≠ 0 := by rintro rfl; simp at h
have hF : F.IsHomogeneous n := by rwa [rename_isHomogeneous_iff hf] at hF
obtain ⟨r, hr⟩ := exists_eval_ne_zero_of_totalDegree_le_card_aux hF hF₀ hnR
obtain ⟨r, rfl⟩ := (Function.factorsThrough_iff _).mp <| (hf.factorsThrough r)
use r
rwa [eval_rename]
/-- See `MvPolynomial.IsHomogeneous.funext`
for a version that assumes `Infinite R`. -/
lemma funext_of_le_card (hF : F.IsHomogeneous n) (hG : G.IsHomogeneous n)
(h : ∀ r : σ → R, eval r F = eval r G) (hnR : n ≤ #R) :
F = G := by
rw [← sub_eq_zero]
apply eq_zero_of_forall_eval_eq_zero_of_le_card (hF.sub hG) _ hnR
simpa [sub_eq_zero] using h
/-- See `MvPolynomial.IsHomogeneous.eq_zero_of_forall_eval_eq_zero_of_le_card`
for a version that assumes `n ≤ #R`. -/
lemma eq_zero_of_forall_eval_eq_zero [Infinite R] {F : MvPolynomial σ R} {n : ℕ}
(hF : F.IsHomogeneous n) (h : ∀ r : σ → R, eval r F = 0) : F = 0 := by
apply eq_zero_of_forall_eval_eq_zero_of_le_card hF h
exact (Cardinal.nat_lt_aleph0 _).le.trans <| Cardinal.infinite_iff.mp ‹Infinite R›
/-- See `MvPolynomial.IsHomogeneous.funext_of_le_card`
for a version that assumes `n ≤ #R`. -/
lemma funext [Infinite R] {F G : MvPolynomial σ R} {n : ℕ}
(hF : F.IsHomogeneous n) (hG : G.IsHomogeneous n)
(h : ∀ r : σ → R, eval r F = eval r G) : F = G := by
apply funext_of_le_card hF hG h
exact (Cardinal.nat_lt_aleph0 _).le.trans <| Cardinal.infinite_iff.mp ‹Infinite R›
end IsDomain
/-- The homogeneous submodules form a graded ring. This instance is used by `DirectSum.commSemiring`
and `DirectSum.algebra`. -/
instance HomogeneousSubmodule.gcommSemiring : SetLike.GradedMonoid (homogeneousSubmodule σ R) where
one_mem := isHomogeneous_one σ R
mul_mem _ _ _ _ := IsHomogeneous.mul
end IsHomogeneous
noncomputable section
open Finset
/-- `homogeneousComponent n φ` is the part of `φ` that is homogeneous of degree `n`.
See `sum_homogeneousComponent` for the statement that `φ` is equal to the sum
of all its homogeneous components. -/
def homogeneousComponent [CommSemiring R] (n : ℕ) : MvPolynomial σ R →ₗ[R] MvPolynomial σ R :=
weightedHomogeneousComponent 1 n
section HomogeneousComponent
open Finset Finsupp
variable (n : ℕ) (φ ψ : MvPolynomial σ R)
theorem homogeneousComponent_mem :
homogeneousComponent n φ ∈ homogeneousSubmodule σ R n :=
weightedHomogeneousComponent_mem _ φ n
theorem coeff_homogeneousComponent (d : σ →₀ ℕ) :
coeff d (homogeneousComponent n φ) = if d.degree = n then coeff d φ else 0 := by
rw [degree_eq_weight_one]
convert coeff_weightedHomogeneousComponent n φ d
theorem homogeneousComponent_apply :
homogeneousComponent n φ =
∑ d ∈ φ.support.filter fun d => d.degree = n, monomial d (coeff d φ) := by
simp_rw [degree_eq_weight_one]
convert weightedHomogeneousComponent_apply n φ
theorem homogeneousComponent_isHomogeneous : (homogeneousComponent n φ).IsHomogeneous n :=
weightedHomogeneousComponent_isWeightedHomogeneous n φ
@[simp]
theorem homogeneousComponent_zero : homogeneousComponent 0 φ = C (coeff 0 φ) :=
weightedHomogeneousComponent_zero φ (fun _ => Nat.succ_ne_zero Nat.zero)
@[simp]
theorem homogeneousComponent_C_mul (n : ℕ) (r : R) :
homogeneousComponent n (C r * φ) = C r * homogeneousComponent n φ :=
weightedHomogeneousComponent_C_mul φ n r
theorem homogeneousComponent_eq_zero'
(h : ∀ d : σ →₀ ℕ, d ∈ φ.support → d.degree ≠ n) :
homogeneousComponent n φ = 0 := by
simp_rw [degree_eq_weight_one] at h
exact weightedHomogeneousComponent_eq_zero' n φ h
theorem homogeneousComponent_eq_zero (h : φ.totalDegree < n) : homogeneousComponent n φ = 0 := by
apply homogeneousComponent_eq_zero'
rw [totalDegree, Finset.sup_lt_iff (lt_of_le_of_lt (Nat.zero_le _) h)] at h
intro d hd; exact ne_of_lt (h d hd)
theorem sum_homogeneousComponent :
(∑ i ∈ range (φ.totalDegree + 1), homogeneousComponent i φ) = φ := by
ext1 d
suffices φ.totalDegree < d.support.sum d → 0 = coeff d φ by
simpa [coeff_sum, coeff_homogeneousComponent]
exact fun h => (coeff_eq_zero_of_totalDegree_lt h).symm
theorem homogeneousComponent_of_mem {m n : ℕ} {p : MvPolynomial σ R}
(h : p ∈ homogeneousSubmodule σ R n) :
homogeneousComponent m p = if m = n then p else 0 :=
weightedHomogeneousComponent_of_mem h
end HomogeneousComponent
end
noncomputable section GradedAlgebra
/-- The homogeneous submodules form a graded ring.
This instance is used by `DirectSum.commSemiring` and `DirectSum.algebra`. -/
lemma HomogeneousSubmodule.gradedMonoid :
SetLike.GradedMonoid (homogeneousSubmodule σ R) :=
WeightedHomogeneousSubmodule.gradedMonoid
/-- The decomposition of `MvPolynomial σ R` into homogeneous submodules. -/
abbrev decomposition :
DirectSum.Decomposition (homogeneousSubmodule σ R) :=
weightedDecomposition R (1 : σ → ℕ)
/-- `MvPolynomial σ R` as a graded algebra, graded by the degree.
We do not make this a global instance because one may want to consider a different
graded algebra structure on `MvPolynomial σ R`, induced by another weight function.
To make it a local instance, you may use
`attribute [local instance] MvPolynomial.gradedAlgebra`.
-/
abbrev gradedAlgebra : GradedAlgebra (homogeneousSubmodule σ R) :=
weightedGradedAlgebra R (1 : σ → ℕ)
theorem decomposition.decompose'_apply (φ : MvPolynomial σ R) (i : ℕ) :
(decomposition.decompose' φ i : MvPolynomial σ R) = homogeneousComponent i φ :=
weightedDecomposition.decompose'_apply R _ φ i
theorem decomposition.decompose'_eq :
decomposition.decompose' = fun φ : MvPolynomial σ R =>
DirectSum.mk (fun i : ℕ => ↥(homogeneousSubmodule σ R i)) (φ.support.image Finsupp.degree)
fun m => ⟨homogeneousComponent m φ, homogeneousComponent_mem m φ⟩ := by
rw [degree_eq_weight_one]
rfl
end GradedAlgebra
end MvPolynomial
|
RingTheory\MvPolynomial\Ideal.lean | /-
Copyright (c) 2023 Eric Wieser. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Eric Wieser
-/
import Mathlib.Algebra.MonoidAlgebra.Ideal
import Mathlib.Algebra.MvPolynomial.Division
/-!
# Lemmas about ideals of `MvPolynomial`
Notably this contains results about monomial ideals.
## Main results
* `MvPolynomial.mem_ideal_span_monomial_image`
* `MvPolynomial.mem_ideal_span_X_image`
-/
variable {σ R : Type*}
namespace MvPolynomial
variable [CommSemiring R]
/-- `x` is in a monomial ideal generated by `s` iff every element of its support dominates one of
the generators. Note that `si ≤ xi` is analogous to saying that the monomial corresponding to `si`
divides the monomial corresponding to `xi`. -/
theorem mem_ideal_span_monomial_image {x : MvPolynomial σ R} {s : Set (σ →₀ ℕ)} :
x ∈ Ideal.span ((fun s => monomial s (1 : R)) '' s) ↔ ∀ xi ∈ x.support, ∃ si ∈ s, si ≤ xi := by
refine AddMonoidAlgebra.mem_ideal_span_of'_image.trans ?_
simp_rw [le_iff_exists_add, add_comm]
rfl
theorem mem_ideal_span_monomial_image_iff_dvd {x : MvPolynomial σ R} {s : Set (σ →₀ ℕ)} :
x ∈ Ideal.span ((fun s => monomial s (1 : R)) '' s) ↔
∀ xi ∈ x.support, ∃ si ∈ s, monomial si 1 ∣ monomial xi (x.coeff xi) := by
refine mem_ideal_span_monomial_image.trans (forall₂_congr fun xi hxi => ?_)
simp_rw [monomial_dvd_monomial, one_dvd, and_true_iff, mem_support_iff.mp hxi, false_or_iff]
/-- `x` is in a monomial ideal generated by variables `X` iff every element of its support
has a component in `s`. -/
theorem mem_ideal_span_X_image {x : MvPolynomial σ R} {s : Set σ} :
x ∈ Ideal.span (MvPolynomial.X '' s : Set (MvPolynomial σ R)) ↔
∀ m ∈ x.support, ∃ i ∈ s, (m : σ →₀ ℕ) i ≠ 0 := by
have := @mem_ideal_span_monomial_image σ R _ x ((fun i => Finsupp.single i 1) '' s)
rw [Set.image_image] at this
refine this.trans ?_
simp [Nat.one_le_iff_ne_zero]
end MvPolynomial
|
RingTheory\MvPolynomial\Localization.lean | /-
Copyright (c) 2024 Christian Merten. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Christian Merten
-/
import Mathlib.Algebra.MvPolynomial.CommRing
import Mathlib.RingTheory.Ideal.QuotientOperations
import Mathlib.RingTheory.Localization.Away.Basic
import Mathlib.RingTheory.Localization.Basic
import Mathlib.RingTheory.MvPolynomial.Basic
/-!
# Localization and multivariate polynomial rings
In this file we show some results connecting multivariate polynomial rings and localization.
## Main results
- `MvPolynomial.isLocalization`: If `S` is the localization of `R` at a submonoid `M`, then
`MvPolynomial σ S` is the localization of `MvPolynomial σ R` at the image of `M` in
`MvPolynomial σ R`.
-/
variable {σ R : Type*} [CommRing R] (M : Submonoid R)
variable (S : Type*) [CommRing S] [Algebra R S]
namespace MvPolynomial
variable [IsLocalization M S]
attribute [local instance] algebraMvPolynomial
/--
If `S` is the localization of `R` at a submonoid `M`, then `MvPolynomial σ S`
is the localization of `MvPolynomial σ R` at `M.map MvPolynomial.C`.
-/
instance isLocalization : IsLocalization (M.map <| C (σ := σ))
(MvPolynomial σ S) where
map_units' := by
rintro ⟨p, q, hq, rfl⟩
simp only [algebraMap_def, map_C]
exact IsUnit.map _ (IsLocalization.map_units _ ⟨q, hq⟩)
surj' p := by
simp only [algebraMap_def, Prod.exists, Subtype.exists,
Submonoid.mem_map, exists_prop, exists_exists_and_eq_and, map_C]
refine induction_on' p ?_ ?_
· intro u s
obtain ⟨⟨r, m⟩, hr⟩ := IsLocalization.surj M s
use monomial u r, m, m.property
simp only [map_monomial]
rw [← hr, mul_comm, C_mul_monomial, mul_comm]
· intro p p' ⟨x, m, hm, hxm⟩ ⟨x', m', hm', hxm'⟩
use x * (C m') + x' * (C m), m * m', Submonoid.mul_mem _ hm hm'
simp only [map_mul, map_add, map_C]
rw [add_mul, ← mul_assoc, hxm, ← mul_assoc, ← hxm, ← hxm']
ring
exists_of_eq {p q} := by
intro h
simp_rw [algebraMap_def, MvPolynomial.ext_iff, coeff_map] at h
choose c hc using (fun m ↦ IsLocalization.exists_of_eq (M := M) (h m))
simp only [Subtype.exists, Submonoid.mem_map, exists_prop, exists_exists_and_eq_and]
classical
refine ⟨Finset.prod (p.support ∪ q.support) (fun m ↦ c m), ?_, ?_⟩
· exact M.prod_mem (fun m _ ↦ (c m).property)
· ext m
simp only [coeff_C_mul]
by_cases h : m ∈ p.support ∪ q.support
· exact Finset.prod_mul_eq_prod_mul_of_exists m h (hc m)
· simp only [Finset.mem_union, mem_support_iff, ne_eq, not_or, Decidable.not_not] at h
rw [h.left, h.right]
lemma isLocalization_C_mk' (a : R) (m : M) :
C (IsLocalization.mk' S a m) = IsLocalization.mk' (MvPolynomial σ S) (C (σ := σ) a)
⟨C m, Submonoid.mem_map_of_mem C m.property⟩ := by
simp_rw [IsLocalization.eq_mk'_iff_mul_eq, algebraMap_def, map_C, ← map_mul,
IsLocalization.mk'_spec]
end MvPolynomial
namespace IsLocalization.Away
open MvPolynomial
variable (r : R) [IsLocalization.Away r S]
/-- The canonical algebra map from `MvPolynomial Unit R` quotiented by
`C r * X () - 1` to the localization of `R` away from `r`. -/
private noncomputable
def auxHom : (MvPolynomial Unit R) ⧸ (Ideal.span { C r * X () - 1 }) →ₐ[R] S :=
Ideal.Quotient.liftₐ (Ideal.span { C r * X () - 1}) (aeval (fun _ ↦ invSelf r)) <| by
intro p hp
refine Submodule.span_induction hp ?_ ?_ ?_ ?_
· rintro p ⟨q, rfl⟩
simp
· simp
· intro p q hp hq
simp [hp, hq]
· intro a x hx
simp [hx]
@[simp]
private lemma auxHom_mk (p : MvPolynomial Unit R) :
auxHom S r p = aeval (S₁ := S) (fun _ ↦ invSelf r) p :=
rfl
private noncomputable
def auxInv : S →+* (MvPolynomial Unit R) ⧸ Ideal.span { C r * X () - 1 } :=
letI g : R →+* MvPolynomial Unit R ⧸ (Ideal.span { C r * X () - 1 }) :=
(Ideal.Quotient.mk _).comp C
IsLocalization.Away.lift (S := S) (g := g) r <| by
simp only [RingHom.coe_comp, Function.comp_apply, g]
rw [isUnit_iff_exists_inv]
use (Ideal.Quotient.mk _ <| X ())
rw [← _root_.map_mul, ← map_one (Ideal.Quotient.mk _), Ideal.Quotient.mk_eq_mk_iff_sub_mem]
exact Ideal.mem_span_singleton_self (C r * X () - 1)
private lemma auxHom_auxInv : (auxHom S r).toRingHom.comp (auxInv S r) = RingHom.id S := by
apply IsLocalization.ringHom_ext (Submonoid.powers r)
ext x
simp [auxInv]
private lemma auxInv_auxHom : (auxInv S r).comp (auxHom (S := S) r).toRingHom = RingHom.id _ := by
rw [← RingHom.cancel_right (Ideal.Quotient.mk_surjective)]
ext x
· simp [auxInv]
· simp only [auxInv, AlgHom.toRingHom_eq_coe, RingHom.coe_comp, RingHom.coe_coe,
Function.comp_apply, auxHom_mk, aeval_X, RingHomCompTriple.comp_eq]
erw [IsLocalization.lift_mk'_spec]
simp only [map_one, RingHom.coe_comp, Function.comp_apply]
rw [← _root_.map_one (Ideal.Quotient.mk _)]
rw [← _root_.map_mul, Ideal.Quotient.mk_eq_mk_iff_sub_mem, ← Ideal.neg_mem_iff, neg_sub]
exact Ideal.mem_span_singleton_self (C r * X x - 1)
/-- The canonical algebra isomorphism from `MvPolynomial Unit R` quotiented by
`C r * X () - 1` to the localization of `R` away from `r`. -/
noncomputable def mvPolynomialQuotientEquiv :
((MvPolynomial Unit R) ⧸ Ideal.span { C r * X () - 1 }) ≃ₐ[R] S where
toFun := auxHom S r
invFun := auxInv S r
left_inv x := by
simpa using congrFun (congrArg DFunLike.coe <| auxInv_auxHom S r) x
right_inv s := by
simpa using congrFun (congrArg DFunLike.coe <| auxHom_auxInv S r) s
map_mul' := by simp
map_add' := by simp
commutes' := by simp
@[simp]
lemma mvPolynomialQuotientEquiv_apply (p : MvPolynomial Unit R) :
mvPolynomialQuotientEquiv S r (Ideal.Quotient.mk _ p) = aeval (S₁ := S) (fun _ ↦ invSelf r) p :=
rfl
end IsLocalization.Away
|
RingTheory\MvPolynomial\NewtonIdentities.lean | /-
Copyright (c) 2023 Michael Lee. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Michael Lee
-/
import Mathlib.Algebra.Algebra.Subalgebra.Basic
import Mathlib.Algebra.MvPolynomial.CommRing
import Mathlib.Algebra.MvPolynomial.Rename
import Mathlib.Data.Finset.Card
import Mathlib.Data.Fintype.Basic
import Mathlib.RingTheory.MvPolynomial.Symmetric
/-!
# Newton's Identities
This file defines `MvPolynomial` power sums as a means of implementing Newton's identities. The
combinatorial proof, due to Zeilberger, defines for `k : ℕ` a subset `pairs` of
`(range k).powerset × range k` and a map `pairMap` such that `pairMap` is an involution on `pairs`,
and a map `weight` which identifies elements of `pairs` with the terms of the summation in Newton's
identities and which satisfies `weight ∘ pairMap = -weight`. The result therefore follows neatly
from an identity implemented in mathlib as `Finset.sum_involution`. Namely, we use
`Finset.sum_involution` to show that `∑ t ∈ pairs σ k, weight σ R k t = 0`. We then identify
`(-1) ^ k * k * esymm σ R k` with the terms of the weight sum for which `t.fst` has
cardinality `k`, and `(-1) ^ i * esymm σ R i * psum σ R (k - i)` with the terms of the weight sum
for which `t.fst` has cardinality `i` for `i < k` , and we thereby derive the main result
`(-1) ^ k * k * esymm σ R k + ∑ i ∈ range k, (-1) ^ i * esymm σ R i * psum σ R (k - i) = 0` (or
rather, two equivalent forms which provide direct definitions for `esymm` and `psum` in lower-degree
terms).
## Main declarations
* `MvPolynomial.mul_esymm_eq_sum`: a recurrence relation for the `k`th elementary
symmetric polynomial in terms of lower-degree elementary symmetric polynomials and power sums.
* `MvPolynomial.psum_eq_mul_esymm_sub_sum`: a recurrence relation for the degree-`k` power sum
in terms of lower-degree elementary symmetric polynomials and power sums.
## References
See [zeilberger1984] for the combinatorial proof of Newton's identities.
-/
open Equiv (Perm)
open MvPolynomial
noncomputable section
namespace MvPolynomial
open Finset Nat
namespace NewtonIdentities
variable (σ : Type*) (R : Type*) [CommRing R]
section DecidableEq
variable [DecidableEq σ]
private def pairMap (t : Finset σ × σ) : Finset σ × σ :=
if h : t.snd ∈ t.fst then (t.fst.erase t.snd, t.snd) else (t.fst.cons t.snd h, t.snd)
private lemma pairMap_ne_self (t : Finset σ × σ) : pairMap σ t ≠ t := by
rw [pairMap]
split_ifs with h1
all_goals by_contra ht; rw [← ht] at h1; simp_all
private lemma pairMap_of_snd_mem_fst {t : Finset σ × σ} (h : t.snd ∈ t.fst) :
pairMap σ t = (t.fst.erase t.snd, t.snd) := by
simp [pairMap, h]
private lemma pairMap_of_snd_nmem_fst {t : Finset σ × σ} (h : t.snd ∉ t.fst) :
pairMap σ t = (t.fst.cons t.snd h, t.snd) := by
simp [pairMap, h]
@[simp]
private theorem pairMap_involutive : (pairMap σ).Involutive := by
intro t
rw [pairMap, pairMap]
split_ifs with h1 h2 h3
· simp at h2
· simp [insert_erase h1]
· simp_all
· simp at h3
variable [Fintype σ]
private def pairs (k : ℕ) : Finset (Finset σ × σ) :=
univ.filter (fun t ↦ card t.fst ≤ k ∧ (card t.fst = k → t.snd ∈ t.fst))
@[simp]
private lemma mem_pairs (k : ℕ) (t : Finset σ × σ) :
t ∈ pairs σ k ↔ card t.fst ≤ k ∧ (card t.fst = k → t.snd ∈ t.fst) := by
simp [pairs]
private def weight (k : ℕ) (t : Finset σ × σ) : MvPolynomial σ R :=
(-1) ^ card t.fst * ((∏ a ∈ t.fst, X a) * X t.snd ^ (k - card t.fst))
private theorem pairMap_mem_pairs {k : ℕ} (t : Finset σ × σ) (h : t ∈ pairs σ k) :
pairMap σ t ∈ pairs σ k := by
rw [mem_pairs] at h ⊢
rcases (em (t.snd ∈ t.fst)) with h1 | h1
· rw [pairMap_of_snd_mem_fst σ h1]
simp only [h1, implies_true, and_true] at h
simp only [card_erase_of_mem h1, tsub_le_iff_right, mem_erase, ne_eq, h1]
refine ⟨le_step h, ?_⟩
by_contra h2
simp only [not_true_eq_false, and_true, not_forall, not_false_eq_true, exists_prop] at h2
rw [← h2] at h
exact not_le_of_lt (sub_lt (card_pos.mpr ⟨t.snd, h1⟩) zero_lt_one) h
· rw [pairMap_of_snd_nmem_fst σ h1]
simp only [h1] at h
simp only [card_cons, mem_cons, true_or, implies_true, and_true]
exact (le_iff_eq_or_lt.mp h.left).resolve_left h.right
private theorem weight_add_weight_pairMap {k : ℕ} (t : Finset σ × σ) (h : t ∈ pairs σ k) :
weight σ R k t + weight σ R k (pairMap σ t) = 0 := by
rw [weight, weight]
rw [mem_pairs] at h
have h2 (n : ℕ) : -(-1 : MvPolynomial σ R) ^ n = (-1) ^ (n + 1) := by
rw [← neg_one_mul ((-1 : MvPolynomial σ R) ^ n), pow_add, pow_one, mul_comm]
rcases (em (t.snd ∈ t.fst)) with h1 | h1
· rw [pairMap_of_snd_mem_fst σ h1]
simp only [← prod_erase_mul t.fst (fun j ↦ (X j : MvPolynomial σ R)) h1,
mul_assoc (∏ a ∈ erase t.fst t.snd, X a), card_erase_of_mem h1]
nth_rewrite 1 [← pow_one (X t.snd)]
simp only [← pow_add, add_comm]
have h3 : 1 ≤ card t.fst := lt_iff_add_one_le.mp (card_pos.mpr ⟨t.snd, h1⟩)
rw [← tsub_tsub_assoc h.left h3, ← neg_neg ((-1 : MvPolynomial σ R) ^ (card t.fst - 1)),
h2 (card t.fst - 1), Nat.sub_add_cancel h3]
simp
· rw [pairMap_of_snd_nmem_fst σ h1]
simp only [mul_comm, mul_assoc (∏ a ∈ t.fst, X a), card_cons, prod_cons]
nth_rewrite 2 [← pow_one (X t.snd)]
simp only [← pow_add, ← Nat.add_sub_assoc (Nat.lt_of_le_of_ne h.left (mt h.right h1)), add_comm,
Nat.succ_eq_add_one, Nat.add_sub_add_right]
rw [← neg_neg ((-1 : MvPolynomial σ R) ^ card t.fst), h2]
simp
private theorem weight_sum (k : ℕ) : ∑ t ∈ pairs σ k, weight σ R k t = 0 :=
sum_involution (fun t _ ↦ pairMap σ t) (weight_add_weight_pairMap σ R)
(fun t _ ↦ (fun _ ↦ pairMap_ne_self σ t)) (pairMap_mem_pairs σ)
(fun t _ ↦ pairMap_involutive σ t)
private theorem sum_filter_pairs_eq_sum_powersetCard_sum (k : ℕ)
(f : Finset σ × σ → MvPolynomial σ R) :
(∑ t ∈ filter (fun t ↦ card t.fst = k) (pairs σ k), f t) =
∑ A ∈ powersetCard k univ, (∑ j ∈ A, f (A, j)) := by
apply sum_finset_product
aesop
private theorem sum_filter_pairs_eq_sum_powersetCard_mem_filter_antidiagonal_sum (k : ℕ) (a : ℕ × ℕ)
(ha : a ∈ (antidiagonal k).filter (fun a ↦ a.fst < k)) (f : Finset σ × σ → MvPolynomial σ R) :
(∑ t ∈ filter (fun t ↦ card t.fst = a.fst) (pairs σ k), f t) =
∑ A ∈ powersetCard a.fst univ, (∑ j, f (A, j)) := by
apply sum_finset_product
simp only [mem_filter, mem_powersetCard_univ, mem_univ, and_true, and_iff_right_iff_imp]
rintro p hp
have : card p.fst ≤ k := by apply le_of_lt; aesop
aesop
private lemma filter_pairs_lt (k : ℕ) :
(pairs σ k).filter (fun (s, _) ↦ s.card < k) =
(range k).disjiUnion (powersetCard · univ) ((pairwise_disjoint_powersetCard _).set_pairwise _)
×ˢ univ := by ext; aesop (add unsafe le_of_lt)
private theorem sum_filter_pairs_eq_sum_filter_antidiagonal_powersetCard_sum (k : ℕ)
(f : Finset σ × σ → MvPolynomial σ R) :
∑ t ∈ (pairs σ k).filter fun t ↦ card t.fst < k, f t =
∑ a ∈ (antidiagonal k).filter fun a ↦ a.fst < k,
∑ A ∈ powersetCard a.fst univ, ∑ j, f (A, j) := by
rw [filter_pairs_lt, sum_product, sum_disjiUnion]
refine sum_nbij' (fun n ↦ (n, k - n)) Prod.fst ?_ ?_ ?_ ?_ ?_ <;>
simp (config := { contextual := true }) [@eq_comm _ _ k, Nat.add_sub_cancel', le_of_lt]
private theorem disjoint_filter_pairs_lt_filter_pairs_eq (k : ℕ) :
Disjoint (filter (fun t ↦ card t.fst < k) (pairs σ k))
(filter (fun t ↦ card t.fst = k) (pairs σ k)) := by
rw [disjoint_filter]
exact fun _ _ h1 h2 ↦ lt_irrefl _ (h2.symm.subst h1)
private theorem disjUnion_filter_pairs_eq_pairs (k : ℕ) :
disjUnion (filter (fun t ↦ card t.fst < k) (pairs σ k))
(filter (fun t ↦ card t.fst = k) (pairs σ k)) (disjoint_filter_pairs_lt_filter_pairs_eq σ k) =
pairs σ k := by
simp only [disjUnion_eq_union, Finset.ext_iff, pairs, filter_filter, mem_filter]
intro a
rw [← filter_or, mem_filter]
refine ⟨fun ha ↦ by tauto, fun ha ↦ ?_⟩
have hacard := le_iff_lt_or_eq.mp ha.2.1
tauto
end DecidableEq
variable [Fintype σ]
private theorem esymm_summand_to_weight (k : ℕ) (A : Finset σ) (h : A ∈ powersetCard k univ) :
∑ j ∈ A, weight σ R k (A, j) = k * (-1) ^ k * (∏ i ∈ A, X i : MvPolynomial σ R) := by
simp [weight, mem_powersetCard_univ.mp h, mul_assoc]
private theorem esymm_to_weight [DecidableEq σ] (k : ℕ) : k * esymm σ R k =
(-1) ^ k * ∑ t ∈ filter (fun t ↦ card t.fst = k) (pairs σ k), weight σ R k t := by
rw [esymm, sum_filter_pairs_eq_sum_powersetCard_sum σ R k (fun t ↦ weight σ R k t),
sum_congr rfl (esymm_summand_to_weight σ R k), mul_comm (k : MvPolynomial σ R) ((-1) ^ k),
← mul_sum, ← mul_assoc, ← mul_assoc, ← pow_add, Even.neg_one_pow ⟨k, rfl⟩, one_mul]
private theorem esymm_mul_psum_summand_to_weight (k : ℕ) (a : ℕ × ℕ) (ha : a ∈ antidiagonal k) :
∑ A ∈ powersetCard a.fst univ, ∑ j, weight σ R k (A, j) =
(-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd := by
simp only [esymm, psum, weight, ← mul_assoc, mul_sum]
rw [sum_comm]
refine sum_congr rfl fun x _ ↦ ?_
rw [sum_mul]
refine sum_congr rfl fun s hs ↦ ?_
rw [mem_powersetCard_univ.mp hs, ← mem_antidiagonal.mp ha, add_sub_self_left]
private theorem esymm_mul_psum_to_weight [DecidableEq σ] (k : ℕ) :
∑ a ∈ (antidiagonal k).filter (fun a ↦ a.fst < k),
(-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd =
∑ t ∈ filter (fun t ↦ card t.fst < k) (pairs σ k), weight σ R k t := by
rw [← sum_congr rfl (fun a ha ↦ esymm_mul_psum_summand_to_weight σ R k a (mem_filter.mp ha).left),
sum_filter_pairs_eq_sum_filter_antidiagonal_powersetCard_sum σ R k]
end NewtonIdentities
variable (σ : Type*) [Fintype σ] (R : Type*) [CommRing R]
/-- **Newton's identities** give a recurrence relation for the kth elementary symmetric polynomial
in terms of lower degree elementary symmetric polynomials and power sums. -/
theorem mul_esymm_eq_sum (k : ℕ) : k * esymm σ R k =
(-1) ^ (k + 1) * ∑ a ∈ (antidiagonal k).filter (fun a ↦ a.fst < k),
(-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd := by
classical
rw [NewtonIdentities.esymm_to_weight σ R k, NewtonIdentities.esymm_mul_psum_to_weight σ R k,
eq_comm, ← sub_eq_zero, sub_eq_add_neg, neg_mul_eq_neg_mul,
neg_eq_neg_one_mul ((-1 : MvPolynomial σ R) ^ k)]
nth_rw 2 [← pow_one (-1 : MvPolynomial σ R)]
rw [← pow_add, add_comm 1 k, ← left_distrib,
← sum_disjUnion (NewtonIdentities.disjoint_filter_pairs_lt_filter_pairs_eq σ k),
NewtonIdentities.disjUnion_filter_pairs_eq_pairs σ k, NewtonIdentities.weight_sum σ R k,
neg_one_pow_mul_eq_zero_iff.mpr rfl]
theorem sum_antidiagonal_card_esymm_psum_eq_zero :
∑ a ∈ antidiagonal (Fintype.card σ), (-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd = 0 := by
let k := Fintype.card σ
suffices (-1 : MvPolynomial σ R) ^ (k + 1) *
∑ a ∈ antidiagonal k, (-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd = 0 by
simpa using this
simp [← sum_filter_add_sum_filter_not (antidiagonal k) (fun a ↦ a.fst < k), ← mul_esymm_eq_sum,
mul_add, ← mul_assoc, ← pow_add, mul_comm ↑k (esymm σ R k)]
/-- A version of Newton's identities which may be more useful in the case that we know the values of
the elementary symmetric polynomials and would like to calculate the values of the power sums. -/
theorem psum_eq_mul_esymm_sub_sum (k : ℕ) (h : 0 < k) : psum σ R k =
(-1) ^ (k + 1) * k * esymm σ R k -
∑ a ∈ (antidiagonal k).filter (fun a ↦ a.fst ∈ Set.Ioo 0 k),
(-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd := by
simp only [Set.Ioo, Set.mem_setOf_eq, and_comm]
have hesymm := mul_esymm_eq_sum σ R k
rw [← (sum_filter_add_sum_filter_not ((antidiagonal k).filter (fun a ↦ a.fst < k))
(fun a ↦ 0 < a.fst) (fun a ↦ (-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd))] at hesymm
have sub_both_sides := congrArg (· - (-1 : MvPolynomial σ R) ^ (k + 1) *
∑ a ∈ ((antidiagonal k).filter (fun a ↦ a.fst < k)).filter (fun a ↦ 0 < a.fst),
(-1) ^ a.fst * esymm σ R a.fst * psum σ R a.snd) hesymm
simp only [left_distrib, add_sub_cancel_left] at sub_both_sides
have sub_both_sides := congrArg ((-1 : MvPolynomial σ R) ^ (k + 1) * ·) sub_both_sides
simp only [mul_sub_left_distrib, ← mul_assoc, ← pow_add, Even.neg_one_pow ⟨k + 1, rfl⟩, one_mul,
not_le, lt_one_iff, filter_filter (fun a : ℕ × ℕ ↦ a.fst < k) (fun a ↦ ¬0 < a.fst)]
at sub_both_sides
have : filter (fun a ↦ a.fst < k ∧ ¬0 < a.fst) (antidiagonal k) = {(0, k)} := by
ext a
rw [mem_filter, mem_antidiagonal, mem_singleton]
refine ⟨?_, by rintro rfl; omega⟩
rintro ⟨ha, ⟨_, ha0⟩⟩
rw [← ha, Nat.eq_zero_of_not_pos ha0, zero_add, ← Nat.eq_zero_of_not_pos ha0]
rw [this, sum_singleton] at sub_both_sides
simp only [_root_.pow_zero, esymm_zero, mul_one, one_mul, filter_filter] at sub_both_sides
exact sub_both_sides.symm
end MvPolynomial
|
RingTheory\MvPolynomial\Symmetric.lean | /-
Copyright (c) 2020 Hanting Zhang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Hanting Zhang, Johan Commelin
-/
import Mathlib.Algebra.Algebra.Subalgebra.Basic
import Mathlib.Algebra.MvPolynomial.CommRing
import Mathlib.Combinatorics.Enumerative.Partition
/-!
# Symmetric Polynomials and Elementary Symmetric Polynomials
This file defines symmetric `MvPolynomial`s and the bases of elementary, complete homogeneous,
power sum, and monomial symmetric `MvPolynomial`s. We also prove some basic facts about them.
## Main declarations
* `MvPolynomial.IsSymmetric`
* `MvPolynomial.symmetricSubalgebra`
* `MvPolynomial.esymm`
* `MvPolynomial.hsymm`
* `MvPolynomial.psum`
* `MvPolynomial.msymm`
## Notation
+ `esymm σ R n` is the `n`th elementary symmetric polynomial in `MvPolynomial σ R`.
+ `hsymm σ R n` is the `n`th complete homogeneous symmetric polynomial in `MvPolynomial σ R`.
+ `psum σ R n` is the degree-`n` power sum in `MvPolynomial σ R`, i.e. the sum of monomials
`(X i)^n` over `i ∈ σ`.
+ `msymm σ R μ` is the monomial symmetric polynomial whose exponents set are the parts
of `μ ⊢ n` in `MvPolynomial σ R`.
As in other polynomial files, we typically use the notation:
+ `σ τ : Type*` (indexing the variables)
+ `R S : Type*` `[CommSemiring R]` `[CommSemiring S]` (the coefficients)
+ `r : R` elements of the coefficient ring
+ `i : σ`, with corresponding monomial `X i`, often denoted `X_i` by mathematicians
+ `φ ψ : MvPolynomial σ R`
-/
open Equiv (Perm)
noncomputable section
namespace Multiset
variable {R : Type*} [CommSemiring R]
/-- The `n`th elementary symmetric function evaluated at the elements of `s` -/
def esymm (s : Multiset R) (n : ℕ) : R :=
((s.powersetCard n).map Multiset.prod).sum
theorem _root_.Finset.esymm_map_val {σ} (f : σ → R) (s : Finset σ) (n : ℕ) :
(s.val.map f).esymm n = (s.powersetCard n).sum fun t => t.prod f := by
simp only [esymm, powersetCard_map, ← Finset.map_val_val_powersetCard, map_map]
rfl
end Multiset
namespace MvPolynomial
variable {σ τ : Type*} {R S : Type*}
/-- A `MvPolynomial φ` is symmetric if it is invariant under
permutations of its variables by the `rename` operation -/
def IsSymmetric [CommSemiring R] (φ : MvPolynomial σ R) : Prop :=
∀ e : Perm σ, rename e φ = φ
/-- The subalgebra of symmetric `MvPolynomial`s. -/
def symmetricSubalgebra (σ R : Type*) [CommSemiring R] : Subalgebra R (MvPolynomial σ R) where
carrier := setOf IsSymmetric
algebraMap_mem' r e := rename_C e r
mul_mem' ha hb e := by rw [map_mul, ha, hb]
add_mem' ha hb e := by rw [map_add, ha, hb]
@[simp]
theorem mem_symmetricSubalgebra [CommSemiring R] (p : MvPolynomial σ R) :
p ∈ symmetricSubalgebra σ R ↔ p.IsSymmetric :=
Iff.rfl
namespace IsSymmetric
section CommSemiring
variable [CommSemiring R] [CommSemiring S] {φ ψ : MvPolynomial σ R}
@[simp]
theorem C (r : R) : IsSymmetric (C r : MvPolynomial σ R) :=
(symmetricSubalgebra σ R).algebraMap_mem r
@[simp]
theorem zero : IsSymmetric (0 : MvPolynomial σ R) :=
(symmetricSubalgebra σ R).zero_mem
@[simp]
theorem one : IsSymmetric (1 : MvPolynomial σ R) :=
(symmetricSubalgebra σ R).one_mem
theorem add (hφ : IsSymmetric φ) (hψ : IsSymmetric ψ) : IsSymmetric (φ + ψ) :=
(symmetricSubalgebra σ R).add_mem hφ hψ
theorem mul (hφ : IsSymmetric φ) (hψ : IsSymmetric ψ) : IsSymmetric (φ * ψ) :=
(symmetricSubalgebra σ R).mul_mem hφ hψ
theorem smul (r : R) (hφ : IsSymmetric φ) : IsSymmetric (r • φ) :=
(symmetricSubalgebra σ R).smul_mem hφ r
@[simp]
theorem map (hφ : IsSymmetric φ) (f : R →+* S) : IsSymmetric (map f φ) := fun e => by
rw [← map_rename, hφ]
protected theorem rename (hφ : φ.IsSymmetric) (e : σ ≃ τ) : (rename e φ).IsSymmetric := fun _ => by
apply rename_injective _ e.symm.injective
simp_rw [rename_rename, ← Equiv.coe_trans, Equiv.self_trans_symm, Equiv.coe_refl, rename_id]
rw [hφ]
@[simp]
theorem _root_.MvPolynomial.isSymmetric_rename {e : σ ≃ τ} :
(MvPolynomial.rename e φ).IsSymmetric ↔ φ.IsSymmetric :=
⟨fun h => by simpa using (IsSymmetric.rename (R := R) h e.symm), (IsSymmetric.rename · e)⟩
end CommSemiring
section CommRing
variable [CommRing R] {φ ψ : MvPolynomial σ R}
theorem neg (hφ : IsSymmetric φ) : IsSymmetric (-φ) :=
(symmetricSubalgebra σ R).neg_mem hφ
theorem sub (hφ : IsSymmetric φ) (hψ : IsSymmetric ψ) : IsSymmetric (φ - ψ) :=
(symmetricSubalgebra σ R).sub_mem hφ hψ
end CommRing
end IsSymmetric
/-- `MvPolynomial.rename` induces an isomorphism between the symmetric subalgebras. -/
@[simps!]
def renameSymmetricSubalgebra [CommSemiring R] (e : σ ≃ τ) :
symmetricSubalgebra σ R ≃ₐ[R] symmetricSubalgebra τ R :=
AlgEquiv.ofAlgHom
(((rename e).comp (symmetricSubalgebra σ R).val).codRestrict _ <| fun x => x.2.rename e)
(((rename e.symm).comp <| Subalgebra.val _).codRestrict _ <| fun x => x.2.rename e.symm)
(AlgHom.ext <| fun p => Subtype.ext <| by simp)
(AlgHom.ext <| fun p => Subtype.ext <| by simp)
variable (σ R : Type*) [CommSemiring R] [CommSemiring S] [Fintype σ] [Fintype τ]
section ElementarySymmetric
open Finset
/-- The `n`th elementary symmetric `MvPolynomial σ R`.
It is the sum over all the degree n squarefree monomials in `MvPolynomial σ R`. -/
def esymm (n : ℕ) : MvPolynomial σ R :=
∑ t ∈ powersetCard n univ, ∏ i ∈ t, X i
/--
`esymmPart` is the product of the symmetric polynomials `esymm μᵢ`,
where `μ = (μ₁, μ₂, ...)` is a partition.
-/
def esymmPart {n : ℕ} (μ : n.Partition) : MvPolynomial σ R := (μ.parts.map (esymm σ R)).prod
/-- The `n`th elementary symmetric `MvPolynomial σ R` is obtained by evaluating the
`n`th elementary symmetric at the `Multiset` of the monomials -/
theorem esymm_eq_multiset_esymm : esymm σ R = (univ.val.map X).esymm := by
exact funext fun n => (esymm_map_val X _ n).symm
theorem aeval_esymm_eq_multiset_esymm [Algebra R S] (n : ℕ) (f : σ → S) :
aeval f (esymm σ R n) = (univ.val.map f).esymm n := by
simp_rw [esymm, aeval_sum, aeval_prod, aeval_X, esymm_map_val]
/-- We can define `esymm σ R n` by summing over a subtype instead of over `powerset_len`. -/
theorem esymm_eq_sum_subtype (n : ℕ) :
esymm σ R n = ∑ t : { s : Finset σ // s.card = n }, ∏ i ∈ (t : Finset σ), X i :=
sum_subtype _ (fun _ => mem_powersetCard_univ) _
/-- We can define `esymm σ R n` as a sum over explicit monomials -/
theorem esymm_eq_sum_monomial (n : ℕ) :
esymm σ R n = ∑ t ∈ powersetCard n univ, monomial (∑ i ∈ t, Finsupp.single i 1) 1 := by
simp_rw [monomial_sum_one]
rfl
@[simp]
theorem esymm_zero : esymm σ R 0 = 1 := by
simp only [esymm, powersetCard_zero, sum_singleton, prod_empty]
@[simp]
theorem esymm_one : esymm σ R 1 = ∑ i, X i := by simp [esymm, powersetCard_one]
theorem esymmPart_zero : esymmPart σ R (.indiscrete 0) = 1 := by simp [esymmPart]
@[simp]
theorem esymmPart_indiscrete (n : ℕ) : esymmPart σ R (.indiscrete n) = esymm σ R n := by
cases n <;> simp [esymmPart]
theorem map_esymm (n : ℕ) (f : R →+* S) : map f (esymm σ R n) = esymm σ S n := by
simp_rw [esymm, map_sum, map_prod, map_X]
theorem rename_esymm (n : ℕ) (e : σ ≃ τ) : rename e (esymm σ R n) = esymm τ R n :=
calc
rename e (esymm σ R n) = ∑ x ∈ powersetCard n univ, ∏ i ∈ x, X (e i) := by
simp_rw [esymm, map_sum, map_prod, rename_X]
_ = ∑ t ∈ powersetCard n (univ.map e.toEmbedding), ∏ i ∈ t, X i := by
simp [powersetCard_map, -map_univ_equiv]
-- Porting note: Why did `mapEmbedding_apply` not work?
dsimp [mapEmbedding, OrderEmbedding.ofMapLEIff]
simp
_ = ∑ t ∈ powersetCard n univ, ∏ i ∈ t, X i := by rw [map_univ_equiv]
theorem esymm_isSymmetric (n : ℕ) : IsSymmetric (esymm σ R n) := by
intro
rw [rename_esymm]
theorem support_esymm'' [DecidableEq σ] [Nontrivial R] (n : ℕ) :
(esymm σ R n).support =
(powersetCard n (univ : Finset σ)).biUnion fun t =>
(Finsupp.single (∑ i ∈ t, Finsupp.single i 1) (1 : R)).support := by
rw [esymm_eq_sum_monomial]
simp only [← single_eq_monomial]
refine Finsupp.support_sum_eq_biUnion (powersetCard n (univ : Finset σ)) ?_
intro s t hst
rw [disjoint_left, Finsupp.support_single_ne_zero _ one_ne_zero]
rw [Finsupp.support_single_ne_zero _ one_ne_zero]
simp only [one_ne_zero, mem_singleton, Finsupp.mem_support_iff]
rintro a h rfl
have := congr_arg Finsupp.support h
rw [Finsupp.support_sum_eq_biUnion, Finsupp.support_sum_eq_biUnion] at this
· have hsingle : ∀ s : Finset σ, ∀ x : σ, x ∈ s → (Finsupp.single x 1).support = {x} := by
intros _ x _
rw [Finsupp.support_single_ne_zero x one_ne_zero]
have hs := biUnion_congr (of_eq_true (eq_self s)) (hsingle s)
have ht := biUnion_congr (of_eq_true (eq_self t)) (hsingle t)
rw [hs, ht] at this
· simp only [biUnion_singleton_eq_self] at this
exact absurd this hst.symm
all_goals intro x y; simp [Finsupp.support_single_disjoint]
theorem support_esymm' [DecidableEq σ] [Nontrivial R] (n : ℕ) : (esymm σ R n).support =
(powersetCard n (univ : Finset σ)).biUnion fun t => {∑ i ∈ t, Finsupp.single i 1} := by
rw [support_esymm'']
congr
funext
exact Finsupp.support_single_ne_zero _ one_ne_zero
theorem support_esymm [DecidableEq σ] [Nontrivial R] (n : ℕ) : (esymm σ R n).support =
(powersetCard n (univ : Finset σ)).image fun t => ∑ i ∈ t, Finsupp.single i 1 := by
rw [support_esymm']
exact biUnion_singleton
theorem degrees_esymm [Nontrivial R] {n : ℕ} (hpos : 0 < n) (hn : n ≤ Fintype.card σ) :
(esymm σ R n).degrees = (univ : Finset σ).val := by
classical
have :
(Finsupp.toMultiset ∘ fun t : Finset σ => ∑ i ∈ t, Finsupp.single i 1) = val := by
funext
simp [Finsupp.toMultiset_sum_single]
rw [degrees_def, support_esymm, sup_image, this]
have : ((powersetCard n univ).sup (fun (x : Finset σ) => x)).val
= sup (powersetCard n univ) val := by
refine comp_sup_eq_sup_comp _ ?_ ?_
· intros
simp only [union_val, sup_eq_union]
congr
· rfl
rw [← this]
obtain ⟨k, rfl⟩ := Nat.exists_eq_succ_of_ne_zero hpos.ne'
simpa using powersetCard_sup _ _ (Nat.lt_of_succ_le hn)
end ElementarySymmetric
section CompleteHomogeneousSymmetric
open Finset Multiset Sym
variable [DecidableEq σ] [DecidableEq τ]
/-- The `n`th complete homogeneous symmetric `MvPolynomial σ R`.
It is the sum over all the degree n monomials in `MvPolynomial σ R`. -/
def hsymm (n : ℕ) : MvPolynomial σ R := ∑ s : Sym σ n, (s.1.map X).prod
/-- `hsymmPart` is the product of the symmetric polynomials `hsymm μᵢ`,
where `μ = (μ₁, μ₂, ...)` is a partition. -/
def hsymmPart {n : ℕ} (μ : n.Partition) : MvPolynomial σ R := (μ.parts.map (hsymm σ R)).prod
@[simp]
theorem hsymm_zero : hsymm σ R 0 = 1 := by simp [hsymm, eq_nil_of_card_zero]
@[simp]
theorem hsymm_one : hsymm σ R 1 = ∑ i, X i := by
symm
apply Fintype.sum_equiv oneEquiv
simp only [oneEquiv_apply, Multiset.map_singleton, Multiset.prod_singleton, implies_true]
theorem hsymmPart_zero : hsymmPart σ R (.indiscrete 0) = 1 := by simp [hsymmPart]
@[simp]
theorem hsymmPart_indiscrete (n : ℕ) : hsymmPart σ R (.indiscrete n) = hsymm σ R n := by
cases n <;> simp [hsymmPart]
theorem map_hsymm (n : ℕ) (f : R →+* S) : map f (hsymm σ R n) = hsymm σ S n := by
simp [hsymm, ← Multiset.prod_hom']
theorem rename_hsymm (n : ℕ) (e : σ ≃ τ) : rename e (hsymm σ R n) = hsymm τ R n := by
simp_rw [hsymm, map_sum, ← prod_hom', rename_X]
apply Fintype.sum_equiv (equivCongr e)
simp
theorem hsymm_isSymmetric (n : ℕ) : IsSymmetric (hsymm σ R n) := rename_hsymm _ _ n
end CompleteHomogeneousSymmetric
section PowerSum
open Finset
/-- The degree-`n` power sum symmetric `MvPolynomial σ R`.
It is the sum over all the `n`-th powers of the variables. -/
def psum (n : ℕ) : MvPolynomial σ R := ∑ i, X i ^ n
/-- `psumPart` is the product of the symmetric polynomials `psum μᵢ`,
where `μ = (μ₁, μ₂, ...)` is a partition. -/
def psumPart {n : ℕ} (μ : n.Partition) : MvPolynomial σ R := (μ.parts.map (psum σ R)).prod
@[simp]
theorem psum_zero : psum σ R 0 = Fintype.card σ := by simp [psum]
@[simp]
theorem psum_one : psum σ R 1 = ∑ i, X i := by simp [psum]
@[simp]
theorem psumPart_zero : psumPart σ R (.indiscrete 0) = 1 := by simp [psumPart]
@[simp]
theorem psumPart_indiscrete {n : ℕ} (npos : n ≠ 0) :
psumPart σ R (.indiscrete n) = psum σ R n := by simp [psumPart, npos]
@[simp]
theorem rename_psum (n : ℕ) (e : σ ≃ τ) : rename e (psum σ R n) = psum τ R n := by
simp_rw [psum, map_sum, map_pow, rename_X, e.sum_comp (X · ^ n)]
theorem psum_isSymmetric (n : ℕ) : IsSymmetric (psum σ R n) := rename_psum _ _ n
end PowerSum
section MonomialSymmetric
variable [DecidableEq σ] [DecidableEq τ] {n : ℕ}
/-- The monomial symmetric `MvPolynomial σ R` with exponent set μ.
It is the sum over all the monomials in `MvPolynomial σ R` such that
the multiset of exponents is equal to the multiset of parts of μ. -/
def msymm (μ : n.Partition) : MvPolynomial σ R :=
∑ s : {a : Sym σ n // .ofSym a = μ}, (s.1.1.map X).prod
@[simp]
theorem msymm_zero : msymm σ R (.indiscrete 0) = 1 := by
rw [msymm, Fintype.sum_subsingleton _ ⟨(Sym.nil : Sym σ 0), by rfl⟩]
simp
@[simp]
theorem msymm_one : msymm σ R (.indiscrete 1) = ∑ i, X i := by
have : (fun (x : Sym σ 1) ↦ x ∈ Set.univ) =
(fun x ↦ Nat.Partition.ofSym x = Nat.Partition.indiscrete 1) := by
simp_rw [Set.mem_univ, Nat.Partition.ofSym_one]
symm
rw [Fintype.sum_equiv (Equiv.trans Sym.oneEquiv (Equiv.Set.univ (Sym σ 1)).symm)
_ (fun s ↦ (s.1.1.map X).prod)]
· apply Fintype.sum_equiv (Equiv.subtypeEquivProp this)
intro x
congr
· intro x
rw [← Multiset.prod_singleton (X x), ← Multiset.map_singleton]
congr
@[simp]
theorem rename_msymm (μ : n.Partition) (e : σ ≃ τ) :
rename e (msymm σ R μ) = msymm τ R μ := by
rw [msymm, map_sum]
apply Fintype.sum_equiv (Nat.Partition.ofSymShapeEquiv μ e)
intro
rw [← Multiset.prod_hom, Multiset.map_map, Nat.Partition.ofSymShapeEquiv]
simp
theorem msymm_isSymmetric (μ : n.Partition) : IsSymmetric (msymm σ R μ) :=
rename_msymm _ _ μ
end MonomialSymmetric
end MvPolynomial
|
RingTheory\MvPolynomial\Tower.lean | /-
Copyright (c) 2022 Yuyang Zhao. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Yuyang Zhao
-/
import Mathlib.Algebra.Algebra.Tower
import Mathlib.Algebra.MvPolynomial.Basic
/-!
# Algebra towers for multivariate polynomial
This file proves some basic results about the algebra tower structure for the type
`MvPolynomial σ R`.
This structure itself is provided elsewhere as `MvPolynomial.isScalarTower`
When you update this file, you can also try to make a corresponding update in
`RingTheory.Polynomial.Tower`.
-/
variable (R A B : Type*) {σ : Type*}
namespace MvPolynomial
section Semiring
variable [CommSemiring R] [CommSemiring A] [CommSemiring B]
variable [Algebra R A] [Algebra A B] [Algebra R B]
variable [IsScalarTower R A B]
variable {R B}
theorem aeval_map_algebraMap (x : σ → B) (p : MvPolynomial σ R) :
aeval x (map (algebraMap R A) p) = aeval x p := by
rw [aeval_def, aeval_def, eval₂_map, IsScalarTower.algebraMap_eq R A B]
end Semiring
section CommSemiring
variable [CommSemiring R] [CommSemiring A] [CommSemiring B]
variable [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B]
variable {R A}
theorem aeval_algebraMap_apply (x : σ → A) (p : MvPolynomial σ R) :
aeval (algebraMap A B ∘ x) p = algebraMap A B (MvPolynomial.aeval x p) := by
rw [aeval_def, aeval_def, ← coe_eval₂Hom, ← coe_eval₂Hom, map_eval₂Hom, ←
IsScalarTower.algebraMap_eq]
-- Porting note: added
simp only [Function.comp]
theorem aeval_algebraMap_eq_zero_iff [NoZeroSMulDivisors A B] [Nontrivial B] (x : σ → A)
(p : MvPolynomial σ R) : aeval (algebraMap A B ∘ x) p = 0 ↔ aeval x p = 0 := by
rw [aeval_algebraMap_apply, Algebra.algebraMap_eq_smul_one, smul_eq_zero,
iff_false_intro (one_ne_zero' B), or_false_iff]
theorem aeval_algebraMap_eq_zero_iff_of_injective {x : σ → A} {p : MvPolynomial σ R}
(h : Function.Injective (algebraMap A B)) :
aeval (algebraMap A B ∘ x) p = 0 ↔ aeval x p = 0 := by
rw [aeval_algebraMap_apply, ← (algebraMap A B).map_zero, h.eq_iff]
end CommSemiring
end MvPolynomial
namespace Subalgebra
open MvPolynomial
section CommSemiring
variable {R A} [CommSemiring R] [CommSemiring A] [Algebra R A]
@[simp]
theorem mvPolynomial_aeval_coe (S : Subalgebra R A) (x : σ → S) (p : MvPolynomial σ R) :
aeval (fun i => (x i : A)) p = aeval x p := by convert aeval_algebraMap_apply A x p
end CommSemiring
end Subalgebra
|
RingTheory\MvPolynomial\WeightedHomogeneous.lean | /-
Copyright (c) 2022 María Inés de Frutos-Fernández. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Antoine Chambert-Loir, María Inés de Frutos-Fernández
-/
import Mathlib.Algebra.DirectSum.Decomposition
import Mathlib.Algebra.GradedMonoid
import Mathlib.Algebra.MvPolynomial.Basic
import Mathlib.Algebra.Order.Monoid.Canonical.Defs
import Mathlib.Data.Finsupp.Weight
import Mathlib.RingTheory.GradedAlgebra.Basic
/-!
# Weighted homogeneous polynomials
It is possible to assign weights (in a commutative additive monoid `M`) to the variables of a
multivariate polynomial ring, so that monomials of the ring then have a weighted degree with
respect to the weights of the variables. The weights are represented by a function `w : σ → M`,
where `σ` are the indeterminates.
A multivariate polynomial `φ` is weighted homogeneous of weighted degree `m : M` if all monomials
occurring in `φ` have the same weighted degree `m`.
## Main definitions/lemmas
* `weightedTotalDegree' w φ` : the weighted total degree of a multivariate polynomial with respect
to the weights `w`, taking values in `WithBot M`.
* `weightedTotalDegree w φ` : When `M` has a `⊥` element, we can define the weighted total degree
of a multivariate polynomial as a function taking values in `M`.
* `IsWeightedHomogeneous w φ m`: a predicate that asserts that `φ` is weighted homogeneous
of weighted degree `m` with respect to the weights `w`.
* `weightedHomogeneousSubmodule R w m`: the submodule of homogeneous polynomials
of weighted degree `m`.
* `weightedHomogeneousComponent w m`: the additive morphism that projects polynomials
onto their summand that is weighted homogeneous of degree `n` with respect to `w`.
* `sum_weightedHomogeneousComponent`: every polynomial is the sum of its weighted homogeneous
components.
-/
noncomputable section
open Set Function Finset Finsupp AddMonoidAlgebra
variable {R M : Type*} [CommSemiring R]
namespace MvPolynomial
variable {σ : Type*}
section AddCommMonoid
variable [AddCommMonoid M]
/-! ### `weight` -/
section SemilatticeSup
variable [SemilatticeSup M]
/-- The weighted total degree of a multivariate polynomial, taking values in `WithBot M`. -/
def weightedTotalDegree' (w : σ → M) (p : MvPolynomial σ R) : WithBot M :=
p.support.sup fun s => weight w s
/-- The `weightedTotalDegree'` of a polynomial `p` is `⊥` if and only if `p = 0`. -/
theorem weightedTotalDegree'_eq_bot_iff (w : σ → M) (p : MvPolynomial σ R) :
weightedTotalDegree' w p = ⊥ ↔ p = 0 := by
simp only [weightedTotalDegree', Finset.sup_eq_bot_iff, mem_support_iff, WithBot.coe_ne_bot,
MvPolynomial.eq_zero_iff]
exact forall_congr' fun _ => Classical.not_not
/-- The `weightedTotalDegree'` of the zero polynomial is `⊥`. -/
theorem weightedTotalDegree'_zero (w : σ → M) :
weightedTotalDegree' w (0 : MvPolynomial σ R) = ⊥ := by
simp only [weightedTotalDegree', support_zero, Finset.sup_empty]
section OrderBot
variable [OrderBot M]
/-- When `M` has a `⊥` element, we can define the weighted total degree of a multivariate
polynomial as a function taking values in `M`. -/
def weightedTotalDegree (w : σ → M) (p : MvPolynomial σ R) : M :=
p.support.sup fun s => weight w s
/-- This lemma relates `weightedTotalDegree` and `weightedTotalDegree'`. -/
theorem weightedTotalDegree_coe (w : σ → M) (p : MvPolynomial σ R) (hp : p ≠ 0) :
weightedTotalDegree' w p = ↑(weightedTotalDegree w p) := by
rw [Ne, ← weightedTotalDegree'_eq_bot_iff w p, ← Ne, WithBot.ne_bot_iff_exists] at hp
obtain ⟨m, hm⟩ := hp
apply le_antisymm
· simp only [weightedTotalDegree, weightedTotalDegree', Finset.sup_le_iff, WithBot.coe_le_coe]
intro b
exact Finset.le_sup
· simp only [weightedTotalDegree]
have hm' : weightedTotalDegree' w p ≤ m := le_of_eq hm.symm
rw [← hm]
simpa [weightedTotalDegree'] using hm'
/-- The `weightedTotalDegree` of the zero polynomial is `⊥`. -/
theorem weightedTotalDegree_zero (w : σ → M) :
weightedTotalDegree w (0 : MvPolynomial σ R) = ⊥ := by
simp only [weightedTotalDegree, support_zero, Finset.sup_empty]
theorem le_weightedTotalDegree (w : σ → M) {φ : MvPolynomial σ R} {d : σ →₀ ℕ}
(hd : d ∈ φ.support) : weight w d ≤ φ.weightedTotalDegree w :=
le_sup hd
end OrderBot
end SemilatticeSup
/-- A multivariate polynomial `φ` is weighted homogeneous of weighted degree `m` if all monomials
occurring in `φ` have weighted degree `m`. -/
def IsWeightedHomogeneous (w : σ → M) (φ : MvPolynomial σ R) (m : M) : Prop :=
∀ ⦃d⦄, coeff d φ ≠ 0 → weight w d = m
variable (R)
/-- The submodule of homogeneous `MvPolynomial`s of degree `n`. -/
def weightedHomogeneousSubmodule (w : σ → M) (m : M) : Submodule R (MvPolynomial σ R) where
carrier := { x | x.IsWeightedHomogeneous w m }
smul_mem' r a ha c hc := by
rw [coeff_smul] at hc
exact ha (right_ne_zero_of_mul hc)
zero_mem' d hd := False.elim (hd <| coeff_zero _)
add_mem' {a} {b} ha hb c hc := by
rw [coeff_add] at hc
obtain h | h : coeff c a ≠ 0 ∨ coeff c b ≠ 0 := by
contrapose! hc
simp only [hc, add_zero]
· exact ha h
· exact hb h
@[simp]
theorem mem_weightedHomogeneousSubmodule (w : σ → M) (m : M) (p : MvPolynomial σ R) :
p ∈ weightedHomogeneousSubmodule R w m ↔ p.IsWeightedHomogeneous w m :=
Iff.rfl
/-- The submodule `weightedHomogeneousSubmodule R w m` of homogeneous `MvPolynomial`s of
degree `n` is equal to the `R`-submodule of all `p : (σ →₀ ℕ) →₀ R` such that
`p.support ⊆ {d | weight w d = m}`. While equal, the former has a
convenient definitional reduction. -/
theorem weightedHomogeneousSubmodule_eq_finsupp_supported (w : σ → M) (m : M) :
weightedHomogeneousSubmodule R w m = Finsupp.supported R R { d | weight w d = m } := by
ext x
rw [mem_supported, Set.subset_def]
simp only [Finsupp.mem_support_iff, mem_coe]
rfl
variable {R}
/-- The submodule generated by products `Pm * Pn` of weighted homogeneous polynomials of degrees `m`
and `n` is contained in the submodule of weighted homogeneous polynomials of degree `m + n`. -/
theorem weightedHomogeneousSubmodule_mul (w : σ → M) (m n : M) :
weightedHomogeneousSubmodule R w m * weightedHomogeneousSubmodule R w n ≤
weightedHomogeneousSubmodule R w (m + n) := by
classical
rw [Submodule.mul_le]
intro φ hφ ψ hψ c hc
rw [coeff_mul] at hc
obtain ⟨⟨d, e⟩, hde, H⟩ := Finset.exists_ne_zero_of_sum_ne_zero hc
have aux : coeff d φ ≠ 0 ∧ coeff e ψ ≠ 0 := by
contrapose! H
by_cases h : coeff d φ = 0 <;>
simp_all only [Ne, not_false_iff, zero_mul, mul_zero]
rw [← mem_antidiagonal.mp hde, ← hφ aux.1, ← hψ aux.2, map_add]
/-- Monomials are weighted homogeneous. -/
theorem isWeightedHomogeneous_monomial (w : σ → M) (d : σ →₀ ℕ) (r : R) {m : M}
(hm : weight w d = m) : IsWeightedHomogeneous w (monomial d r) m := by
classical
intro c hc
rw [coeff_monomial] at hc
split_ifs at hc with h
· subst c
exact hm
· contradiction
/-- A polynomial of weightedTotalDegree `⊥` is weighted_homogeneous of degree `⊥`. -/
theorem isWeightedHomogeneous_of_total_degree_zero [SemilatticeSup M] [OrderBot M] (w : σ → M)
{p : MvPolynomial σ R} (hp : weightedTotalDegree w p = (⊥ : M)) :
IsWeightedHomogeneous w p (⊥ : M) := by
intro d hd
have h := weightedTotalDegree_coe w p (MvPolynomial.ne_zero_iff.mpr ⟨d, hd⟩)
simp only [weightedTotalDegree', hp] at h
rw [eq_bot_iff, ← WithBot.coe_le_coe, ← h]
apply Finset.le_sup (mem_support_iff.mpr hd)
/-- Constant polynomials are weighted homogeneous of degree 0. -/
theorem isWeightedHomogeneous_C (w : σ → M) (r : R) :
IsWeightedHomogeneous w (C r : MvPolynomial σ R) 0 :=
isWeightedHomogeneous_monomial _ _ _ (map_zero _)
variable (R)
/-- 0 is weighted homogeneous of any degree. -/
theorem isWeightedHomogeneous_zero (w : σ → M) (m : M) :
IsWeightedHomogeneous w (0 : MvPolynomial σ R) m :=
(weightedHomogeneousSubmodule R w m).zero_mem
/-- 1 is weighted homogeneous of degree 0. -/
theorem isWeightedHomogeneous_one (w : σ → M) : IsWeightedHomogeneous w (1 : MvPolynomial σ R) 0 :=
isWeightedHomogeneous_C _ _
/-- An indeterminate `i : σ` is weighted homogeneous of degree `w i`. -/
theorem isWeightedHomogeneous_X (w : σ → M) (i : σ) :
IsWeightedHomogeneous w (X i : MvPolynomial σ R) (w i) := by
apply isWeightedHomogeneous_monomial
simp only [weight, LinearMap.toAddMonoidHom_coe, total_single, one_nsmul]
namespace IsWeightedHomogeneous
variable {R}
variable {φ ψ : MvPolynomial σ R} {m n : M}
/-- The weighted degree of a weighted homogeneous polynomial controls its support. -/
theorem coeff_eq_zero {w : σ → M} (hφ : IsWeightedHomogeneous w φ n) (d : σ →₀ ℕ)
(hd : weight w d ≠ n) : coeff d φ = 0 := by
have aux := mt (@hφ d) hd
rwa [Classical.not_not] at aux
/-- The weighted degree of a nonzero weighted homogeneous polynomial is well-defined. -/
theorem inj_right {w : σ → M} (hφ : φ ≠ 0) (hm : IsWeightedHomogeneous w φ m)
(hn : IsWeightedHomogeneous w φ n) : m = n := by
obtain ⟨d, hd⟩ : ∃ d, coeff d φ ≠ 0 := exists_coeff_ne_zero hφ
rw [← hm hd, ← hn hd]
/-- The sum of two weighted homogeneous polynomials of degree `n` is weighted homogeneous of
weighted degree `n`. -/
theorem add {w : σ → M} (hφ : IsWeightedHomogeneous w φ n) (hψ : IsWeightedHomogeneous w ψ n) :
IsWeightedHomogeneous w (φ + ψ) n :=
(weightedHomogeneousSubmodule R w n).add_mem hφ hψ
/-- The sum of weighted homogeneous polynomials of degree `n` is weighted homogeneous of
weighted degree `n`. -/
theorem sum {ι : Type*} (s : Finset ι) (φ : ι → MvPolynomial σ R) (n : M) {w : σ → M}
(h : ∀ i ∈ s, IsWeightedHomogeneous w (φ i) n) : IsWeightedHomogeneous w (∑ i ∈ s, φ i) n :=
(weightedHomogeneousSubmodule R w n).sum_mem h
/-- The product of weighted homogeneous polynomials of weighted degrees `m` and `n` is weighted
homogeneous of weighted degree `m + n`. -/
theorem mul {w : σ → M} (hφ : IsWeightedHomogeneous w φ m) (hψ : IsWeightedHomogeneous w ψ n) :
IsWeightedHomogeneous w (φ * ψ) (m + n) :=
weightedHomogeneousSubmodule_mul w m n <| Submodule.mul_mem_mul hφ hψ
/-- A product of weighted homogeneous polynomials is weighted homogeneous, with weighted degree
equal to the sum of the weighted degrees. -/
theorem prod {ι : Type*} (s : Finset ι) (φ : ι → MvPolynomial σ R) (n : ι → M) {w : σ → M} :
(∀ i ∈ s, IsWeightedHomogeneous w (φ i) (n i)) →
IsWeightedHomogeneous w (∏ i ∈ s, φ i) (∑ i ∈ s, n i) := by
classical
refine Finset.induction_on s ?_ ?_
· intro
simp only [isWeightedHomogeneous_one, Finset.sum_empty, Finset.prod_empty]
· intro i s his IH h
simp only [his, Finset.prod_insert, Finset.sum_insert, not_false_iff]
apply (h i (Finset.mem_insert_self _ _)).mul (IH _)
intro j hjs
exact h j (Finset.mem_insert_of_mem hjs)
/-- A non zero weighted homogeneous polynomial of weighted degree `n` has weighted total degree
`n`. -/
theorem weighted_total_degree [SemilatticeSup M] {w : σ → M} (hφ : IsWeightedHomogeneous w φ n)
(h : φ ≠ 0) : weightedTotalDegree' w φ = n := by
simp only [weightedTotalDegree']
apply le_antisymm
· simp only [Finset.sup_le_iff, mem_support_iff, WithBot.coe_le_coe]
exact fun d hd => le_of_eq (hφ hd)
· obtain ⟨d, hd⟩ : ∃ d, coeff d φ ≠ 0 := exists_coeff_ne_zero h
simp only [← hφ hd, Finsupp.sum]
replace hd := Finsupp.mem_support_iff.mpr hd
apply Finset.le_sup hd
end IsWeightedHomogeneous
variable {R}
/-- The weighted homogeneous submodules form a graded monoid. -/
lemma WeightedHomogeneousSubmodule.gradedMonoid {w : σ → M} :
SetLike.GradedMonoid (weightedHomogeneousSubmodule R w) where
one_mem := isWeightedHomogeneous_one R w
mul_mem _ _ _ _ := IsWeightedHomogeneous.mul
/-- `weightedHomogeneousComponent w n φ` is the part of `φ` that is weighted homogeneous of
weighted degree `n`, with respect to the weights `w`.
See `sum_weightedHomogeneousComponent` for the statement that `φ` is equal to the sum
of all its weighted homogeneous components. -/
def weightedHomogeneousComponent (w : σ → M) (n : M) : MvPolynomial σ R →ₗ[R] MvPolynomial σ R :=
letI := Classical.decEq M
(Submodule.subtype _).comp <| Finsupp.restrictDom _ _ { d | weight w d = n }
section WeightedHomogeneousComponent
variable {w : σ → M} (n : M) (φ ψ : MvPolynomial σ R)
theorem coeff_weightedHomogeneousComponent [DecidableEq M] (d : σ →₀ ℕ) :
coeff d (weightedHomogeneousComponent w n φ) =
if weight w d = n then coeff d φ else 0 :=
letI := Classical.decEq M
Finsupp.filter_apply (fun d : σ →₀ ℕ => weight w d = n) φ d |>.trans <| by convert rfl
theorem weightedHomogeneousComponent_apply [DecidableEq M] :
weightedHomogeneousComponent w n φ =
∑ d ∈ φ.support.filter fun d => weight w d = n, monomial d (coeff d φ) :=
letI := Classical.decEq M
Finsupp.filter_eq_sum (fun d : σ →₀ ℕ => weight w d = n) φ |>.trans <| by convert rfl
/-- The `n` weighted homogeneous component of a polynomial is weighted homogeneous of
weighted degree `n`. -/
theorem weightedHomogeneousComponent_isWeightedHomogeneous :
(weightedHomogeneousComponent w n φ).IsWeightedHomogeneous w n := by
classical
intro d hd
contrapose! hd
rw [coeff_weightedHomogeneousComponent, if_neg hd]
theorem weightedHomogeneousComponent_mem (w : σ → M) (φ : MvPolynomial σ R) (m : M) :
weightedHomogeneousComponent w m φ ∈ weightedHomogeneousSubmodule R w m := by
rw [mem_weightedHomogeneousSubmodule]
exact weightedHomogeneousComponent_isWeightedHomogeneous m φ
@[simp]
theorem weightedHomogeneousComponent_C_mul (n : M) (r : R) :
weightedHomogeneousComponent w n (C r * φ) = C r * weightedHomogeneousComponent w n φ := by
simp only [C_mul', LinearMap.map_smul]
theorem weightedHomogeneousComponent_eq_zero'
(h : ∀ d : σ →₀ ℕ, d ∈ φ.support → weight w d ≠ n) :
weightedHomogeneousComponent w n φ = 0 := by
classical
rw [weightedHomogeneousComponent_apply, sum_eq_zero]
intro d hd; rw [mem_filter] at hd
exfalso; exact h _ hd.1 hd.2
theorem weightedHomogeneousComponent_eq_zero [SemilatticeSup M] [OrderBot M]
(h : weightedTotalDegree w φ < n) : weightedHomogeneousComponent w n φ = 0 := by
classical
rw [weightedHomogeneousComponent_apply, sum_eq_zero]
intro d hd
rw [Finset.mem_filter] at hd
exfalso
apply lt_irrefl n
nth_rw 1 [← hd.2]
exact lt_of_le_of_lt (le_weightedTotalDegree w hd.1) h
theorem weightedHomogeneousComponent_finsupp :
(Function.support fun m => weightedHomogeneousComponent w m φ).Finite := by
suffices
(Function.support fun m => weightedHomogeneousComponent w m φ) ⊆
(fun d => weight w d) '' φ.support by
exact Finite.subset ((fun d : σ →₀ ℕ => (weight w) d) '' ↑(support φ)).toFinite this
intro m hm
by_contra hm'
apply hm
simp only [mem_support, Ne] at hm
simp only [Set.mem_image, not_exists, not_and] at hm'
exact weightedHomogeneousComponent_eq_zero' m φ hm'
variable (w)
/-- Every polynomial is the sum of its weighted homogeneous components. -/
theorem sum_weightedHomogeneousComponent :
(finsum fun m => weightedHomogeneousComponent w m φ) = φ := by
classical
rw [finsum_eq_sum _ (weightedHomogeneousComponent_finsupp φ)]
ext1 d
simp only [coeff_sum, coeff_weightedHomogeneousComponent]
rw [Finset.sum_eq_single (weight w d)]
· rw [if_pos rfl]
· intro m _ hm'
rw [if_neg hm'.symm]
· intro hm
rw [if_pos rfl]
simp only [Finite.mem_toFinset, mem_support, Ne, Classical.not_not] at hm
have := coeff_weightedHomogeneousComponent (w := w) (weight w d) φ d
rw [hm, if_pos rfl, coeff_zero] at this
exact this.symm
theorem finsum_weightedHomogeneousComponent :
(finsum fun m => weightedHomogeneousComponent w m φ) = φ := by
rw [sum_weightedHomogeneousComponent]
variable {w}
theorem IsWeightedHomogeneous.weightedHomogeneousComponent_same {m : M} {p : MvPolynomial σ R}
(hp : IsWeightedHomogeneous w p m) :
weightedHomogeneousComponent w m p = p := by
classical
ext x
rw [coeff_weightedHomogeneousComponent]
by_cases zero_coeff : coeff x p = 0
· split_ifs
· rfl
rw [zero_coeff]
· rw [hp zero_coeff, if_pos rfl]
theorem IsWeightedHomogeneous.weightedHomogeneousComponent_ne {m : M} (n : M)
{p : MvPolynomial σ R} (hp : IsWeightedHomogeneous w p m) :
n ≠ m → weightedHomogeneousComponent w n p = 0 := by
classical
intro hn
ext x
rw [coeff_weightedHomogeneousComponent]
by_cases zero_coeff : coeff x p = 0
· simp [zero_coeff]
· rw [if_neg]
· rw [coeff_zero]
· rw [hp zero_coeff]; exact Ne.symm hn
/-- The weighted homogeneous components of a weighted homogeneous polynomial. -/
theorem weightedHomogeneousComponent_of_mem [DecidableEq M] {m n : M}
{p : MvPolynomial σ R} (h : p ∈ weightedHomogeneousSubmodule R w n) :
weightedHomogeneousComponent w m p = if m = n then p else 0 := by
simp only [mem_weightedHomogeneousSubmodule] at h
ext x
rw [coeff_weightedHomogeneousComponent]
by_cases zero_coeff : coeff x p = 0
· split_ifs <;>
simp only [zero_coeff, coeff_zero]
· rw [h zero_coeff]
simp only [show n = m ↔ m = n from eq_comm]
split_ifs with h1
· rfl
· simp only [coeff_zero]
theorem weightedHomogeneousComponent_of_isWeightedHomogeneous_same
{m : M} {p : MvPolynomial σ R} (hp : IsWeightedHomogeneous w p m) :
weightedHomogeneousComponent w m p = p := by
classical
ext x
rw [coeff_weightedHomogeneousComponent]
by_cases zero_coeff : coeff x p = 0
· simp [zero_coeff]
· rw [hp zero_coeff, if_pos rfl]
theorem weightedHomogeneousComponent_of_isWeightedHomogeneous_ne
{m n : M} {p : MvPolynomial σ R} (hp : IsWeightedHomogeneous w p m) (hn : n ≠ m) :
weightedHomogeneousComponent w n p = 0 := by
classical
ext x
rw [coeff_weightedHomogeneousComponent]
by_cases zero_coeff : coeff x p = 0
· simp [zero_coeff]
· rw [if_neg (by simp only [hp zero_coeff, hn.symm, not_false_eq_true]), coeff_zero]
variable (R w)
open DirectSum
theorem DirectSum.coeLinearMap_eq_dfinsupp_sum [DecidableEq σ] [DecidableEq R] [DecidableEq M]
(x : DirectSum M fun i : M => ↥(weightedHomogeneousSubmodule R w i)) :
(coeLinearMap fun i : M => weightedHomogeneousSubmodule R w i) x =
DFinsupp.sum x (fun _ x => ↑x) := by
rw [_root_.DirectSum.coeLinearMap_eq_dfinsupp_sum]
theorem DirectSum.coeAddMonoidHom_eq_support_sum [DecidableEq σ] [DecidableEq R] [DecidableEq M]
(x : DirectSum M fun i : M => ↥(weightedHomogeneousSubmodule R w i)) :
(DirectSum.coeAddMonoidHom fun i : M => weightedHomogeneousSubmodule R w i) x =
DFinsupp.sum x (fun _ x => ↑x) :=
DirectSum.coeLinearMap_eq_dfinsupp_sum R w x
theorem DirectSum.coeLinearMap_eq_finsum [DecidableEq M]
(x : DirectSum M fun i : M => ↥(weightedHomogeneousSubmodule R w i)) :
(DirectSum.coeLinearMap fun i : M => weightedHomogeneousSubmodule R w i) x =
finsum fun m => x m := by
classical
rw [DirectSum.coeLinearMap_eq_dfinsupp_sum, DFinsupp.sum, finsum_eq_sum_of_support_subset]
apply DirectSum.support_subset
theorem weightedHomogeneousComponent_directSum [DecidableEq M]
(x : DirectSum M fun i : M => ↥(weightedHomogeneousSubmodule R w i)) (m : M) :
(weightedHomogeneousComponent w m)
((DirectSum.coeLinearMap fun i : M => weightedHomogeneousSubmodule R w i) x) = x m := by
classical
rw [DirectSum.coeLinearMap_eq_dfinsupp_sum, DFinsupp.sum, map_sum]
convert @Finset.sum_eq_single M (MvPolynomial σ R) _ (DFinsupp.support x) _ m _ _
· rw [weightedHomogeneousComponent_of_isWeightedHomogeneous_same (x m).prop]
· intro n _ hmn
rw [weightedHomogeneousComponent_of_isWeightedHomogeneous_ne (x n).prop hmn.symm]
· rw [DFinsupp.not_mem_support_iff]
intro hm; rw [hm, Submodule.coe_zero, map_zero]
end WeightedHomogeneousComponent
end AddCommMonoid
section CanonicallyOrderedAddCommMonoid
variable [CanonicallyOrderedAddCommMonoid M] {w : σ → M} (φ : MvPolynomial σ R)
/-- If `M` is a `CanonicallyOrderedAddCommMonoid`, then the `weightedHomogeneousComponent`
of weighted degree `0` of a polynomial is its constant coefficient. -/
@[simp]
theorem weightedHomogeneousComponent_zero [NoZeroSMulDivisors ℕ M] (hw : ∀ i : σ, w i ≠ 0) :
weightedHomogeneousComponent w 0 φ = C (coeff 0 φ) := by
classical
ext1 d
rcases Classical.em (d = 0) with (rfl | hd)
· simp only [coeff_weightedHomogeneousComponent, if_pos, map_zero, coeff_zero_C]
· rw [coeff_weightedHomogeneousComponent, if_neg, coeff_C, if_neg (Ne.symm hd)]
simp only [weight, LinearMap.toAddMonoidHom_coe, Finsupp.total_apply, Finsupp.sum,
sum_eq_zero_iff, Finsupp.mem_support_iff, Ne, smul_eq_zero, not_forall, not_or,
and_self_left, exists_prop]
simp only [DFunLike.ext_iff, Finsupp.coe_zero, Pi.zero_apply, not_forall] at hd
obtain ⟨i, hi⟩ := hd
exact ⟨i, hi, hw i⟩
/-- A weight function is nontorsion if its values are not torsion. -/
def NonTorsionWeight (w : σ → M) :=
∀ n x, n • w x = (0 : M) → n = 0
theorem nonTorsionWeight_of [NoZeroSMulDivisors ℕ M] (hw : ∀ i : σ, w i ≠ 0) :
NonTorsionWeight w :=
fun _ x hnx => (smul_eq_zero_iff_left (hw x)).mp hnx
end CanonicallyOrderedAddCommMonoid
section CanonicallyLinearOrderedMonoid
variable [CanonicallyLinearOrderedAddCommMonoid M] {w : σ → M} (φ : MvPolynomial σ R)
/-- If `w` is a nontorsion weight function, then the finitely supported function `m : σ →₀ ℕ`
has weighted degree zero if and only if `∀ x : σ, m x = 0`. -/
theorem weightedDegree_eq_zero_iff (hw : NonTorsionWeight w) {m : σ →₀ ℕ} :
weight w m = 0 ↔ ∀ x : σ, m x = 0 := by
simp only [weight, Finsupp.total, LinearMap.toAddMonoidHom_coe, coe_lsum,
LinearMap.coe_smulRight, LinearMap.id_coe, id_eq]
rw [Finsupp.sum, Finset.sum_eq_zero_iff]
apply forall_congr'
intro x
rw [Finsupp.mem_support_iff]
constructor
· intro hx
by_contra hx'
exact absurd (hw _ _ (hx hx')) hx'
· intro hax _
simp only [hax, zero_smul]
/-- A multivatiate polynomial is weighted homogeneous of weighted degree zero if and only if
its weighted total degree is equal to zero. -/
theorem isWeightedHomogeneous_zero_iff_weightedTotalDegree_eq_zero {p : MvPolynomial σ R} :
IsWeightedHomogeneous w p 0 ↔ p.weightedTotalDegree w = 0 := by
rw [weightedTotalDegree, ← bot_eq_zero, Finset.sup_eq_bot_iff, bot_eq_zero, IsWeightedHomogeneous]
apply forall_congr'
intro m
rw [mem_support_iff]
/-- If `w` is a nontorsion weight function, then a multivariate polynomial has weighted total
degree zero if and only if for every `m ∈ p.support` and `x : σ`, `m x = 0`. -/
theorem weightedTotalDegree_eq_zero_iff (hw : NonTorsionWeight w) (p : MvPolynomial σ R) :
p.weightedTotalDegree w = 0 ↔ ∀ (m : σ →₀ ℕ) (_ : m ∈ p.support) (x : σ), m x = 0 := by
rw [← isWeightedHomogeneous_zero_iff_weightedTotalDegree_eq_zero, IsWeightedHomogeneous]
apply forall_congr'
intro m
rw [mem_support_iff]
apply forall_congr'
intro _
exact weightedDegree_eq_zero_iff hw
end CanonicallyLinearOrderedMonoid
section GradedAlgebra
/- Here, given a weight `w : σ → M`, where `M` is an additive and commutative monoid, we endow the
ring of multivariate polynomials `MvPolynomial σ R` with the structure of a graded algebra -/
variable (w : σ → M) [AddCommMonoid M]
theorem weightedHomogeneousComponent_eq_zero_of_not_mem [DecidableEq M]
(φ : MvPolynomial σ R) (i : M) (hi : i ∉ Finset.image (weight w) φ.support) :
weightedHomogeneousComponent w i φ = 0 := by
apply weightedHomogeneousComponent_eq_zero'
simp only [Finset.mem_image, mem_support_iff, ne_eq, exists_prop, not_exists, not_and] at hi
exact fun m hm ↦ hi m (mem_support_iff.mp hm)
variable (R)
/-- The `decompose'` argument of `weightedDecomposition`. -/
def decompose' [DecidableEq M] := fun φ : MvPolynomial σ R =>
DirectSum.mk (fun i : M => ↥(weightedHomogeneousSubmodule R w i))
(Finset.image (weight w) φ.support) fun m =>
⟨weightedHomogeneousComponent w m φ, weightedHomogeneousComponent_mem w φ m⟩
theorem decompose'_apply [DecidableEq M] (φ : MvPolynomial σ R) (m : M) :
(decompose' R w φ m : MvPolynomial σ R) = weightedHomogeneousComponent w m φ := by
rw [decompose']
by_cases hm : m ∈ Finset.image (weight w) φ.support
· simp only [DirectSum.mk_apply_of_mem hm, Subtype.coe_mk]
· rw [DirectSum.mk_apply_of_not_mem hm, Submodule.coe_zero,
weightedHomogeneousComponent_eq_zero_of_not_mem w φ m hm]
/-- Given a weight `w`, the decomposition of `MvPolynomial σ R` into weighted homogeneous
submodules -/
def weightedDecomposition [DecidableEq M] :
DirectSum.Decomposition (weightedHomogeneousSubmodule R w) where
decompose' := decompose' R w
left_inv φ := by
classical
conv_rhs => rw [← sum_weightedHomogeneousComponent w φ]
rw [← DirectSum.sum_support_of (decompose' R w φ)]
simp only [DirectSum.coeAddMonoidHom_of, MvPolynomial.coeff_sum, map_sum,
finsum_eq_sum _ (weightedHomogeneousComponent_finsupp φ)]
apply Finset.sum_congr _ (fun m _ ↦ by rw [decompose'_apply])
ext m
simp only [DFinsupp.mem_support_toFun, ne_eq, Set.Finite.mem_toFinset, Function.mem_support,
not_iff_not]
conv_lhs => rw [← Subtype.coe_inj]
rw [decompose'_apply, Submodule.coe_zero]
right_inv x := by
classical
apply DFinsupp.ext
intro m
rw [← Subtype.coe_inj, decompose'_apply]
exact weightedHomogeneousComponent_directSum R w x m
/-- Given a weight, `MvPolynomial` as a graded algebra -/
def weightedGradedAlgebra [DecidableEq M] :
GradedAlgebra (weightedHomogeneousSubmodule R w) where
toDecomposition := weightedDecomposition R w
toGradedMonoid := WeightedHomogeneousSubmodule.gradedMonoid
theorem weightedDecomposition.decompose'_eq [DecidableEq M] :
(weightedDecomposition R w).decompose' = fun φ : MvPolynomial σ R =>
DirectSum.mk (fun i : M => ↥(weightedHomogeneousSubmodule R w i))
(Finset.image (weight w) φ.support) fun m =>
⟨weightedHomogeneousComponent w m φ, weightedHomogeneousComponent_mem w φ m⟩ := rfl
theorem weightedDecomposition.decompose'_apply [DecidableEq M]
(φ : MvPolynomial σ R) (m : M) :
((weightedDecomposition R w).decompose' φ m : MvPolynomial σ R) =
weightedHomogeneousComponent w m φ :=
MvPolynomial.decompose'_apply R w φ m
end GradedAlgebra
end MvPolynomial
|
RingTheory\MvPowerSeries\Basic.lean | /-
Copyright (c) 2019 Johan Commelin. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johan Commelin, Kenny Lau
-/
import Mathlib.Algebra.MvPolynomial.Basic
import Mathlib.Algebra.Order.Antidiag.Finsupp
import Mathlib.Data.Finsupp.Antidiagonal
import Mathlib.Data.Finsupp.Weight
import Mathlib.LinearAlgebra.StdBasis
import Mathlib.Tactic.Linarith
/-!
# Formal (multivariate) power series
This file defines multivariate formal power series
and develops the basic properties of these objects.
A formal power series is to a polynomial like an infinite sum is to a finite sum.
We provide the natural inclusion from multivariate polynomials to multivariate formal power series.
## Main definitions
- `MvPowerSeries.C`: constant power series
- `MvPowerSeries.X`: the indeterminates
- `MvPowerSeries.coeff`, `MvPowerSeries.constantCoeff`:
the coefficients of a `MvPowerSeries`, its constant coefficient
- `MvPowerSeries.monomial`: the monomials
- `MvPowerSeries.coeff_mul`: computes the coefficients of the product of two `MvPowerSeries`
- `MvPowerSeries.coeff_prod` : computes the coefficients of products of `MvPowerSeries`
- `MvPowerSeries.coeff_pow` : computes the coefficients of powers of a `MvPowerSeries`
- `MvPowerSeries.coeff_eq_zero_of_constantCoeff_nilpotent`: if the constant coefficient
of a `MvPowerSeries` is nilpotent, then some coefficients of its powers are automatically zero
- `MvPowerSeries.map`: apply a `RingHom` to the coefficients of a `MvPowerSeries` (as a `RingHom)
- `MvPowerSeries.X_pow_dvd_iff`, `MvPowerSeries.X_dvd_iff`: equivalent
conditions for (a power of) an indeterminate to divide a `MvPowerSeries`
- `MvPolynomial.toMvPowerSeries`: the canonical coercion from `MvPolynomial` to `MvPowerSeries`
## Note
This file sets up the (semi)ring structure on multivariate power series:
additional results are in:
* `Mathlib.RingTheory.MvPowerSeries.Inverse` : invertibility,
formal power series over a local ring form a local ring;
* `Mathlib.RingTheory.MvPowerSeries.Trunc`: truncation of power series.
In `Mathlib.RingTheory.PowerSeries.Basic`, formal power series in one variable
will be obtained as a particular case, defined by
`PowerSeries R := MvPowerSeries Unit R`.
See that file for a specific description.
## Implementation notes
In this file we define multivariate formal power series with
variables indexed by `σ` and coefficients in `R` as
`MvPowerSeries σ R := (σ →₀ ℕ) → R`.
Unfortunately there is not yet enough API to show that they are the completion
of the ring of multivariate polynomials. However, we provide most of the infrastructure
that is needed to do this. Once I-adic completion (topological or algebraic) is available
it should not be hard to fill in the details.
-/
noncomputable section
open Finset (antidiagonal mem_antidiagonal)
/-- Multivariate formal power series, where `σ` is the index set of the variables
and `R` is the coefficient ring. -/
def MvPowerSeries (σ : Type*) (R : Type*) :=
(σ →₀ ℕ) → R
namespace MvPowerSeries
open Finsupp
variable {σ R : Type*}
instance [Inhabited R] : Inhabited (MvPowerSeries σ R) :=
⟨fun _ => default⟩
instance [Zero R] : Zero (MvPowerSeries σ R) :=
Pi.instZero
instance [AddMonoid R] : AddMonoid (MvPowerSeries σ R) :=
Pi.addMonoid
instance [AddGroup R] : AddGroup (MvPowerSeries σ R) :=
Pi.addGroup
instance [AddCommMonoid R] : AddCommMonoid (MvPowerSeries σ R) :=
Pi.addCommMonoid
instance [AddCommGroup R] : AddCommGroup (MvPowerSeries σ R) :=
Pi.addCommGroup
instance [Nontrivial R] : Nontrivial (MvPowerSeries σ R) :=
Function.nontrivial
instance {A} [Semiring R] [AddCommMonoid A] [Module R A] : Module R (MvPowerSeries σ A) :=
Pi.module _ _ _
instance {A S} [Semiring R] [Semiring S] [AddCommMonoid A] [Module R A] [Module S A] [SMul R S]
[IsScalarTower R S A] : IsScalarTower R S (MvPowerSeries σ A) :=
Pi.isScalarTower
section Semiring
variable (R) [Semiring R]
/-- The `n`th monomial as multivariate formal power series:
it is defined as the `R`-linear map from `R` to the semi-ring
of multivariate formal power series associating to each `a`
the map sending `n : σ →₀ ℕ` to the value `a`
and sending all other `x : σ →₀ ℕ` different from `n` to `0`. -/
def monomial (n : σ →₀ ℕ) : R →ₗ[R] MvPowerSeries σ R :=
letI := Classical.decEq σ
LinearMap.stdBasis R (fun _ ↦ R) n
/-- The `n`th coefficient of a multivariate formal power series. -/
def coeff (n : σ →₀ ℕ) : MvPowerSeries σ R →ₗ[R] R :=
LinearMap.proj n
variable {R}
/-- Two multivariate formal power series are equal if all their coefficients are equal. -/
@[ext]
theorem ext {φ ψ} (h : ∀ n : σ →₀ ℕ, coeff R n φ = coeff R n ψ) : φ = ψ :=
funext h
/-- Two multivariate formal power series are equal
if and only if all their coefficients are equal. -/
add_decl_doc MvPowerSeries.ext_iff
theorem monomial_def [DecidableEq σ] (n : σ →₀ ℕ) :
(monomial R n) = LinearMap.stdBasis R (fun _ ↦ R) n := by
rw [monomial]
-- unify the `Decidable` arguments
convert rfl
theorem coeff_monomial [DecidableEq σ] (m n : σ →₀ ℕ) (a : R) :
coeff R m (monomial R n a) = if m = n then a else 0 := by
-- This used to be `rw`, but we need `erw` after leanprover/lean4#2644
erw [coeff, monomial_def, LinearMap.proj_apply (i := m)]
-- This used to be `rw`, but we need `erw` after leanprover/lean4#2644
erw [LinearMap.stdBasis_apply, Function.update_apply, Pi.zero_apply]
@[simp]
theorem coeff_monomial_same (n : σ →₀ ℕ) (a : R) : coeff R n (monomial R n a) = a := by
classical
rw [monomial_def]
exact LinearMap.stdBasis_same R (fun _ ↦ R) n a
theorem coeff_monomial_ne {m n : σ →₀ ℕ} (h : m ≠ n) (a : R) : coeff R m (monomial R n a) = 0 := by
classical
rw [monomial_def]
exact LinearMap.stdBasis_ne R (fun _ ↦ R) _ _ h a
theorem eq_of_coeff_monomial_ne_zero {m n : σ →₀ ℕ} {a : R} (h : coeff R m (monomial R n a) ≠ 0) :
m = n :=
by_contra fun h' => h <| coeff_monomial_ne h' a
@[simp]
theorem coeff_comp_monomial (n : σ →₀ ℕ) : (coeff R n).comp (monomial R n) = LinearMap.id :=
LinearMap.ext <| coeff_monomial_same n
-- Porting note (#10618): simp can prove this.
-- @[simp]
theorem coeff_zero (n : σ →₀ ℕ) : coeff R n (0 : MvPowerSeries σ R) = 0 :=
rfl
variable (m n : σ →₀ ℕ) (φ ψ : MvPowerSeries σ R)
instance : One (MvPowerSeries σ R) :=
⟨monomial R (0 : σ →₀ ℕ) 1⟩
theorem coeff_one [DecidableEq σ] : coeff R n (1 : MvPowerSeries σ R) = if n = 0 then 1 else 0 :=
coeff_monomial _ _ _
theorem coeff_zero_one : coeff R (0 : σ →₀ ℕ) 1 = 1 :=
coeff_monomial_same 0 1
theorem monomial_zero_one : monomial R (0 : σ →₀ ℕ) 1 = 1 :=
rfl
instance : AddMonoidWithOne (MvPowerSeries σ R) :=
{ show AddMonoid (MvPowerSeries σ R) by infer_instance with
natCast := fun n => monomial R 0 n
natCast_zero := by simp [Nat.cast]
natCast_succ := by simp [Nat.cast, monomial_zero_one]
one := 1 }
instance : Mul (MvPowerSeries σ R) :=
letI := Classical.decEq σ
⟨fun φ ψ n => ∑ p ∈ antidiagonal n, coeff R p.1 φ * coeff R p.2 ψ⟩
theorem coeff_mul [DecidableEq σ] :
coeff R n (φ * ψ) = ∑ p ∈ antidiagonal n, coeff R p.1 φ * coeff R p.2 ψ := by
refine Finset.sum_congr ?_ fun _ _ => rfl
rw [Subsingleton.elim (Classical.decEq σ) ‹DecidableEq σ›]
protected theorem zero_mul : (0 : MvPowerSeries σ R) * φ = 0 :=
ext fun n => by classical simp [coeff_mul]
protected theorem mul_zero : φ * 0 = 0 :=
ext fun n => by classical simp [coeff_mul]
theorem coeff_monomial_mul (a : R) :
coeff R m (monomial R n a * φ) = if n ≤ m then a * coeff R (m - n) φ else 0 := by
classical
have :
∀ p ∈ antidiagonal m,
coeff R (p : (σ →₀ ℕ) × (σ →₀ ℕ)).1 (monomial R n a) * coeff R p.2 φ ≠ 0 → p.1 = n :=
fun p _ hp => eq_of_coeff_monomial_ne_zero (left_ne_zero_of_mul hp)
rw [coeff_mul, ← Finset.sum_filter_of_ne this, Finset.filter_fst_eq_antidiagonal _ n,
Finset.sum_ite_index]
simp only [Finset.sum_singleton, coeff_monomial_same, Finset.sum_empty]
theorem coeff_mul_monomial (a : R) :
coeff R m (φ * monomial R n a) = if n ≤ m then coeff R (m - n) φ * a else 0 := by
classical
have :
∀ p ∈ antidiagonal m,
coeff R (p : (σ →₀ ℕ) × (σ →₀ ℕ)).1 φ * coeff R p.2 (monomial R n a) ≠ 0 → p.2 = n :=
fun p _ hp => eq_of_coeff_monomial_ne_zero (right_ne_zero_of_mul hp)
rw [coeff_mul, ← Finset.sum_filter_of_ne this, Finset.filter_snd_eq_antidiagonal _ n,
Finset.sum_ite_index]
simp only [Finset.sum_singleton, coeff_monomial_same, Finset.sum_empty]
theorem coeff_add_monomial_mul (a : R) :
coeff R (m + n) (monomial R m a * φ) = a * coeff R n φ := by
rw [coeff_monomial_mul, if_pos, add_tsub_cancel_left]
exact le_add_right le_rfl
theorem coeff_add_mul_monomial (a : R) :
coeff R (m + n) (φ * monomial R n a) = coeff R m φ * a := by
rw [coeff_mul_monomial, if_pos, add_tsub_cancel_right]
exact le_add_left le_rfl
@[simp]
theorem commute_monomial {a : R} {n} :
Commute φ (monomial R n a) ↔ ∀ m, Commute (coeff R m φ) a := by
rw [commute_iff_eq, MvPowerSeries.ext_iff]
refine ⟨fun h m => ?_, fun h m => ?_⟩
· have := h (m + n)
rwa [coeff_add_mul_monomial, add_comm, coeff_add_monomial_mul] at this
· rw [coeff_mul_monomial, coeff_monomial_mul]
split_ifs <;> [apply h; rfl]
protected theorem one_mul : (1 : MvPowerSeries σ R) * φ = φ :=
ext fun n => by simpa using coeff_add_monomial_mul 0 n φ 1
protected theorem mul_one : φ * 1 = φ :=
ext fun n => by simpa using coeff_add_mul_monomial n 0 φ 1
protected theorem mul_add (φ₁ φ₂ φ₃ : MvPowerSeries σ R) : φ₁ * (φ₂ + φ₃) = φ₁ * φ₂ + φ₁ * φ₃ :=
ext fun n => by
classical simp only [coeff_mul, mul_add, Finset.sum_add_distrib, LinearMap.map_add]
protected theorem add_mul (φ₁ φ₂ φ₃ : MvPowerSeries σ R) : (φ₁ + φ₂) * φ₃ = φ₁ * φ₃ + φ₂ * φ₃ :=
ext fun n => by
classical simp only [coeff_mul, add_mul, Finset.sum_add_distrib, LinearMap.map_add]
protected theorem mul_assoc (φ₁ φ₂ φ₃ : MvPowerSeries σ R) : φ₁ * φ₂ * φ₃ = φ₁ * (φ₂ * φ₃) := by
ext1 n
classical
simp only [coeff_mul, Finset.sum_mul, Finset.mul_sum, Finset.sum_sigma']
apply Finset.sum_nbij' (fun ⟨⟨_i, j⟩, ⟨k, l⟩⟩ ↦ ⟨(k, l + j), (l, j)⟩)
(fun ⟨⟨i, _j⟩, ⟨k, l⟩⟩ ↦ ⟨(i + k, l), (i, k)⟩) <;> aesop (add simp [add_assoc, mul_assoc])
instance : Semiring (MvPowerSeries σ R) :=
{ inferInstanceAs (AddMonoidWithOne (MvPowerSeries σ R)),
inferInstanceAs (Mul (MvPowerSeries σ R)),
inferInstanceAs (AddCommMonoid (MvPowerSeries σ R)) with
mul_one := MvPowerSeries.mul_one
one_mul := MvPowerSeries.one_mul
mul_assoc := MvPowerSeries.mul_assoc
mul_zero := MvPowerSeries.mul_zero
zero_mul := MvPowerSeries.zero_mul
left_distrib := MvPowerSeries.mul_add
right_distrib := MvPowerSeries.add_mul }
end Semiring
instance [CommSemiring R] : CommSemiring (MvPowerSeries σ R) :=
{ show Semiring (MvPowerSeries σ R) by infer_instance with
mul_comm := fun φ ψ =>
ext fun n => by
classical
simpa only [coeff_mul, mul_comm] using
sum_antidiagonal_swap n fun a b => coeff R a φ * coeff R b ψ }
instance [Ring R] : Ring (MvPowerSeries σ R) :=
{ inferInstanceAs (Semiring (MvPowerSeries σ R)),
inferInstanceAs (AddCommGroup (MvPowerSeries σ R)) with }
instance [CommRing R] : CommRing (MvPowerSeries σ R) :=
{ inferInstanceAs (CommSemiring (MvPowerSeries σ R)),
inferInstanceAs (AddCommGroup (MvPowerSeries σ R)) with }
section Semiring
variable [Semiring R]
theorem monomial_mul_monomial (m n : σ →₀ ℕ) (a b : R) :
monomial R m a * monomial R n b = monomial R (m + n) (a * b) := by
classical
ext k
simp only [coeff_mul_monomial, coeff_monomial]
split_ifs with h₁ h₂ h₃ h₃ h₂ <;> try rfl
· rw [← h₂, tsub_add_cancel_of_le h₁] at h₃
exact (h₃ rfl).elim
· rw [h₃, add_tsub_cancel_right] at h₂
exact (h₂ rfl).elim
· exact zero_mul b
· rw [h₂] at h₁
exact (h₁ <| le_add_left le_rfl).elim
variable (σ) (R)
/-- The constant multivariate formal power series. -/
def C : R →+* MvPowerSeries σ R :=
{ monomial R (0 : σ →₀ ℕ) with
map_one' := rfl
map_mul' := fun a b => (monomial_mul_monomial 0 0 a b).symm
map_zero' := (monomial R (0 : _)).map_zero }
variable {σ} {R}
@[simp]
theorem monomial_zero_eq_C : ⇑(monomial R (0 : σ →₀ ℕ)) = C σ R :=
rfl
theorem monomial_zero_eq_C_apply (a : R) : monomial R (0 : σ →₀ ℕ) a = C σ R a :=
rfl
theorem coeff_C [DecidableEq σ] (n : σ →₀ ℕ) (a : R) :
coeff R n (C σ R a) = if n = 0 then a else 0 :=
coeff_monomial _ _ _
theorem coeff_zero_C (a : R) : coeff R (0 : σ →₀ ℕ) (C σ R a) = a :=
coeff_monomial_same 0 a
/-- The variables of the multivariate formal power series ring. -/
def X (s : σ) : MvPowerSeries σ R :=
monomial R (single s 1) 1
theorem coeff_X [DecidableEq σ] (n : σ →₀ ℕ) (s : σ) :
coeff R n (X s : MvPowerSeries σ R) = if n = single s 1 then 1 else 0 :=
coeff_monomial _ _ _
theorem coeff_index_single_X [DecidableEq σ] (s t : σ) :
coeff R (single t 1) (X s : MvPowerSeries σ R) = if t = s then 1 else 0 := by
simp only [coeff_X, single_left_inj (one_ne_zero : (1 : ℕ) ≠ 0)]
@[simp]
theorem coeff_index_single_self_X (s : σ) : coeff R (single s 1) (X s : MvPowerSeries σ R) = 1 :=
coeff_monomial_same _ _
theorem coeff_zero_X (s : σ) : coeff R (0 : σ →₀ ℕ) (X s : MvPowerSeries σ R) = 0 := by
classical
rw [coeff_X, if_neg]
intro h
exact one_ne_zero (single_eq_zero.mp h.symm)
theorem commute_X (φ : MvPowerSeries σ R) (s : σ) : Commute φ (X s) :=
φ.commute_monomial.mpr fun _m => Commute.one_right _
theorem X_def (s : σ) : X s = monomial R (single s 1) 1 :=
rfl
theorem X_pow_eq (s : σ) (n : ℕ) : (X s : MvPowerSeries σ R) ^ n = monomial R (single s n) 1 := by
induction' n with n ih
· simp
· rw [pow_succ, ih, Finsupp.single_add, X, monomial_mul_monomial, one_mul]
theorem coeff_X_pow [DecidableEq σ] (m : σ →₀ ℕ) (s : σ) (n : ℕ) :
coeff R m ((X s : MvPowerSeries σ R) ^ n) = if m = single s n then 1 else 0 := by
rw [X_pow_eq s n, coeff_monomial]
@[simp]
theorem coeff_mul_C (n : σ →₀ ℕ) (φ : MvPowerSeries σ R) (a : R) :
coeff R n (φ * C σ R a) = coeff R n φ * a := by simpa using coeff_add_mul_monomial n 0 φ a
@[simp]
theorem coeff_C_mul (n : σ →₀ ℕ) (φ : MvPowerSeries σ R) (a : R) :
coeff R n (C σ R a * φ) = a * coeff R n φ := by simpa using coeff_add_monomial_mul 0 n φ a
theorem coeff_zero_mul_X (φ : MvPowerSeries σ R) (s : σ) : coeff R (0 : σ →₀ ℕ) (φ * X s) = 0 := by
have : ¬single s 1 ≤ 0 := fun h => by simpa using h s
simp only [X, coeff_mul_monomial, if_neg this]
theorem coeff_zero_X_mul (φ : MvPowerSeries σ R) (s : σ) : coeff R (0 : σ →₀ ℕ) (X s * φ) = 0 := by
rw [← (φ.commute_X s).eq, coeff_zero_mul_X]
variable (σ) (R)
/-- The constant coefficient of a formal power series. -/
def constantCoeff : MvPowerSeries σ R →+* R :=
{ coeff R (0 : σ →₀ ℕ) with
toFun := coeff R (0 : σ →₀ ℕ)
map_one' := coeff_zero_one
map_mul' := fun φ ψ => by classical simp [coeff_mul, support_single_ne_zero]
map_zero' := LinearMap.map_zero _ }
variable {σ} {R}
@[simp]
theorem coeff_zero_eq_constantCoeff : ⇑(coeff R (0 : σ →₀ ℕ)) = constantCoeff σ R :=
rfl
theorem coeff_zero_eq_constantCoeff_apply (φ : MvPowerSeries σ R) :
coeff R (0 : σ →₀ ℕ) φ = constantCoeff σ R φ :=
rfl
@[simp]
theorem constantCoeff_C (a : R) : constantCoeff σ R (C σ R a) = a :=
rfl
@[simp]
theorem constantCoeff_comp_C : (constantCoeff σ R).comp (C σ R) = RingHom.id R :=
rfl
-- Porting note (#10618): simp can prove this.
-- @[simp]
theorem constantCoeff_zero : constantCoeff σ R 0 = 0 :=
rfl
-- Porting note (#10618): simp can prove this.
-- @[simp]
theorem constantCoeff_one : constantCoeff σ R 1 = 1 :=
rfl
@[simp]
theorem constantCoeff_X (s : σ) : constantCoeff σ R (X s) = 0 :=
coeff_zero_X s
/-- If a multivariate formal power series is invertible,
then so is its constant coefficient. -/
theorem isUnit_constantCoeff (φ : MvPowerSeries σ R) (h : IsUnit φ) :
IsUnit (constantCoeff σ R φ) :=
h.map _
-- Porting note (#10618): simp can prove this.
-- @[simp]
theorem coeff_smul (f : MvPowerSeries σ R) (n) (a : R) : coeff _ n (a • f) = a * coeff _ n f :=
rfl
theorem smul_eq_C_mul (f : MvPowerSeries σ R) (a : R) : a • f = C σ R a * f := by
ext
simp
theorem X_inj [Nontrivial R] {s t : σ} : (X s : MvPowerSeries σ R) = X t ↔ s = t :=
⟨by
classical
intro h
replace h := congr_arg (coeff R (single s 1)) h
rw [coeff_X, if_pos rfl, coeff_X] at h
split_ifs at h with H
· rw [Finsupp.single_eq_single_iff] at H
cases' H with H H
· exact H.1
· exfalso
exact one_ne_zero H.1
· exfalso
exact one_ne_zero h, congr_arg X⟩
end Semiring
section Map
variable {S T : Type*} [Semiring R] [Semiring S] [Semiring T]
variable (f : R →+* S) (g : S →+* T)
variable (σ)
/-- The map between multivariate formal power series induced by a map on the coefficients. -/
def map : MvPowerSeries σ R →+* MvPowerSeries σ S where
toFun φ n := f <| coeff R n φ
map_zero' := ext fun _n => f.map_zero
map_one' :=
ext fun n =>
show f ((coeff R n) 1) = (coeff S n) 1 by
classical
rw [coeff_one, coeff_one]
split_ifs with h
· simp only [RingHom.map_ite_one_zero, ite_true, map_one, h]
· simp only [RingHom.map_ite_one_zero, ite_false, map_zero, h]
map_add' φ ψ :=
ext fun n => show f ((coeff R n) (φ + ψ)) = f ((coeff R n) φ) + f ((coeff R n) ψ) by simp
map_mul' φ ψ :=
ext fun n =>
show f _ = _ by
classical
rw [coeff_mul, map_sum, coeff_mul]
apply Finset.sum_congr rfl
rintro ⟨i, j⟩ _; rw [f.map_mul]; rfl
variable {σ}
@[simp]
theorem map_id : map σ (RingHom.id R) = RingHom.id _ :=
rfl
theorem map_comp : map σ (g.comp f) = (map σ g).comp (map σ f) :=
rfl
@[simp]
theorem coeff_map (n : σ →₀ ℕ) (φ : MvPowerSeries σ R) : coeff S n (map σ f φ) = f (coeff R n φ) :=
rfl
@[simp]
theorem constantCoeff_map (φ : MvPowerSeries σ R) :
constantCoeff σ S (map σ f φ) = f (constantCoeff σ R φ) :=
rfl
@[simp]
theorem map_monomial (n : σ →₀ ℕ) (a : R) : map σ f (monomial R n a) = monomial S n (f a) := by
classical
ext m
simp [coeff_monomial, apply_ite f]
@[simp]
theorem map_C (a : R) : map σ f (C σ R a) = C σ S (f a) :=
map_monomial _ _ _
@[simp]
theorem map_X (s : σ) : map σ f (X s) = X s := by simp [MvPowerSeries.X]
end Map
section Semiring
variable [Semiring R]
theorem X_pow_dvd_iff {s : σ} {n : ℕ} {φ : MvPowerSeries σ R} :
(X s : MvPowerSeries σ R) ^ n ∣ φ ↔ ∀ m : σ →₀ ℕ, m s < n → coeff R m φ = 0 := by
classical
constructor
· rintro ⟨φ, rfl⟩ m h
rw [coeff_mul, Finset.sum_eq_zero]
rintro ⟨i, j⟩ hij
rw [coeff_X_pow, if_neg, zero_mul]
contrapose! h
dsimp at h
subst i
rw [mem_antidiagonal] at hij
rw [← hij, Finsupp.add_apply, Finsupp.single_eq_same]
exact Nat.le_add_right n _
· intro h
refine ⟨fun m => coeff R (m + single s n) φ, ?_⟩
ext m
by_cases H : m - single s n + single s n = m
· rw [coeff_mul, Finset.sum_eq_single (single s n, m - single s n)]
· rw [coeff_X_pow, if_pos rfl, one_mul]
simpa using congr_arg (fun m : σ →₀ ℕ => coeff R m φ) H.symm
· rintro ⟨i, j⟩ hij hne
rw [mem_antidiagonal] at hij
rw [coeff_X_pow]
split_ifs with hi
· exfalso
apply hne
rw [← hij, ← hi, Prod.mk.inj_iff]
refine ⟨rfl, ?_⟩
ext t
simp only [add_tsub_cancel_left, Finsupp.add_apply, Finsupp.tsub_apply]
· exact zero_mul _
· intro hni
exfalso
apply hni
rwa [mem_antidiagonal, add_comm]
· rw [h, coeff_mul, Finset.sum_eq_zero]
· rintro ⟨i, j⟩ hij
rw [mem_antidiagonal] at hij
rw [coeff_X_pow]
split_ifs with hi
· exfalso
apply H
rw [← hij, hi]
ext
rw [coe_add, coe_add, Pi.add_apply, Pi.add_apply, add_tsub_cancel_left, add_comm]
· exact zero_mul _
· contrapose! H
ext t
by_cases hst : s = t
· subst t
simpa using tsub_add_cancel_of_le H
· simp [Finsupp.single_apply, hst]
theorem X_dvd_iff {s : σ} {φ : MvPowerSeries σ R} :
(X s : MvPowerSeries σ R) ∣ φ ↔ ∀ m : σ →₀ ℕ, m s = 0 → coeff R m φ = 0 := by
rw [← pow_one (X s : MvPowerSeries σ R), X_pow_dvd_iff]
constructor <;> intro h m hm
· exact h m (hm.symm ▸ zero_lt_one)
· exact h m (Nat.eq_zero_of_le_zero <| Nat.le_of_succ_le_succ hm)
end Semiring
section CommSemiring
open Finset.HasAntidiagonal Finset
variable {R : Type*} [CommSemiring R] {ι : Type*} [DecidableEq ι]
/-- Coefficients of a product of power series -/
theorem coeff_prod [DecidableEq σ]
(f : ι → MvPowerSeries σ R) (d : σ →₀ ℕ) (s : Finset ι) :
coeff R d (∏ j ∈ s, f j) =
∑ l ∈ finsuppAntidiag s d,
∏ i ∈ s, coeff R (l i) (f i) := by
induction s using Finset.induction_on generalizing d with
| empty =>
simp only [prod_empty, sum_const, nsmul_eq_mul, mul_one, coeff_one, finsuppAntidiag_empty]
split_ifs
· simp only [card_singleton, Nat.cast_one]
· simp only [card_empty, Nat.cast_zero]
| @insert a s ha ih =>
rw [finsuppAntidiag_insert ha, prod_insert ha, coeff_mul, sum_biUnion]
· apply Finset.sum_congr rfl
simp only [mem_antidiagonal, sum_map, Function.Embedding.coeFn_mk, coe_update, Prod.forall]
rintro u v rfl
rw [ih, Finset.mul_sum, ← Finset.sum_attach]
apply Finset.sum_congr rfl
simp only [mem_attach, Finset.prod_insert ha, Function.update_same, forall_true_left,
Subtype.forall]
rintro x -
rw [Finset.prod_congr rfl]
intro i hi
rw [Function.update_noteq]
exact ne_of_mem_of_not_mem hi ha
· simp only [Set.PairwiseDisjoint, Set.Pairwise, mem_coe, mem_antidiagonal, ne_eq,
disjoint_left, mem_map, mem_attach, Function.Embedding.coeFn_mk, true_and, Subtype.exists,
exists_prop, not_exists, not_and, forall_exists_index, and_imp, forall_apply_eq_imp_iff₂,
Prod.forall, Prod.mk.injEq]
rintro u v rfl u' v' huv h k - l - hkl
obtain rfl : u' = u := by
simpa only [Finsupp.coe_update, Function.update_same] using DFunLike.congr_fun hkl a
simp only [add_right_inj] at huv
exact h rfl huv.symm
/-- The `d`th coefficient of a power of a multivariate power series
is the sum, indexed by `finsuppAntidiag (Finset.range n) d`, of products of coefficients -/
theorem coeff_pow [DecidableEq σ] (f : MvPowerSeries σ R) {n : ℕ} (d : σ →₀ ℕ) :
coeff R d (f ^ n) =
∑ l in finsuppAntidiag (Finset.range n) d,
∏ i in Finset.range n, coeff R (l i) f := by
suffices f ^ n = (Finset.range n).prod fun _ ↦ f by
rw [this, coeff_prod]
rw [Finset.prod_const, card_range]
/-- Vanishing of coefficients of powers of multivariate power series
when the constant coefficient is nilpotent
[N. Bourbaki, *Algebra {II}*, Chapter 4, §4, n°2, proposition 3][bourbaki1981] -/
theorem coeff_eq_zero_of_constantCoeff_nilpotent
{f : MvPowerSeries σ R} {m : ℕ} (hf : constantCoeff σ R f ^ m = 0)
{d : σ →₀ ℕ} {n : ℕ} (hn : m + degree d ≤ n) : coeff R d (f ^ n) = 0 := by
classical
rw [coeff_pow]
apply sum_eq_zero
intro k hk
rw [mem_finsuppAntidiag] at hk
set s := (range n).filter fun i ↦ k i = 0 with hs_def
have hs : s ⊆ range n := filter_subset _ _
have hs' (i : ℕ) (hi : i ∈ s) : coeff R (k i) f = constantCoeff σ R f := by
simp only [hs_def, mem_filter] at hi
rw [hi.2, coeff_zero_eq_constantCoeff]
have hs'' (i : ℕ) (hi : i ∈ s) : k i = 0 := by
simp only [hs_def, mem_filter] at hi
rw [hi.2]
rw [← prod_sdiff (s₁ := s) (filter_subset _ _)]
apply mul_eq_zero_of_right
rw [prod_congr rfl hs', prod_const]
suffices m ≤ s.card by
obtain ⟨m', hm'⟩ := Nat.exists_eq_add_of_le this
rw [hm', pow_add, hf, MulZeroClass.zero_mul]
rw [← Nat.add_le_add_iff_right, add_comm s.card,
Finset.card_sdiff_add_card_eq_card (filter_subset _ _), card_range]
apply le_trans _ hn
simp only [add_comm m, Nat.add_le_add_iff_right, ← hk.1,
← sum_sdiff (hs), sum_eq_zero (s := s) hs'', add_zero]
rw [← hs_def]
convert Finset.card_nsmul_le_sum (range n \ s) (fun x ↦ degree (k x)) 1 _
· simp only [Algebra.id.smul_eq_mul, mul_one]
· simp only [degree_eq_weight_one, map_sum]
· simp only [hs_def, mem_filter, mem_sdiff, mem_range, not_and, and_imp]
intro i hi hi'
rw [← not_lt, Nat.lt_one_iff, degree_eq_zero_iff]
exact hi' hi
end CommSemiring
section Algebra
variable {A : Type*} [CommSemiring R] [Semiring A] [Algebra R A]
instance : Algebra R (MvPowerSeries σ A) :=
{
show Module R (MvPowerSeries σ A) by infer_instance with
commutes' := fun a φ => by
ext n
simp [Algebra.commutes]
smul_def' := fun a σ => by
ext n
simp [(coeff A n).map_smul_of_tower a, Algebra.smul_def]
toRingHom := (MvPowerSeries.map σ (algebraMap R A)).comp (C σ R) }
theorem c_eq_algebraMap : C σ R = algebraMap R (MvPowerSeries σ R) :=
rfl
theorem algebraMap_apply {r : R} :
algebraMap R (MvPowerSeries σ A) r = C σ A (algebraMap R A r) := by
change (MvPowerSeries.map σ (algebraMap R A)).comp (C σ R) r = _
simp
instance [Nonempty σ] [Nontrivial R] : Nontrivial (Subalgebra R (MvPowerSeries σ R)) :=
⟨⟨⊥, ⊤, by
classical
rw [Ne, SetLike.ext_iff, not_forall]
inhabit σ
refine ⟨X default, ?_⟩
simp only [Algebra.mem_bot, not_exists, Set.mem_range, iff_true_iff, Algebra.mem_top]
intro x
rw [MvPowerSeries.ext_iff, not_forall]
refine ⟨Finsupp.single default 1, ?_⟩
simp [algebraMap_apply, coeff_C]⟩⟩
end Algebra
end MvPowerSeries
namespace MvPolynomial
open Finsupp
variable {σ : Type*} {R : Type*} [CommSemiring R] (φ ψ : MvPolynomial σ R)
-- Porting note: added so we can add the `@[coe]` attribute
/-- The natural inclusion from multivariate polynomials into multivariate formal power series. -/
@[coe]
def toMvPowerSeries : MvPolynomial σ R → MvPowerSeries σ R :=
fun φ n => coeff n φ
/-- The natural inclusion from multivariate polynomials into multivariate formal power series. -/
instance coeToMvPowerSeries : Coe (MvPolynomial σ R) (MvPowerSeries σ R) :=
⟨toMvPowerSeries⟩
theorem coe_def : (φ : MvPowerSeries σ R) = fun n => coeff n φ :=
rfl
@[simp, norm_cast]
theorem coeff_coe (n : σ →₀ ℕ) : MvPowerSeries.coeff R n ↑φ = coeff n φ :=
rfl
@[simp, norm_cast]
theorem coe_monomial (n : σ →₀ ℕ) (a : R) :
(monomial n a : MvPowerSeries σ R) = MvPowerSeries.monomial R n a :=
MvPowerSeries.ext fun m => by
classical
rw [coeff_coe, coeff_monomial, MvPowerSeries.coeff_monomial]
split_ifs with h₁ h₂ <;> first |rfl|subst m; contradiction
@[simp, norm_cast]
theorem coe_zero : ((0 : MvPolynomial σ R) : MvPowerSeries σ R) = 0 :=
rfl
@[simp, norm_cast]
theorem coe_one : ((1 : MvPolynomial σ R) : MvPowerSeries σ R) = 1 :=
coe_monomial _ _
@[simp, norm_cast]
theorem coe_add : ((φ + ψ : MvPolynomial σ R) : MvPowerSeries σ R) = φ + ψ :=
rfl
@[simp, norm_cast]
theorem coe_mul : ((φ * ψ : MvPolynomial σ R) : MvPowerSeries σ R) = φ * ψ :=
MvPowerSeries.ext fun n => by
classical
simp only [coeff_coe, MvPowerSeries.coeff_mul, coeff_mul]
@[simp, norm_cast]
theorem coe_C (a : R) : ((C a : MvPolynomial σ R) : MvPowerSeries σ R) = MvPowerSeries.C σ R a :=
coe_monomial _ _
@[simp, norm_cast]
theorem coe_X (s : σ) : ((X s : MvPolynomial σ R) : MvPowerSeries σ R) = MvPowerSeries.X s :=
coe_monomial _ _
variable (σ R)
theorem coe_injective : Function.Injective (Coe.coe : MvPolynomial σ R → MvPowerSeries σ R) :=
fun x y h => by
ext
simp_rw [← coeff_coe]
congr
variable {σ R φ ψ}
@[simp, norm_cast]
theorem coe_inj : (φ : MvPowerSeries σ R) = ψ ↔ φ = ψ :=
(coe_injective σ R).eq_iff
@[simp]
theorem coe_eq_zero_iff : (φ : MvPowerSeries σ R) = 0 ↔ φ = 0 := by rw [← coe_zero, coe_inj]
@[simp]
theorem coe_eq_one_iff : (φ : MvPowerSeries σ R) = 1 ↔ φ = 1 := by rw [← coe_one, coe_inj]
/-- The coercion from multivariate polynomials to multivariate power series
as a ring homomorphism.
-/
def coeToMvPowerSeries.ringHom : MvPolynomial σ R →+* MvPowerSeries σ R where
toFun := (Coe.coe : MvPolynomial σ R → MvPowerSeries σ R)
map_zero' := coe_zero
map_one' := coe_one
map_add' := coe_add
map_mul' := coe_mul
@[simp, norm_cast]
theorem coe_pow (n : ℕ) :
((φ ^ n : MvPolynomial σ R) : MvPowerSeries σ R) = (φ : MvPowerSeries σ R) ^ n :=
coeToMvPowerSeries.ringHom.map_pow _ _
variable (φ ψ)
@[simp]
theorem coeToMvPowerSeries.ringHom_apply : coeToMvPowerSeries.ringHom φ = φ :=
rfl
section Algebra
variable (A : Type*) [CommSemiring A] [Algebra R A]
/-- The coercion from multivariate polynomials to multivariate power series
as an algebra homomorphism.
-/
def coeToMvPowerSeries.algHom : MvPolynomial σ R →ₐ[R] MvPowerSeries σ A :=
{ (MvPowerSeries.map σ (algebraMap R A)).comp coeToMvPowerSeries.ringHom with
commutes' := fun r => by simp [algebraMap_apply, MvPowerSeries.algebraMap_apply] }
@[simp]
theorem coeToMvPowerSeries.algHom_apply :
coeToMvPowerSeries.algHom A φ = MvPowerSeries.map σ (algebraMap R A) ↑φ :=
rfl
end Algebra
end MvPolynomial
namespace MvPowerSeries
variable {σ R A : Type*} [CommSemiring R] [CommSemiring A] [Algebra R A] (f : MvPowerSeries σ R)
instance algebraMvPolynomial : Algebra (MvPolynomial σ R) (MvPowerSeries σ A) :=
RingHom.toAlgebra (MvPolynomial.coeToMvPowerSeries.algHom A).toRingHom
instance algebraMvPowerSeries : Algebra (MvPowerSeries σ R) (MvPowerSeries σ A) :=
(map σ (algebraMap R A)).toAlgebra
variable (A)
theorem algebraMap_apply' (p : MvPolynomial σ R) :
algebraMap (MvPolynomial σ R) (MvPowerSeries σ A) p = map σ (algebraMap R A) p :=
rfl
theorem algebraMap_apply'' :
algebraMap (MvPowerSeries σ R) (MvPowerSeries σ A) f = map σ (algebraMap R A) f :=
rfl
end MvPowerSeries
end
|
RingTheory\MvPowerSeries\Inverse.lean | /-
Copyright (c) 2019 Johan Commelin. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johan Commelin, Kenny Lau
-/
import Mathlib.Algebra.Group.Units
import Mathlib.RingTheory.MvPowerSeries.Basic
import Mathlib.RingTheory.MvPowerSeries.NoZeroDivisors
import Mathlib.RingTheory.LocalRing.RingHom.Basic
/-!
# Formal (multivariate) power series - Inverses
This file defines multivariate formal power series and develops the basic
properties of these objects, when it comes about multiplicative inverses.
For `φ : MvPowerSeries σ R` and `u : Rˣ` is the constant coefficient of `φ`,
`MvPowerSeries.invOfUnit φ u` is a formal power series such,
and `MvPowerSeries.mul_invOfUnit` proves that `φ * invOfUnit φ u = 1`.
The construction of the power series `invOfUnit` is done by writing that
relation and solving and for its coefficients by induction.
Over a field, all power series `φ` have an “inverse” `MvPowerSeries.inv φ`,
which is `0` if and only if the constant coefficient of `φ` is zero
(by `MvPowerSeries.inv_eq_zero`),
and `MvPowerSeries.mul_inv_cancel` asserts the equality `φ * φ⁻¹ = 1` when
the constant coefficient of `φ` is nonzero.
Instances are defined:
* Formal power series over a local ring form a local ring.
* The morphism `MvPowerSeries.map σ f : MvPowerSeries σ A →* MvPowerSeries σ B`
induced by a local morphism `f : A →+* B` (`IsLocalRingHom f`)
of commutative rings is a *local* morphism.
-/
noncomputable section
open Finset (antidiagonal mem_antidiagonal)
namespace MvPowerSeries
open Finsupp
variable {σ R : Type*}
section Ring
variable [Ring R]
/-
The inverse of a multivariate formal power series is defined by
well-founded recursion on the coefficients of the inverse.
-/
/-- Auxiliary definition that unifies
the totalised inverse formal power series `(_)⁻¹` and
the inverse formal power series that depends on
an inverse of the constant coefficient `invOfUnit`. -/
protected noncomputable def inv.aux (a : R) (φ : MvPowerSeries σ R) : MvPowerSeries σ R
| n =>
letI := Classical.decEq σ
if n = 0 then a
else
-a *
∑ x ∈ antidiagonal n, if _ : x.2 < n then coeff R x.1 φ * inv.aux a φ x.2 else 0
termination_by n => n
theorem coeff_inv_aux [DecidableEq σ] (n : σ →₀ ℕ) (a : R) (φ : MvPowerSeries σ R) :
coeff R n (inv.aux a φ) =
if n = 0 then a
else
-a *
∑ x ∈ antidiagonal n, if x.2 < n then coeff R x.1 φ * coeff R x.2 (inv.aux a φ) else 0 :=
show inv.aux a φ n = _ by
cases Subsingleton.elim ‹DecidableEq σ› (Classical.decEq σ)
rw [inv.aux]
rfl
/-- A multivariate formal power series is invertible if the constant coefficient is invertible. -/
def invOfUnit (φ : MvPowerSeries σ R) (u : Rˣ) : MvPowerSeries σ R :=
inv.aux (↑u⁻¹) φ
theorem coeff_invOfUnit [DecidableEq σ] (n : σ →₀ ℕ) (φ : MvPowerSeries σ R) (u : Rˣ) :
coeff R n (invOfUnit φ u) =
if n = 0 then ↑u⁻¹
else
-↑u⁻¹ *
∑ x ∈ antidiagonal n,
if x.2 < n then coeff R x.1 φ * coeff R x.2 (invOfUnit φ u) else 0 := by
convert coeff_inv_aux n (↑u⁻¹) φ
@[simp]
theorem constantCoeff_invOfUnit (φ : MvPowerSeries σ R) (u : Rˣ) :
constantCoeff σ R (invOfUnit φ u) = ↑u⁻¹ := by
classical
rw [← coeff_zero_eq_constantCoeff_apply, coeff_invOfUnit, if_pos rfl]
@[simp]
theorem mul_invOfUnit (φ : MvPowerSeries σ R) (u : Rˣ) (h : constantCoeff σ R φ = u) :
φ * invOfUnit φ u = 1 :=
ext fun n =>
letI := Classical.decEq (σ →₀ ℕ)
if H : n = 0 then by
rw [H]
simp [coeff_mul, support_single_ne_zero, h]
else by
classical
have : ((0 : σ →₀ ℕ), n) ∈ antidiagonal n := by rw [mem_antidiagonal, zero_add]
rw [coeff_one, if_neg H, coeff_mul, ← Finset.insert_erase this,
Finset.sum_insert (Finset.not_mem_erase _ _), coeff_zero_eq_constantCoeff_apply, h,
coeff_invOfUnit, if_neg H, neg_mul, mul_neg, Units.mul_inv_cancel_left, ←
Finset.insert_erase this, Finset.sum_insert (Finset.not_mem_erase _ _),
Finset.insert_erase this, if_neg (not_lt_of_ge <| le_rfl), zero_add, add_comm, ←
sub_eq_add_neg, sub_eq_zero, Finset.sum_congr rfl]
rintro ⟨i, j⟩ hij
rw [Finset.mem_erase, mem_antidiagonal] at hij
cases' hij with h₁ h₂
subst n
rw [if_pos]
suffices (0 : _) + j < i + j by simpa
apply add_lt_add_right
constructor
· intro s
exact Nat.zero_le _
· intro H
apply h₁
suffices i = 0 by simp [this]
ext1 s
exact Nat.eq_zero_of_le_zero (H s)
-- TODO : can one prove equivalence?
@[simp]
theorem invOfUnit_mul (φ : MvPowerSeries σ R) (u : Rˣ) (h : constantCoeff σ R φ = u) :
invOfUnit φ u * φ = 1 := by
rw [← mul_cancel_right_mem_nonZeroDivisors (r := φ.invOfUnit u), mul_assoc, one_mul,
mul_invOfUnit _ _ h, mul_one]
apply mem_nonZeroDivisors_of_constantCoeff
simp only [constantCoeff_invOfUnit, IsUnit.mem_nonZeroDivisors (Units.isUnit u⁻¹)]
theorem isUnit_iff_constantCoeff {φ : MvPowerSeries σ R} :
IsUnit φ ↔ IsUnit (constantCoeff σ R φ) := by
constructor
· exact IsUnit.map _
· intro ⟨u, hu⟩
exact ⟨⟨_, φ.invOfUnit u, mul_invOfUnit φ u hu.symm, invOfUnit_mul φ u hu.symm⟩, rfl⟩
end Ring
section CommRing
variable [CommRing R]
/-- Multivariate formal power series over a local ring form a local ring. -/
instance [LocalRing R] : LocalRing (MvPowerSeries σ R) :=
LocalRing.of_isUnit_or_isUnit_one_sub_self <| by
intro φ
rcases LocalRing.isUnit_or_isUnit_one_sub_self (constantCoeff σ R φ) with (⟨u, h⟩ | ⟨u, h⟩) <;>
[left; right] <;>
· refine isUnit_of_mul_eq_one _ _ (mul_invOfUnit _ u ?_)
simpa using h.symm
-- TODO(jmc): once adic topology lands, show that this is complete
end CommRing
section LocalRing
variable {S : Type*} [CommRing R] [CommRing S] (f : R →+* S) [IsLocalRingHom f]
-- Thanks to the linter for informing us that this instance does
-- not actually need R and S to be local rings!
/-- The map between multivariate formal power series over the same indexing set
induced by a local ring hom `A → B` is local -/
instance map.isLocalRingHom : IsLocalRingHom (map σ f) :=
⟨by
rintro φ ⟨ψ, h⟩
replace h := congr_arg (constantCoeff σ S) h
rw [constantCoeff_map] at h
have : IsUnit (constantCoeff σ S ↑ψ) := isUnit_constantCoeff (↑ψ) ψ.isUnit
rw [h] at this
rcases isUnit_of_map_unit f _ this with ⟨c, hc⟩
exact isUnit_of_mul_eq_one φ (invOfUnit φ c) (mul_invOfUnit φ c hc.symm)⟩
end LocalRing
section Field
open MvPowerSeries
variable {k : Type*} [Field k]
/-- The inverse `1/f` of a multivariable power series `f` over a field -/
protected def inv (φ : MvPowerSeries σ k) : MvPowerSeries σ k :=
inv.aux (constantCoeff σ k φ)⁻¹ φ
instance : Inv (MvPowerSeries σ k) :=
⟨MvPowerSeries.inv⟩
theorem coeff_inv [DecidableEq σ] (n : σ →₀ ℕ) (φ : MvPowerSeries σ k) :
coeff k n φ⁻¹ =
if n = 0 then (constantCoeff σ k φ)⁻¹
else
-(constantCoeff σ k φ)⁻¹ *
∑ x ∈ antidiagonal n, if x.2 < n then coeff k x.1 φ * coeff k x.2 φ⁻¹ else 0 :=
coeff_inv_aux n _ φ
@[simp]
theorem constantCoeff_inv (φ : MvPowerSeries σ k) :
constantCoeff σ k φ⁻¹ = (constantCoeff σ k φ)⁻¹ := by
classical
rw [← coeff_zero_eq_constantCoeff_apply, coeff_inv, if_pos rfl]
theorem inv_eq_zero {φ : MvPowerSeries σ k} : φ⁻¹ = 0 ↔ constantCoeff σ k φ = 0 :=
⟨fun h => by simpa using congr_arg (constantCoeff σ k) h, fun h =>
ext fun n => by
classical
rw [coeff_inv]
split_ifs <;>
simp only [h, map_zero, zero_mul, inv_zero, neg_zero]⟩
@[simp]
theorem zero_inv : (0 : MvPowerSeries σ k)⁻¹ = 0 := by
rw [inv_eq_zero, constantCoeff_zero]
-- Porting note (#10618): simp can prove this.
-- @[simp]
theorem invOfUnit_eq (φ : MvPowerSeries σ k) (h : constantCoeff σ k φ ≠ 0) :
invOfUnit φ (Units.mk0 _ h) = φ⁻¹ :=
rfl
@[simp]
theorem invOfUnit_eq' (φ : MvPowerSeries σ k) (u : Units k) (h : constantCoeff σ k φ = u) :
invOfUnit φ u = φ⁻¹ := by
rw [← invOfUnit_eq φ (h.symm ▸ u.ne_zero)]
apply congrArg (invOfUnit φ)
rw [Units.ext_iff]
exact h.symm
@[simp]
protected theorem mul_inv_cancel (φ : MvPowerSeries σ k) (h : constantCoeff σ k φ ≠ 0) :
φ * φ⁻¹ = 1 := by rw [← invOfUnit_eq φ h, mul_invOfUnit φ (Units.mk0 _ h) rfl]
@[simp]
protected theorem inv_mul_cancel (φ : MvPowerSeries σ k) (h : constantCoeff σ k φ ≠ 0) :
φ⁻¹ * φ = 1 := by rw [mul_comm, φ.mul_inv_cancel h]
protected theorem eq_mul_inv_iff_mul_eq {φ₁ φ₂ φ₃ : MvPowerSeries σ k}
(h : constantCoeff σ k φ₃ ≠ 0) : φ₁ = φ₂ * φ₃⁻¹ ↔ φ₁ * φ₃ = φ₂ :=
⟨fun k => by simp [k, mul_assoc, MvPowerSeries.inv_mul_cancel _ h], fun k => by
simp [← k, mul_assoc, MvPowerSeries.mul_inv_cancel _ h]⟩
protected theorem eq_inv_iff_mul_eq_one {φ ψ : MvPowerSeries σ k} (h : constantCoeff σ k ψ ≠ 0) :
φ = ψ⁻¹ ↔ φ * ψ = 1 := by rw [← MvPowerSeries.eq_mul_inv_iff_mul_eq h, one_mul]
protected theorem inv_eq_iff_mul_eq_one {φ ψ : MvPowerSeries σ k} (h : constantCoeff σ k ψ ≠ 0) :
ψ⁻¹ = φ ↔ φ * ψ = 1 := by rw [eq_comm, MvPowerSeries.eq_inv_iff_mul_eq_one h]
@[simp]
protected theorem mul_inv_rev (φ ψ : MvPowerSeries σ k) :
(φ * ψ)⁻¹ = ψ⁻¹ * φ⁻¹ := by
by_cases h : constantCoeff σ k (φ * ψ) = 0
· rw [inv_eq_zero.mpr h]
simp only [map_mul, mul_eq_zero] at h
-- we don't have `NoZeroDivisors (MvPowerSeries σ k)` yet,
cases' h with h h <;> simp [inv_eq_zero.mpr h]
· rw [MvPowerSeries.inv_eq_iff_mul_eq_one h]
simp only [not_or, map_mul, mul_eq_zero] at h
rw [← mul_assoc, mul_assoc _⁻¹, MvPowerSeries.inv_mul_cancel _ h.left, mul_one,
MvPowerSeries.inv_mul_cancel _ h.right]
instance : InvOneClass (MvPowerSeries σ k) :=
{ inferInstanceAs (One (MvPowerSeries σ k)),
inferInstanceAs (Inv (MvPowerSeries σ k)) with
inv_one := by
rw [MvPowerSeries.inv_eq_iff_mul_eq_one, mul_one]
simp }
@[simp]
theorem C_inv (r : k) : (C σ k r)⁻¹ = C σ k r⁻¹ := by
rcases eq_or_ne r 0 with (rfl | hr)
· simp
rw [MvPowerSeries.inv_eq_iff_mul_eq_one, ← map_mul, inv_mul_cancel hr, map_one]
simpa using hr
@[simp]
theorem X_inv (s : σ) : (X s : MvPowerSeries σ k)⁻¹ = 0 := by
rw [inv_eq_zero, constantCoeff_X]
@[simp]
theorem smul_inv (r : k) (φ : MvPowerSeries σ k) : (r • φ)⁻¹ = r⁻¹ • φ⁻¹ := by
simp [smul_eq_C_mul, mul_comm]
end Field
end MvPowerSeries
end
|
RingTheory\MvPowerSeries\NoZeroDivisors.lean | /-
Copyright (c) 2024 Antoine Chambert-Loir. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Antoine Chambert-Loir
-/
import Mathlib.RingTheory.MvPowerSeries.Basic
import Mathlib.Data.Finsupp.WellFounded
/-! # ZeroDivisors in a MvPowerSeries ring
- `mem_nonZeroDivisors_of_constantCoeff` proves that
a multivariate power series whose constant coefficient is not a zero divisor
is itself not a zero divisor
## TODO
* A subsequent PR #14571 proves that if `R` has no zero divisors,
then so does `MvPowerSeries σ R`.
* Transfer/adapt these results to `HahnSeries`.
## Remark
The analogue of `Polynomial.nmem_nonZeroDivisors_iff`
(McCoy theorem) holds for power series over a noetherian ring,
but not in general. See [Fields1971]
-/
noncomputable section
open Finset (antidiagonal mem_antidiagonal)
namespace MvPowerSeries
open Finsupp
variable {σ R : Type*}
section Semiring
variable [Semiring R]
/-- A multivariate power series is not a zero divisor
when its constant coefficient is not a zero divisor -/
theorem mem_nonZeroDivisors_of_constantCoeff {φ : MvPowerSeries σ R}
(hφ : constantCoeff σ R φ ∈ nonZeroDivisors R) :
φ ∈ nonZeroDivisors (MvPowerSeries σ R) := by
classical
intro x hx
ext d
apply WellFoundedLT.induction d
intro e he
rw [map_zero, ← mul_right_mem_nonZeroDivisors_eq_zero_iff hφ, ← map_zero (f := coeff R e), ← hx]
convert (coeff_mul e x φ).symm
rw [Finset.sum_eq_single (e,0), coeff_zero_eq_constantCoeff]
· rintro ⟨u, _⟩ huv _
suffices u < e by simp only [he u this, zero_mul, map_zero]
apply lt_of_le_of_ne
· simp only [← mem_antidiagonal.mp huv, le_add_iff_nonneg_right, zero_le]
· rintro rfl
simp_all
· simp only [mem_antidiagonal, add_zero, not_true_eq_false, coeff_zero_eq_constantCoeff,
false_implies]
end Semiring
end MvPowerSeries
end
|
RingTheory\MvPowerSeries\Trunc.lean | /-
Copyright (c) 2019 Johan Commelin. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johan Commelin, Kenny Lau
-/
import Mathlib.RingTheory.MvPowerSeries.Basic
import Mathlib.Data.Finsupp.Interval
/-!
# Formal (multivariate) power series - Truncation
`MvPowerSeries.trunc n φ` truncates a formal multivariate power series
to the multivariate polynomial that has the same coefficients as `φ`,
for all `m < n`, and `0` otherwise.
Note that here, `m` and `n` have types `σ →₀ ℕ`,
so that `m < n` means that `m ≠ n` and `m s ≤ n s` for all `s : σ`.
-/
noncomputable section
open Finset (antidiagonal mem_antidiagonal)
namespace MvPowerSeries
open Finsupp
variable {σ R : Type*}
section Trunc
variable [CommSemiring R] (n : σ →₀ ℕ)
/-- Auxiliary definition for the truncation function. -/
def truncFun (φ : MvPowerSeries σ R) : MvPolynomial σ R :=
∑ m ∈ Finset.Iio n, MvPolynomial.monomial m (coeff R m φ)
theorem coeff_truncFun (m : σ →₀ ℕ) (φ : MvPowerSeries σ R) :
(truncFun n φ).coeff m = if m < n then coeff R m φ else 0 := by
classical
simp [truncFun, MvPolynomial.coeff_sum]
variable (R)
/-- The `n`th truncation of a multivariate formal power series to a multivariate polynomial -/
def trunc : MvPowerSeries σ R →+ MvPolynomial σ R where
toFun := truncFun n
map_zero' := by
classical
ext
simp [coeff_truncFun]
map_add' := by
classical
intros x y
ext m
simp only [coeff_truncFun, MvPolynomial.coeff_add]
split_ifs
· rw [map_add]
· rw [zero_add]
variable {R}
theorem coeff_trunc (m : σ →₀ ℕ) (φ : MvPowerSeries σ R) :
(trunc R n φ).coeff m = if m < n then coeff R m φ else 0 := by
classical simp [trunc, coeff_truncFun]
@[simp]
theorem trunc_one (n : σ →₀ ℕ) (hnn : n ≠ 0) : trunc R n 1 = 1 :=
MvPolynomial.ext _ _ fun m => by
classical
rw [coeff_trunc, coeff_one]
split_ifs with H H'
· subst m
simp
· symm
rw [MvPolynomial.coeff_one]
exact if_neg (Ne.symm H')
· symm
rw [MvPolynomial.coeff_one]
refine if_neg ?_
rintro rfl
apply H
exact Ne.bot_lt hnn
@[simp]
theorem trunc_c (n : σ →₀ ℕ) (hnn : n ≠ 0) (a : R) : trunc R n (C σ R a) = MvPolynomial.C a :=
MvPolynomial.ext _ _ fun m => by
classical
rw [coeff_trunc, coeff_C, MvPolynomial.coeff_C]
split_ifs with H <;> first |rfl|try simp_all
exfalso; apply H; subst m; exact Ne.bot_lt hnn
end Trunc
end MvPowerSeries
end
|
RingTheory\Nilpotent\Basic.lean | /-
Copyright (c) 2021 Oliver Nash. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Oliver Nash
-/
import Mathlib.Algebra.Associated.Basic
import Mathlib.Algebra.GeomSum
import Mathlib.Algebra.GroupWithZero.NonZeroDivisors
import Mathlib.Algebra.Module.Defs
import Mathlib.Algebra.SMulWithZero
import Mathlib.Data.Nat.Choose.Sum
import Mathlib.Data.Nat.Lattice
import Mathlib.RingTheory.Nilpotent.Defs
/-!
# Nilpotent elements
This file develops the basic theory of nilpotent elements. In particular it shows that the
nilpotent elements are closed under many operations.
For the definition of `nilradical`, see `Mathlib.RingTheory.Nilpotent.Lemmas`.
## Main definitions
* `isNilpotent_neg_iff`
* `Commute.isNilpotent_add`
* `Commute.isNilpotent_sub`
-/
universe u v
open Function Set
variable {R S : Type*} {x y : R}
theorem IsNilpotent.neg [Ring R] (h : IsNilpotent x) : IsNilpotent (-x) := by
obtain ⟨n, hn⟩ := h
use n
rw [neg_pow, hn, mul_zero]
@[simp]
theorem isNilpotent_neg_iff [Ring R] : IsNilpotent (-x) ↔ IsNilpotent x :=
⟨fun h => neg_neg x ▸ h.neg, fun h => h.neg⟩
lemma IsNilpotent.smul [MonoidWithZero R] [MonoidWithZero S] [MulActionWithZero R S]
[SMulCommClass R S S] [IsScalarTower R S S] {a : S} (ha : IsNilpotent a) (t : R) :
IsNilpotent (t • a) := by
obtain ⟨k, ha⟩ := ha
use k
rw [smul_pow, ha, smul_zero]
theorem IsNilpotent.isUnit_sub_one [Ring R] {r : R} (hnil : IsNilpotent r) : IsUnit (r - 1) := by
obtain ⟨n, hn⟩ := hnil
refine ⟨⟨r - 1, -∑ i ∈ Finset.range n, r ^ i, ?_, ?_⟩, rfl⟩
· simp [mul_geom_sum, hn]
· simp [geom_sum_mul, hn]
theorem IsNilpotent.isUnit_one_sub [Ring R] {r : R} (hnil : IsNilpotent r) : IsUnit (1 - r) := by
rw [← IsUnit.neg_iff, neg_sub]
exact isUnit_sub_one hnil
theorem IsNilpotent.isUnit_add_one [Ring R] {r : R} (hnil : IsNilpotent r) : IsUnit (r + 1) := by
rw [← IsUnit.neg_iff, neg_add']
exact isUnit_sub_one hnil.neg
theorem IsNilpotent.isUnit_one_add [Ring R] {r : R} (hnil : IsNilpotent r) : IsUnit (1 + r) :=
add_comm r 1 ▸ isUnit_add_one hnil
theorem IsNilpotent.isUnit_add_left_of_commute [Ring R] {r u : R}
(hnil : IsNilpotent r) (hu : IsUnit u) (h_comm : Commute r u) :
IsUnit (u + r) := by
rw [← Units.isUnit_mul_units _ hu.unit⁻¹, add_mul, IsUnit.mul_val_inv]
replace h_comm : Commute r (↑hu.unit⁻¹) := Commute.units_inv_right h_comm
refine IsNilpotent.isUnit_one_add ?_
exact (hu.unit⁻¹.isUnit.isNilpotent_mul_unit_of_commute_iff h_comm).mpr hnil
theorem IsNilpotent.isUnit_add_right_of_commute [Ring R] {r u : R}
(hnil : IsNilpotent r) (hu : IsUnit u) (h_comm : Commute r u) :
IsUnit (r + u) :=
add_comm r u ▸ hnil.isUnit_add_left_of_commute hu h_comm
instance [Zero R] [Pow R ℕ] [Zero S] [Pow S ℕ] [IsReduced R] [IsReduced S] : IsReduced (R × S) where
eq_zero _ := fun ⟨n, hn⟩ ↦ have hn := Prod.ext_iff.1 hn
Prod.ext (IsReduced.eq_zero _ ⟨n, hn.1⟩) (IsReduced.eq_zero _ ⟨n, hn.2⟩)
theorem Prime.isRadical [CommMonoidWithZero R] {y : R} (hy : Prime y) : IsRadical y :=
fun _ _ ↦ hy.dvd_of_dvd_pow
theorem zero_isRadical_iff [MonoidWithZero R] : IsRadical (0 : R) ↔ IsReduced R := by
simp_rw [isReduced_iff, IsNilpotent, exists_imp, ← zero_dvd_iff]
exact forall_swap
theorem isReduced_iff_pow_one_lt [MonoidWithZero R] (k : ℕ) (hk : 1 < k) :
IsReduced R ↔ ∀ x : R, x ^ k = 0 → x = 0 := by
simp_rw [← zero_isRadical_iff, isRadical_iff_pow_one_lt k hk, zero_dvd_iff]
theorem IsRadical.of_dvd [CancelCommMonoidWithZero R] {x y : R} (hy : IsRadical y) (h0 : y ≠ 0)
(hxy : x ∣ y) : IsRadical x := (isRadical_iff_pow_one_lt 2 one_lt_two).2 <| by
obtain ⟨z, rfl⟩ := hxy
refine fun w dvd ↦ ((mul_dvd_mul_iff_right <| right_ne_zero_of_mul h0).mp <| hy 2 _ ?_)
rw [mul_pow, sq z]; exact mul_dvd_mul dvd (dvd_mul_left z z)
namespace Commute
section Semiring
variable [Semiring R]
theorem add_pow_eq_zero_of_add_le_succ_of_pow_eq_zero (h_comm : Commute x y) {m n k : ℕ}
(hx : x ^ m = 0) (hy : y ^ n = 0) (h : m + n ≤ k + 1) :
(x + y) ^ k = 0 := by
rw [h_comm.add_pow']
apply Finset.sum_eq_zero
rintro ⟨i, j⟩ hij
suffices x ^ i * y ^ j = 0 by simp only [this, nsmul_eq_mul, mul_zero]
by_cases hi : m ≤ i
· rw [pow_eq_zero_of_le hi hx, zero_mul]
rw [pow_eq_zero_of_le ?_ hy, mul_zero]
linarith [Finset.mem_antidiagonal.mp hij]
theorem add_pow_add_eq_zero_of_pow_eq_zero (h_comm : Commute x y) {m n : ℕ}
(hx : x ^ m = 0) (hy : y ^ n = 0) :
(x + y) ^ (m + n - 1) = 0 :=
h_comm.add_pow_eq_zero_of_add_le_succ_of_pow_eq_zero hx hy <| by rw [← Nat.sub_le_iff_le_add]
theorem isNilpotent_add (h_comm : Commute x y) (hx : IsNilpotent x) (hy : IsNilpotent y) :
IsNilpotent (x + y) := by
obtain ⟨n, hn⟩ := hx
obtain ⟨m, hm⟩ := hy
exact ⟨_, add_pow_add_eq_zero_of_pow_eq_zero h_comm hn hm⟩
protected lemma isNilpotent_sum {ι : Type*} {s : Finset ι} {f : ι → R}
(hnp : ∀ i ∈ s, IsNilpotent (f i)) (h_comm : ∀ i j, i ∈ s → j ∈ s → Commute (f i) (f j)) :
IsNilpotent (∑ i ∈ s, f i) := by
classical
induction s using Finset.induction with
| empty => simp
| @insert j s hj ih => ?_
rw [Finset.sum_insert hj]
apply Commute.isNilpotent_add
· exact Commute.sum_right _ _ _ (fun i hi ↦ h_comm _ _ (by simp) (by simp [hi]))
· apply hnp; simp
· exact ih (fun i hi ↦ hnp i (by simp [hi]))
(fun i j hi hj ↦ h_comm i j (by simp [hi]) (by simp [hj]))
protected lemma isNilpotent_mul_left_iff (h_comm : Commute x y) (hy : y ∈ nonZeroDivisorsLeft R) :
IsNilpotent (x * y) ↔ IsNilpotent x := by
refine ⟨?_, h_comm.isNilpotent_mul_left⟩
rintro ⟨k, hk⟩
rw [mul_pow h_comm] at hk
exact ⟨k, (nonZeroDivisorsLeft R).pow_mem hy k _ hk⟩
protected lemma isNilpotent_mul_right_iff (h_comm : Commute x y) (hx : x ∈ nonZeroDivisorsRight R) :
IsNilpotent (x * y) ↔ IsNilpotent y := by
refine ⟨?_, h_comm.isNilpotent_mul_right⟩
rintro ⟨k, hk⟩
rw [mul_pow h_comm] at hk
exact ⟨k, (nonZeroDivisorsRight R).pow_mem hx k _ hk⟩
end Semiring
section Ring
variable [Ring R]
theorem isNilpotent_sub (h_comm : Commute x y) (hx : IsNilpotent x) (hy : IsNilpotent y) :
IsNilpotent (x - y) := by
rw [← neg_right_iff] at h_comm
rw [← isNilpotent_neg_iff] at hy
rw [sub_eq_add_neg]
exact h_comm.isNilpotent_add hx hy
end Ring
end Commute
section CommSemiring
variable [CommSemiring R] {x y : R}
lemma isNilpotent_sum {ι : Type*} {s : Finset ι} {f : ι → R}
(hnp : ∀ i ∈ s, IsNilpotent (f i)) :
IsNilpotent (∑ i ∈ s, f i) :=
Commute.isNilpotent_sum hnp fun _ _ _ _ ↦ Commute.all _ _
end CommSemiring
lemma NoZeroSMulDivisors.isReduced (R M : Type*)
[MonoidWithZero R] [Zero M] [MulActionWithZero R M] [Nontrivial M] [NoZeroSMulDivisors R M] :
IsReduced R := by
refine ⟨fun x ⟨k, hk⟩ ↦ ?_⟩
induction' k with k ih
· rw [pow_zero] at hk
exact eq_zero_of_zero_eq_one hk.symm x
· obtain ⟨m : M, hm : m ≠ 0⟩ := exists_ne (0 : M)
have : x ^ (k + 1) • m = 0 := by simp only [hk, zero_smul]
rw [pow_succ', mul_smul] at this
rcases eq_zero_or_eq_zero_of_smul_eq_zero this with rfl | hx
· rfl
· exact ih <| (eq_zero_or_eq_zero_of_smul_eq_zero hx).resolve_right hm
|
RingTheory\Nilpotent\Defs.lean | /-
Copyright (c) 2021 Oliver Nash. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Oliver Nash
-/
import Mathlib.Algebra.GroupWithZero.Hom
import Mathlib.Algebra.GroupWithZero.Units.Basic
import Mathlib.Algebra.Ring.Defs
import Mathlib.Data.Nat.Lattice
/-!
# Definition of nilpotent elements
This file defines the notion of a nilpotent element and proves the immediate consequences.
For results that require further theory, see `Mathlib.RingTheory.Nilpotent.Basic`
and `Mathlib.RingTheory.Nilpotent.Lemmas`.
## Main definitions
* `IsNilpotent`
* `Commute.isNilpotent_mul_left`
* `Commute.isNilpotent_mul_right`
* `nilpotencyClass`
-/
universe u v
open Function Set
variable {R S : Type*} {x y : R}
/-- An element is said to be nilpotent if some natural-number-power of it equals zero.
Note that we require only the bare minimum assumptions for the definition to make sense. Even
`MonoidWithZero` is too strong since nilpotency is important in the study of rings that are only
power-associative. -/
def IsNilpotent [Zero R] [Pow R ℕ] (x : R) : Prop :=
∃ n : ℕ, x ^ n = 0
theorem IsNilpotent.mk [Zero R] [Pow R ℕ] (x : R) (n : ℕ) (e : x ^ n = 0) : IsNilpotent x :=
⟨n, e⟩
@[simp] lemma isNilpotent_of_subsingleton [Zero R] [Pow R ℕ] [Subsingleton R] : IsNilpotent x :=
⟨0, Subsingleton.elim _ _⟩
@[simp] theorem IsNilpotent.zero [MonoidWithZero R] : IsNilpotent (0 : R) :=
⟨1, pow_one 0⟩
theorem not_isNilpotent_one [MonoidWithZero R] [Nontrivial R] :
¬ IsNilpotent (1 : R) := fun ⟨_, H⟩ ↦ zero_ne_one (H.symm.trans (one_pow _))
lemma IsNilpotent.pow_succ (n : ℕ) {S : Type*} [MonoidWithZero S] {x : S}
(hx : IsNilpotent x) : IsNilpotent (x ^ n.succ) := by
obtain ⟨N,hN⟩ := hx
use N
rw [← pow_mul, Nat.succ_mul, pow_add, hN, mul_zero]
theorem IsNilpotent.of_pow [MonoidWithZero R] {x : R} {m : ℕ}
(h : IsNilpotent (x ^ m)) : IsNilpotent x := by
obtain ⟨n, h⟩ := h
use m*n
rw [← h, pow_mul x m n]
lemma IsNilpotent.pow_of_pos {n} {S : Type*} [MonoidWithZero S] {x : S}
(hx : IsNilpotent x) (hn : n ≠ 0) : IsNilpotent (x ^ n) := by
cases n with
| zero => contradiction
| succ => exact IsNilpotent.pow_succ _ hx
@[simp]
lemma IsNilpotent.pow_iff_pos {n} {S : Type*} [MonoidWithZero S] {x : S}
(hn : n ≠ 0) : IsNilpotent (x ^ n) ↔ IsNilpotent x :=
⟨fun h => of_pow h, fun h => pow_of_pos h hn⟩
theorem IsNilpotent.map [MonoidWithZero R] [MonoidWithZero S] {r : R} {F : Type*}
[FunLike F R S] [MonoidWithZeroHomClass F R S] (hr : IsNilpotent r) (f : F) :
IsNilpotent (f r) := by
use hr.choose
rw [← map_pow, hr.choose_spec, map_zero]
lemma IsNilpotent.map_iff [MonoidWithZero R] [MonoidWithZero S] {r : R} {F : Type*}
[FunLike F R S] [MonoidWithZeroHomClass F R S] {f : F} (hf : Function.Injective f) :
IsNilpotent (f r) ↔ IsNilpotent r :=
⟨fun ⟨k, hk⟩ ↦ ⟨k, (map_eq_zero_iff f hf).mp <| by rwa [map_pow]⟩, fun h ↦ h.map f⟩
theorem IsUnit.isNilpotent_mul_unit_of_commute_iff [MonoidWithZero R] {r u : R}
(hu : IsUnit u) (h_comm : Commute r u) :
IsNilpotent (r * u) ↔ IsNilpotent r :=
exists_congr fun n ↦ by rw [h_comm.mul_pow, (hu.pow n).mul_left_eq_zero]
theorem IsUnit.isNilpotent_unit_mul_of_commute_iff [MonoidWithZero R] {r u : R}
(hu : IsUnit u) (h_comm : Commute r u) :
IsNilpotent (u * r) ↔ IsNilpotent r :=
h_comm ▸ hu.isNilpotent_mul_unit_of_commute_iff h_comm
section NilpotencyClass
section ZeroPow
variable [Zero R] [Pow R ℕ]
variable (x) in
/-- If `x` is nilpotent, the nilpotency class is the smallest natural number `k` such that
`x ^ k = 0`. If `x` is not nilpotent, the nilpotency class takes the junk value `0`. -/
noncomputable def nilpotencyClass : ℕ := sInf {k | x ^ k = 0}
@[simp] lemma nilpotencyClass_eq_zero_of_subsingleton [Subsingleton R] :
nilpotencyClass x = 0 := by
let s : Set ℕ := {k | x ^ k = 0}
suffices s = univ by change sInf _ = 0; simp [s] at this; simp [this]
exact eq_univ_iff_forall.mpr fun k ↦ Subsingleton.elim _ _
lemma isNilpotent_of_pos_nilpotencyClass (hx : 0 < nilpotencyClass x) :
IsNilpotent x := by
let s : Set ℕ := {k | x ^ k = 0}
change s.Nonempty
change 0 < sInf s at hx
by_contra contra
simp [not_nonempty_iff_eq_empty.mp contra] at hx
lemma pow_nilpotencyClass (hx : IsNilpotent x) : x ^ (nilpotencyClass x) = 0 :=
Nat.sInf_mem hx
end ZeroPow
section MonoidWithZero
variable [MonoidWithZero R]
lemma nilpotencyClass_eq_succ_iff {k : ℕ} :
nilpotencyClass x = k + 1 ↔ x ^ (k + 1) = 0 ∧ x ^ k ≠ 0 := by
let s : Set ℕ := {k | x ^ k = 0}
have : ∀ k₁ k₂ : ℕ, k₁ ≤ k₂ → k₁ ∈ s → k₂ ∈ s := fun k₁ k₂ h_le hk₁ ↦ pow_eq_zero_of_le h_le hk₁
simp [s, nilpotencyClass, Nat.sInf_upward_closed_eq_succ_iff this]
@[simp] lemma nilpotencyClass_zero [Nontrivial R] :
nilpotencyClass (0 : R) = 1 :=
nilpotencyClass_eq_succ_iff.mpr <| by constructor <;> simp
@[simp] lemma pos_nilpotencyClass_iff [Nontrivial R] :
0 < nilpotencyClass x ↔ IsNilpotent x := by
refine ⟨isNilpotent_of_pos_nilpotencyClass, fun hx ↦ Nat.pos_of_ne_zero fun hx' ↦ ?_⟩
replace hx := pow_nilpotencyClass hx
rw [hx', pow_zero] at hx
exact one_ne_zero hx
lemma pow_pred_nilpotencyClass [Nontrivial R] (hx : IsNilpotent x) :
x ^ (nilpotencyClass x - 1) ≠ 0 :=
(nilpotencyClass_eq_succ_iff.mp <| Nat.eq_add_of_sub_eq (pos_nilpotencyClass_iff.mpr hx) rfl).2
lemma eq_zero_of_nilpotencyClass_eq_one (hx : nilpotencyClass x = 1) :
x = 0 := by
have : IsNilpotent x := isNilpotent_of_pos_nilpotencyClass (hx ▸ Nat.one_pos)
rw [← pow_nilpotencyClass this, hx, pow_one]
@[simp] lemma nilpotencyClass_eq_one [Nontrivial R] :
nilpotencyClass x = 1 ↔ x = 0 :=
⟨eq_zero_of_nilpotencyClass_eq_one, fun hx ↦ hx ▸ nilpotencyClass_zero⟩
end MonoidWithZero
end NilpotencyClass
/-- A structure that has zero and pow is reduced if it has no nonzero nilpotent elements. -/
@[mk_iff]
class IsReduced (R : Type*) [Zero R] [Pow R ℕ] : Prop where
/-- A reduced structure has no nonzero nilpotent elements. -/
eq_zero : ∀ x : R, IsNilpotent x → x = 0
instance (priority := 900) isReduced_of_noZeroDivisors [MonoidWithZero R] [NoZeroDivisors R] :
IsReduced R :=
⟨fun _ ⟨_, hn⟩ => pow_eq_zero hn⟩
instance (priority := 900) isReduced_of_subsingleton [Zero R] [Pow R ℕ] [Subsingleton R] :
IsReduced R :=
⟨fun _ _ => Subsingleton.elim _ _⟩
theorem IsNilpotent.eq_zero [Zero R] [Pow R ℕ] [IsReduced R] (h : IsNilpotent x) : x = 0 :=
IsReduced.eq_zero x h
@[simp]
theorem isNilpotent_iff_eq_zero [MonoidWithZero R] [IsReduced R] : IsNilpotent x ↔ x = 0 :=
⟨fun h => h.eq_zero, fun h => h.symm ▸ IsNilpotent.zero⟩
theorem isReduced_of_injective [MonoidWithZero R] [MonoidWithZero S] {F : Type*}
[FunLike F R S] [MonoidWithZeroHomClass F R S]
(f : F) (hf : Function.Injective f) [IsReduced S] :
IsReduced R := by
constructor
intro x hx
apply hf
rw [map_zero]
exact (hx.map f).eq_zero
instance (ι) (R : ι → Type*) [∀ i, Zero (R i)] [∀ i, Pow (R i) ℕ]
[∀ i, IsReduced (R i)] : IsReduced (∀ i, R i) where
eq_zero _ := fun ⟨n, hn⟩ ↦ funext fun i ↦ IsReduced.eq_zero _ ⟨n, congr_fun hn i⟩
/-- An element `y` in a monoid is radical if for any element `x`, `y` divides `x` whenever it
divides a power of `x`. -/
def IsRadical [Dvd R] [Pow R ℕ] (y : R) : Prop :=
∀ (n : ℕ) (x), y ∣ x ^ n → y ∣ x
theorem isRadical_iff_pow_one_lt [MonoidWithZero R] (k : ℕ) (hk : 1 < k) :
IsRadical y ↔ ∀ x, y ∣ x ^ k → y ∣ x :=
⟨(· k), k.pow_imp_self_of_one_lt hk _ fun _ _ h ↦ .inl (dvd_mul_of_dvd_left h _)⟩
namespace Commute
section Semiring
variable [Semiring R]
theorem isNilpotent_mul_left (h_comm : Commute x y) (h : IsNilpotent x) : IsNilpotent (x * y) := by
obtain ⟨n, hn⟩ := h
use n
rw [h_comm.mul_pow, hn, zero_mul]
theorem isNilpotent_mul_right (h_comm : Commute x y) (h : IsNilpotent y) : IsNilpotent (x * y) := by
rw [h_comm.eq]
exact h_comm.symm.isNilpotent_mul_left h
end Semiring
end Commute
|
RingTheory\Nilpotent\Lemmas.lean | /-
Copyright (c) 2021 Oliver Nash. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Oliver Nash
-/
import Mathlib.LinearAlgebra.Matrix.ToLin
import Mathlib.LinearAlgebra.Quotient
import Mathlib.RingTheory.Ideal.Maps
import Mathlib.RingTheory.Nilpotent.Defs
/-!
# Nilpotent elements
This file contains results about nilpotent elements that involve ring theory.
-/
universe u v
open Function Set
variable {R S : Type*} {x y : R}
theorem RingHom.ker_isRadical_iff_reduced_of_surjective {S F} [CommSemiring R] [CommRing S]
[FunLike F R S] [RingHomClass F R S] {f : F} (hf : Function.Surjective f) :
(RingHom.ker f).IsRadical ↔ IsReduced S := by
simp_rw [isReduced_iff, hf.forall, IsNilpotent, ← map_pow, ← RingHom.mem_ker]
rfl
theorem isRadical_iff_span_singleton [CommSemiring R] :
IsRadical y ↔ (Ideal.span ({y} : Set R)).IsRadical := by
simp_rw [IsRadical, ← Ideal.mem_span_singleton]
exact forall_swap.trans (forall_congr' fun r => exists_imp.symm)
namespace Commute
section Semiring
variable [Semiring R] (h_comm : Commute x y)
end Semiring
end Commute
section CommSemiring
variable [CommSemiring R] {x y : R}
/-- The nilradical of a commutative semiring is the ideal of nilpotent elements. -/
def nilradical (R : Type*) [CommSemiring R] : Ideal R :=
(0 : Ideal R).radical
theorem mem_nilradical : x ∈ nilradical R ↔ IsNilpotent x :=
Iff.rfl
theorem nilradical_eq_sInf (R : Type*) [CommSemiring R] :
nilradical R = sInf { J : Ideal R | J.IsPrime } :=
(Ideal.radical_eq_sInf ⊥).trans <| by simp_rw [and_iff_right bot_le]
theorem nilpotent_iff_mem_prime : IsNilpotent x ↔ ∀ J : Ideal R, J.IsPrime → x ∈ J := by
rw [← mem_nilradical, nilradical_eq_sInf, Submodule.mem_sInf]
rfl
theorem nilradical_le_prime (J : Ideal R) [H : J.IsPrime] : nilradical R ≤ J :=
(nilradical_eq_sInf R).symm ▸ sInf_le H
@[simp]
theorem nilradical_eq_zero (R : Type*) [CommSemiring R] [IsReduced R] : nilradical R = 0 :=
Ideal.ext fun _ => isNilpotent_iff_eq_zero
end CommSemiring
namespace LinearMap
variable (R) {A : Type v} [CommSemiring R] [Semiring A] [Algebra R A]
@[simp]
theorem isNilpotent_mulLeft_iff (a : A) : IsNilpotent (mulLeft R a) ↔ IsNilpotent a := by
constructor <;> rintro ⟨n, hn⟩ <;> use n <;>
simp only [mulLeft_eq_zero_iff, pow_mulLeft] at hn ⊢ <;>
exact hn
@[simp]
theorem isNilpotent_mulRight_iff (a : A) : IsNilpotent (mulRight R a) ↔ IsNilpotent a := by
constructor <;> rintro ⟨n, hn⟩ <;> use n <;>
simp only [mulRight_eq_zero_iff, pow_mulRight] at hn ⊢ <;>
exact hn
variable {R}
variable {ι M : Type*} [Fintype ι] [DecidableEq ι] [AddCommMonoid M] [Module R M]
@[simp]
lemma isNilpotent_toMatrix_iff (b : Basis ι R M) (f : M →ₗ[R] M) :
IsNilpotent (toMatrix b b f) ↔ IsNilpotent f := by
refine exists_congr fun k ↦ ?_
rw [toMatrix_pow]
exact (toMatrix b b).map_eq_zero_iff
end LinearMap
namespace Module.End
lemma isNilpotent.restrict {R M : Type*} [Semiring R] [AddCommMonoid M] [Module R M]
{f : M →ₗ[R] M} {p : Submodule R M} (hf : MapsTo f p p) (hnil : IsNilpotent f) :
IsNilpotent (f.restrict hf) := by
obtain ⟨n, hn⟩ := hnil
exact ⟨n, LinearMap.ext fun m ↦ by simp [LinearMap.pow_restrict n, LinearMap.restrict_apply, hn]⟩
variable {M : Type v} [Ring R] [AddCommGroup M] [Module R M]
variable {f : Module.End R M} {p : Submodule R M} (hp : p ≤ p.comap f)
theorem IsNilpotent.mapQ (hnp : IsNilpotent f) : IsNilpotent (p.mapQ p f hp) := by
obtain ⟨k, hk⟩ := hnp
use k
simp [← p.mapQ_pow, hk]
end Module.End
|
RingTheory\NonUnitalSubring\Basic.lean | /-
Copyright (c) 2023 Jireh Loreaux. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jireh Loreaux
-/
import Mathlib.Algebra.Group.Subgroup.Basic
import Mathlib.GroupTheory.Subsemigroup.Center
import Mathlib.RingTheory.NonUnitalSubsemiring.Basic
/-!
# `NonUnitalSubring`s
Let `R` be a non-unital ring. This file defines the "bundled" non-unital subring type
`NonUnitalSubring R`, a type whose terms correspond to non-unital subrings of `R`.
This is the preferred way to talk about non-unital subrings in mathlib.
We prove that non-unital subrings are a complete lattice, and that you can `map` (pushforward) and
`comap` (pull back) them along ring homomorphisms.
We define the `closure` construction from `Set R` to `NonUnitalSubring R`, sending a subset of
`R` to the non-unital subring it generates, and prove that it is a Galois insertion.
## Main definitions
Notation used here:
`(R : Type u) [NonUnitalRing R] (S : Type u) [NonUnitalRing S] (f g : R →ₙ+* S)`
`(A : NonUnitalSubring R) (B : NonUnitalSubring S) (s : Set R)`
* `NonUnitalSubring R` : the type of non-unital subrings of a ring `R`.
* `instance : CompleteLattice (NonUnitalSubring R)` : the complete lattice structure on the
non-unital subrings.
* `NonUnitalSubring.center` : the center of a non-unital ring `R`.
* `NonUnitalSubring.closure` : non-unital subring closure of a set, i.e., the smallest
non-unital subring that includes the set.
* `NonUnitalSubring.gi` : `closure : Set M → NonUnitalSubring M` and coercion
`coe : NonUnitalSubring M → Set M`
form a `GaloisInsertion`.
* `comap f B : NonUnitalSubring A` : the preimage of a non-unital subring `B` along the
non-unital ring homomorphism `f`
* `map f A : NonUnitalSubring B` : the image of a non-unital subring `A` along the
non-unital ring homomorphism `f`.
* `Prod A B : NonUnitalSubring (R × S)` : the product of non-unital subrings
* `f.range : NonUnitalSubring B` : the range of the non-unital ring homomorphism `f`.
* `eq_locus f g : NonUnitalSubring R` : given non-unital ring homomorphisms `f g : R →ₙ+* S`,
the non-unital subring of `R` where `f x = g x`
## Implementation notes
A non-unital subring is implemented as a `NonUnitalSubsemiring` which is also an
additive subgroup.
Lattice inclusion (e.g. `≤` and `⊓`) is used rather than set notation (`⊆` and `∩`), although
`∈` is defined as membership of a non-unital subring's underlying set.
## Tags
non-unital subring
-/
universe u v w
section Basic
variable {R : Type u} {S : Type v} {T : Type w} [NonUnitalNonAssocRing R]
section NonUnitalSubringClass
/-- `NonUnitalSubringClass S R` states that `S` is a type of subsets `s ⊆ R` that
are both a multiplicative submonoid and an additive subgroup. -/
class NonUnitalSubringClass (S : Type*) (R : Type u) [NonUnitalNonAssocRing R]
[SetLike S R] extends NonUnitalSubsemiringClass S R, NegMemClass S R : Prop where
-- See note [lower instance priority]
instance (priority := 100) NonUnitalSubringClass.addSubgroupClass (S : Type*) (R : Type u)
[SetLike S R] [NonUnitalNonAssocRing R] [h : NonUnitalSubringClass S R] :
AddSubgroupClass S R :=
{ h with }
variable [SetLike S R] [hSR : NonUnitalSubringClass S R] (s : S)
namespace NonUnitalSubringClass
-- Prefer subclasses of `NonUnitalRing` over subclasses of `NonUnitalSubringClass`.
/-- A non-unital subring of a non-unital ring inherits a non-unital ring structure -/
instance (priority := 75) toNonUnitalNonAssocRing : NonUnitalNonAssocRing s :=
Subtype.val_injective.nonUnitalNonAssocRing _ rfl (fun _ _ => rfl) (fun _ _ => rfl) (fun _ => rfl)
(fun _ _ => rfl) (fun _ _ => rfl) fun _ _ => rfl
-- Prefer subclasses of `NonUnitalRing` over subclasses of `NonUnitalSubringClass`.
/-- A non-unital subring of a non-unital ring inherits a non-unital ring structure -/
instance (priority := 75) toNonUnitalRing {R : Type*} [NonUnitalRing R] [SetLike S R]
[NonUnitalSubringClass S R] (s : S) : NonUnitalRing s :=
Subtype.val_injective.nonUnitalRing _ rfl (fun _ _ => rfl) (fun _ _ => rfl) (fun _ => rfl)
(fun _ _ => rfl) (fun _ _ => rfl) fun _ _ => rfl
-- Prefer subclasses of `NonUnitalRing` over subclasses of `NonUnitalSubringClass`.
/-- A non-unital subring of a `NonUnitalCommRing` is a `NonUnitalCommRing`. -/
instance (priority := 75) toNonUnitalCommRing {R} [NonUnitalCommRing R] [SetLike S R]
[NonUnitalSubringClass S R] : NonUnitalCommRing s :=
Subtype.val_injective.nonUnitalCommRing _ rfl (fun _ _ => rfl) (fun _ _ => rfl) (fun _ => rfl)
(fun _ _ => rfl) (fun _ _ => rfl) fun _ _ => rfl
/-- The natural non-unital ring hom from a non-unital subring of a non-unital ring `R` to `R`. -/
def subtype (s : S) : s →ₙ+* R :=
{ NonUnitalSubsemiringClass.subtype s,
AddSubgroupClass.subtype s with
toFun := Subtype.val }
@[simp]
theorem coe_subtype : (subtype s : s → R) = Subtype.val :=
rfl
end NonUnitalSubringClass
end NonUnitalSubringClass
variable [NonUnitalNonAssocRing S] [NonUnitalNonAssocRing T]
/-- `NonUnitalSubring R` is the type of non-unital subrings of `R`. A non-unital subring of `R`
is a subset `s` that is a multiplicative subsemigroup and an additive subgroup. Note in particular
that it shares the same 0 as R. -/
structure NonUnitalSubring (R : Type u) [NonUnitalNonAssocRing R] extends
NonUnitalSubsemiring R, AddSubgroup R
/-- Reinterpret a `NonUnitalSubring` as a `NonUnitalSubsemiring`. -/
add_decl_doc NonUnitalSubring.toNonUnitalSubsemiring
/-- Reinterpret a `NonUnitalSubring` as an `AddSubgroup`. -/
add_decl_doc NonUnitalSubring.toAddSubgroup
namespace NonUnitalSubring
/-- The underlying submonoid of a `NonUnitalSubring`. -/
def toSubsemigroup (s : NonUnitalSubring R) : Subsemigroup R :=
{ s.toNonUnitalSubsemiring.toSubsemigroup with carrier := s.carrier }
instance : SetLike (NonUnitalSubring R) R where
coe s := s.carrier
coe_injective' p q h := by cases p; cases q; congr; exact SetLike.coe_injective h
instance : NonUnitalSubringClass (NonUnitalSubring R) R where
zero_mem s := s.zero_mem'
add_mem {s} := s.add_mem'
mul_mem {s} := s.mul_mem'
neg_mem {s} := s.neg_mem'
theorem mem_carrier {s : NonUnitalSubring R} {x : R} : x ∈ s.toNonUnitalSubsemiring ↔ x ∈ s :=
Iff.rfl
@[simp]
theorem mem_mk {S : NonUnitalSubsemiring R} {x : R} (h) :
x ∈ (⟨S, h⟩ : NonUnitalSubring R) ↔ x ∈ S :=
Iff.rfl
@[simp]
theorem coe_set_mk (S : NonUnitalSubsemiring R) (h) :
((⟨S, h⟩ : NonUnitalSubring R) : Set R) = S :=
rfl
@[simp]
theorem mk_le_mk {S S' : NonUnitalSubsemiring R} (h h') :
(⟨S, h⟩ : NonUnitalSubring R) ≤ (⟨S', h'⟩ : NonUnitalSubring R) ↔ S ≤ S' :=
Iff.rfl
/-- Two non-unital subrings are equal if they have the same elements. -/
@[ext]
theorem ext {S T : NonUnitalSubring R} (h : ∀ x, x ∈ S ↔ x ∈ T) : S = T :=
SetLike.ext h
/-- Copy of a non-unital subring with a new `carrier` equal to the old one. Useful to fix
definitional equalities. -/
protected def copy (S : NonUnitalSubring R) (s : Set R) (hs : s = ↑S) : NonUnitalSubring R :=
{ S.toNonUnitalSubsemiring.copy s hs with
carrier := s
neg_mem' := hs.symm ▸ S.neg_mem' }
@[simp]
theorem coe_copy (S : NonUnitalSubring R) (s : Set R) (hs : s = ↑S) : (S.copy s hs : Set R) = s :=
rfl
theorem copy_eq (S : NonUnitalSubring R) (s : Set R) (hs : s = ↑S) : S.copy s hs = S :=
SetLike.coe_injective hs
theorem toNonUnitalSubsemiring_injective :
Function.Injective (toNonUnitalSubsemiring : NonUnitalSubring R → NonUnitalSubsemiring R)
| _r, _s, h => ext (SetLike.ext_iff.mp h : _)
@[mono]
theorem toNonUnitalSubsemiring_strictMono :
StrictMono (toNonUnitalSubsemiring : NonUnitalSubring R → NonUnitalSubsemiring R) := fun _ _ =>
id
@[mono]
theorem toNonUnitalSubsemiring_mono :
Monotone (toNonUnitalSubsemiring : NonUnitalSubring R → NonUnitalSubsemiring R) :=
toNonUnitalSubsemiring_strictMono.monotone
theorem toAddSubgroup_injective :
Function.Injective (toAddSubgroup : NonUnitalSubring R → AddSubgroup R)
| _r, _s, h => ext (SetLike.ext_iff.mp h : _)
@[mono]
theorem toAddSubgroup_strictMono :
StrictMono (toAddSubgroup : NonUnitalSubring R → AddSubgroup R) := fun _ _ => id
@[mono]
theorem toAddSubgroup_mono : Monotone (toAddSubgroup : NonUnitalSubring R → AddSubgroup R) :=
toAddSubgroup_strictMono.monotone
theorem toSubsemigroup_injective :
Function.Injective (toSubsemigroup : NonUnitalSubring R → Subsemigroup R)
| _r, _s, h => ext (SetLike.ext_iff.mp h : _)
@[mono]
theorem toSubsemigroup_strictMono :
StrictMono (toSubsemigroup : NonUnitalSubring R → Subsemigroup R) := fun _ _ => id
@[mono]
theorem toSubsemigroup_mono : Monotone (toSubsemigroup : NonUnitalSubring R → Subsemigroup R) :=
toSubsemigroup_strictMono.monotone
/-- Construct a `NonUnitalSubring R` from a set `s`, a subsemigroup `sm`, and an additive
subgroup `sa` such that `x ∈ s ↔ x ∈ sm ↔ x ∈ sa`. -/
protected def mk' (s : Set R) (sm : Subsemigroup R) (sa : AddSubgroup R) (hm : ↑sm = s)
(ha : ↑sa = s) : NonUnitalSubring R :=
{ sm.copy s hm.symm, sa.copy s ha.symm with }
@[simp]
theorem coe_mk' {s : Set R} {sm : Subsemigroup R} (hm : ↑sm = s) {sa : AddSubgroup R}
(ha : ↑sa = s) : (NonUnitalSubring.mk' s sm sa hm ha : Set R) = s :=
rfl
@[simp]
theorem mem_mk' {s : Set R} {sm : Subsemigroup R} (hm : ↑sm = s) {sa : AddSubgroup R} (ha : ↑sa = s)
{x : R} : x ∈ NonUnitalSubring.mk' s sm sa hm ha ↔ x ∈ s :=
Iff.rfl
@[simp]
theorem mk'_toSubsemigroup {s : Set R} {sm : Subsemigroup R} (hm : ↑sm = s) {sa : AddSubgroup R}
(ha : ↑sa = s) : (NonUnitalSubring.mk' s sm sa hm ha).toSubsemigroup = sm :=
SetLike.coe_injective hm.symm
@[simp]
theorem mk'_toAddSubgroup {s : Set R} {sm : Subsemigroup R} (hm : ↑sm = s) {sa : AddSubgroup R}
(ha : ↑sa = s) : (NonUnitalSubring.mk' s sm sa hm ha).toAddSubgroup = sa :=
SetLike.coe_injective ha.symm
end NonUnitalSubring
namespace NonUnitalSubring
variable (s : NonUnitalSubring R)
/-- A non-unital subring contains the ring's 0. -/
protected theorem zero_mem : (0 : R) ∈ s :=
zero_mem _
/-- A non-unital subring is closed under multiplication. -/
protected theorem mul_mem {x y : R} : x ∈ s → y ∈ s → x * y ∈ s :=
mul_mem
/-- A non-unital subring is closed under addition. -/
protected theorem add_mem {x y : R} : x ∈ s → y ∈ s → x + y ∈ s :=
add_mem
/-- A non-unital subring is closed under negation. -/
protected theorem neg_mem {x : R} : x ∈ s → -x ∈ s :=
neg_mem
/-- A non-unital subring is closed under subtraction -/
protected theorem sub_mem {x y : R} (hx : x ∈ s) (hy : y ∈ s) : x - y ∈ s :=
sub_mem hx hy
/-- Sum of a list of elements in a non-unital subring is in the non-unital subring. -/
protected theorem list_sum_mem {l : List R} : (∀ x ∈ l, x ∈ s) → l.sum ∈ s :=
list_sum_mem
/-- Sum of a multiset of elements in a `NonUnitalSubring` of a `NonUnitalRing` is
in the `NonUnitalSubring`. -/
protected theorem multiset_sum_mem {R} [NonUnitalNonAssocRing R] (s : NonUnitalSubring R)
(m : Multiset R) : (∀ a ∈ m, a ∈ s) → m.sum ∈ s :=
multiset_sum_mem _
/-- Sum of elements in a `NonUnitalSubring` of a `NonUnitalRing` indexed by a `Finset`
is in the `NonUnitalSubring`. -/
protected theorem sum_mem {R : Type*} [NonUnitalNonAssocRing R] (s : NonUnitalSubring R)
{ι : Type*} {t : Finset ι} {f : ι → R} (h : ∀ c ∈ t, f c ∈ s) : (∑ i ∈ t, f i) ∈ s :=
sum_mem h
/-- A non-unital subring of a non-unital ring inherits a non-unital ring structure -/
instance toNonUnitalRing {R : Type*} [NonUnitalRing R] (s : NonUnitalSubring R) :
NonUnitalRing s :=
Subtype.coe_injective.nonUnitalRing _ rfl (fun _ _ => rfl) (fun _ _ => rfl) (fun _ => rfl)
(fun _ _ => rfl) (fun _ _ => rfl) fun _ _ => rfl
protected theorem zsmul_mem {x : R} (hx : x ∈ s) (n : ℤ) : n • x ∈ s :=
zsmul_mem hx n
@[simp, norm_cast]
theorem val_add (x y : s) : (↑(x + y) : R) = ↑x + ↑y :=
rfl
@[simp, norm_cast]
theorem val_neg (x : s) : (↑(-x) : R) = -↑x :=
rfl
@[simp, norm_cast]
theorem val_mul (x y : s) : (↑(x * y) : R) = ↑x * ↑y :=
rfl
@[simp, norm_cast]
theorem val_zero : ((0 : s) : R) = 0 :=
rfl
theorem coe_eq_zero_iff {x : s} : (x : R) = 0 ↔ x = 0 := by
simp
/-- A non-unital subring of a `NonUnitalCommRing` is a `NonUnitalCommRing`. -/
instance toNonUnitalCommRing {R} [NonUnitalCommRing R] (s : NonUnitalSubring R) :
NonUnitalCommRing s :=
Subtype.coe_injective.nonUnitalCommRing _ rfl (fun _ _ => rfl) (fun _ _ => rfl) (fun _ => rfl)
(fun _ _ => rfl) (fun _ _ => rfl) fun _ _ => rfl
/-! ## Partial order -/
@[simp]
theorem mem_toSubsemigroup {s : NonUnitalSubring R} {x : R} : x ∈ s.toSubsemigroup ↔ x ∈ s :=
Iff.rfl
@[simp]
theorem coe_toSubsemigroup (s : NonUnitalSubring R) : (s.toSubsemigroup : Set R) = s :=
rfl
@[simp]
theorem mem_toAddSubgroup {s : NonUnitalSubring R} {x : R} : x ∈ s.toAddSubgroup ↔ x ∈ s :=
Iff.rfl
@[simp]
theorem coe_toAddSubgroup (s : NonUnitalSubring R) : (s.toAddSubgroup : Set R) = s :=
rfl
@[simp]
theorem mem_toNonUnitalSubsemiring {s : NonUnitalSubring R} {x : R} :
x ∈ s.toNonUnitalSubsemiring ↔ x ∈ s :=
Iff.rfl
@[simp]
theorem coe_toNonUnitalSubsemiring (s : NonUnitalSubring R) :
(s.toNonUnitalSubsemiring : Set R) = s :=
rfl
/-! ## top -/
/-- The non-unital subring `R` of the ring `R`. -/
instance : Top (NonUnitalSubring R) :=
⟨{ (⊤ : Subsemigroup R), (⊤ : AddSubgroup R) with }⟩
@[simp]
theorem mem_top (x : R) : x ∈ (⊤ : NonUnitalSubring R) :=
Set.mem_univ x
@[simp]
theorem coe_top : ((⊤ : NonUnitalSubring R) : Set R) = Set.univ :=
rfl
/-- The ring equiv between the top element of `NonUnitalSubring R` and `R`. -/
@[simps!]
def topEquiv : (⊤ : NonUnitalSubring R) ≃+* R := NonUnitalSubsemiring.topEquiv
end NonUnitalSubring
end Basic
section Hom
namespace NonUnitalSubring
variable {F : Type w} {R : Type u} {S : Type v} {T : Type*} {SR : Type*}
[NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S] [NonUnitalNonAssocRing T]
[FunLike F R S] [NonUnitalRingHomClass F R S] (s : NonUnitalSubring R)
/-! ## comap -/
/-- The preimage of a `NonUnitalSubring` along a ring homomorphism is a `NonUnitalSubring`. -/
def comap {F : Type w} {R : Type u} {S : Type v} [NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S]
[FunLike F R S] [NonUnitalRingHomClass F R S] (f : F) (s : NonUnitalSubring S) :
NonUnitalSubring R :=
{ s.toSubsemigroup.comap (f : R →ₙ* S), s.toAddSubgroup.comap (f : R →+ S) with
carrier := f ⁻¹' s.carrier }
@[simp]
theorem coe_comap (s : NonUnitalSubring S) (f : F) : (s.comap f : Set R) = f ⁻¹' s :=
rfl
@[simp]
theorem mem_comap {s : NonUnitalSubring S} {f : F} {x : R} : x ∈ s.comap f ↔ f x ∈ s :=
Iff.rfl
theorem comap_comap (s : NonUnitalSubring T) (g : S →ₙ+* T) (f : R →ₙ+* S) :
(s.comap g).comap f = s.comap (g.comp f) :=
rfl
/-! ## map -/
/-- The image of a `NonUnitalSubring` along a ring homomorphism is a `NonUnitalSubring`. -/
def map {F : Type w} {R : Type u} {S : Type v} [NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S]
[FunLike F R S] [NonUnitalRingHomClass F R S] (f : F) (s : NonUnitalSubring R) :
NonUnitalSubring S :=
{ s.toSubsemigroup.map (f : R →ₙ* S), s.toAddSubgroup.map (f : R →+ S) with
carrier := f '' s.carrier }
@[simp]
theorem coe_map (f : F) (s : NonUnitalSubring R) : (s.map f : Set S) = f '' s :=
rfl
@[simp]
theorem mem_map {f : F} {s : NonUnitalSubring R} {y : S} : y ∈ s.map f ↔ ∃ x ∈ s, f x = y :=
Set.mem_image _ _ _
@[simp]
theorem map_id : s.map (NonUnitalRingHom.id R) = s :=
SetLike.coe_injective <| Set.image_id _
theorem map_map (g : S →ₙ+* T) (f : R →ₙ+* S) : (s.map f).map g = s.map (g.comp f) :=
SetLike.coe_injective <| Set.image_image _ _ _
theorem map_le_iff_le_comap {f : F} {s : NonUnitalSubring R} {t : NonUnitalSubring S} :
s.map f ≤ t ↔ s ≤ t.comap f :=
Set.image_subset_iff
theorem gc_map_comap (f : F) :
GaloisConnection (map f : NonUnitalSubring R → NonUnitalSubring S) (comap f) := fun _S _T =>
map_le_iff_le_comap
/-- A `NonUnitalSubring` is isomorphic to its image under an injective function -/
noncomputable def equivMapOfInjective (f : F) (hf : Function.Injective (f : R → S)) :
s ≃+* s.map f :=
{
Equiv.Set.image f s
hf with
map_mul' := fun _ _ => Subtype.ext (map_mul f _ _)
map_add' := fun _ _ => Subtype.ext (map_add f _ _) }
@[simp]
theorem coe_equivMapOfInjective_apply (f : F) (hf : Function.Injective f) (x : s) :
(equivMapOfInjective s f hf x : S) = f x :=
rfl
end NonUnitalSubring
namespace NonUnitalRingHom
variable {R : Type u} {S : Type v} {T : Type*}
[NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S] [NonUnitalNonAssocRing T]
(g : S →ₙ+* T) (f : R →ₙ+* S)
/-! ## range -/
/-- The range of a ring homomorphism, as a `NonUnitalSubring` of the target.
See Note [range copy pattern]. -/
def range {R : Type u} {S : Type v} [NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S]
(f : R →ₙ+* S) : NonUnitalSubring S :=
((⊤ : NonUnitalSubring R).map f).copy (Set.range f) Set.image_univ.symm
@[simp]
theorem coe_range : (f.range : Set S) = Set.range f :=
rfl
@[simp]
theorem mem_range {f : R →ₙ+* S} {y : S} : y ∈ f.range ↔ ∃ x, f x = y :=
Iff.rfl
theorem range_eq_map (f : R →ₙ+* S) : f.range = NonUnitalSubring.map f ⊤ := by ext; simp
theorem mem_range_self (f : R →ₙ+* S) (x : R) : f x ∈ f.range :=
mem_range.mpr ⟨x, rfl⟩
theorem map_range : f.range.map g = (g.comp f).range := by
simpa only [range_eq_map] using (⊤ : NonUnitalSubring R).map_map g f
/-- The range of a ring homomorphism is a fintype, if the domain is a fintype.
Note: this instance can form a diamond with `Subtype.fintype` in the
presence of `Fintype S`. -/
instance fintypeRange [Fintype R] [DecidableEq S] (f : R →ₙ+* S) : Fintype (range f) :=
Set.fintypeRange f
end NonUnitalRingHom
namespace NonUnitalSubring
section Order
variable {F : Type w} {R : Type u} {S : Type v} {T : Type*}
[NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S] [NonUnitalNonAssocRing T]
[FunLike F R S] [NonUnitalRingHomClass F R S]
(g : S →ₙ+* T) (f : R →ₙ+* S)
/-! ## bot -/
instance : Bot (NonUnitalSubring R) :=
⟨(0 : R →ₙ+* R).range⟩
instance : Inhabited (NonUnitalSubring R) :=
⟨⊥⟩
theorem coe_bot : ((⊥ : NonUnitalSubring R) : Set R) = {0} :=
(NonUnitalRingHom.coe_range (0 : R →ₙ+* R)).trans (@Set.range_const R R _ 0)
theorem mem_bot {x : R} : x ∈ (⊥ : NonUnitalSubring R) ↔ x = 0 :=
show x ∈ ((⊥ : NonUnitalSubring R) : Set R) ↔ x = 0 by rw [coe_bot, Set.mem_singleton_iff]
/-! ## inf -/
/-- The inf of two `NonUnitalSubring`s is their intersection. -/
instance : Inf (NonUnitalSubring R) :=
⟨fun s t =>
{ s.toSubsemigroup ⊓ t.toSubsemigroup, s.toAddSubgroup ⊓ t.toAddSubgroup with
carrier := s ∩ t }⟩
@[simp]
theorem coe_inf (p p' : NonUnitalSubring R) :
((p ⊓ p' : NonUnitalSubring R) : Set R) = (p : Set R) ∩ p' :=
rfl
@[simp]
theorem mem_inf {p p' : NonUnitalSubring R} {x : R} : x ∈ p ⊓ p' ↔ x ∈ p ∧ x ∈ p' :=
Iff.rfl
instance : InfSet (NonUnitalSubring R) :=
⟨fun s =>
NonUnitalSubring.mk' (⋂ t ∈ s, ↑t) (⨅ t ∈ s, NonUnitalSubring.toSubsemigroup t)
(⨅ t ∈ s, NonUnitalSubring.toAddSubgroup t) (by simp) (by simp)⟩
@[simp, norm_cast]
theorem coe_sInf (S : Set (NonUnitalSubring R)) :
((sInf S : NonUnitalSubring R) : Set R) = ⋂ s ∈ S, ↑s :=
rfl
theorem mem_sInf {S : Set (NonUnitalSubring R)} {x : R} : x ∈ sInf S ↔ ∀ p ∈ S, x ∈ p :=
Set.mem_iInter₂
@[simp, norm_cast]
theorem coe_iInf {ι : Sort*} {S : ι → NonUnitalSubring R} : (↑(⨅ i, S i) : Set R) = ⋂ i, S i := by
simp only [iInf, coe_sInf, Set.biInter_range]
theorem mem_iInf {ι : Sort*} {S : ι → NonUnitalSubring R} {x : R} :
(x ∈ ⨅ i, S i) ↔ ∀ i, x ∈ S i := by simp only [iInf, mem_sInf, Set.forall_mem_range]
@[simp]
theorem sInf_toSubsemigroup (s : Set (NonUnitalSubring R)) :
(sInf s).toSubsemigroup = ⨅ t ∈ s, NonUnitalSubring.toSubsemigroup t :=
mk'_toSubsemigroup _ _
@[simp]
theorem sInf_toAddSubgroup (s : Set (NonUnitalSubring R)) :
(sInf s).toAddSubgroup = ⨅ t ∈ s, NonUnitalSubring.toAddSubgroup t :=
mk'_toAddSubgroup _ _
/-- `NonUnitalSubring`s of a ring form a complete lattice. -/
instance : CompleteLattice (NonUnitalSubring R) :=
{ completeLatticeOfInf (NonUnitalSubring R) fun _s =>
IsGLB.of_image (@fun _ _ : NonUnitalSubring R => SetLike.coe_subset_coe)
isGLB_biInf with
bot := ⊥
bot_le := fun s _x hx => (mem_bot.mp hx).symm ▸ zero_mem s
top := ⊤
le_top := fun _ _ _ => trivial
inf := (· ⊓ ·)
inf_le_left := fun _ _ _ => And.left
inf_le_right := fun _ _ _ => And.right
le_inf := fun _s _t₁ _t₂ h₁ h₂ _x hx => ⟨h₁ hx, h₂ hx⟩ }
theorem eq_top_iff' (A : NonUnitalSubring R) : A = ⊤ ↔ ∀ x : R, x ∈ A :=
eq_top_iff.trans ⟨fun h m => h <| mem_top m, fun h m _ => h m⟩
end Order
/-! ## Center of a ring -/
section Center
variable {R : Type u}
section NonUnitalNonAssocRing
variable (R) [NonUnitalNonAssocRing R]
/-- The center of a ring `R` is the set of elements that commute with everything in `R` -/
def center : NonUnitalSubring R :=
{ NonUnitalSubsemiring.center R with
neg_mem' := Set.neg_mem_center }
theorem coe_center : ↑(center R) = Set.center R :=
rfl
@[simp]
theorem center_toNonUnitalSubsemiring :
(center R).toNonUnitalSubsemiring = NonUnitalSubsemiring.center R :=
rfl
/-- The center is commutative and associative. -/
instance center.instNonUnitalCommRing : NonUnitalCommRing (center R) :=
{ NonUnitalSubsemiring.center.instNonUnitalCommSemiring R,
inferInstanceAs <| NonUnitalNonAssocRing (center R) with }
end NonUnitalNonAssocRing
section NonUnitalRing
variable [NonUnitalRing R]
-- no instance diamond, unlike the unital version
example : (center.instNonUnitalCommRing _).toNonUnitalRing =
NonUnitalSubringClass.toNonUnitalRing (center R) := by
with_reducible_and_instances rfl
theorem mem_center_iff {z : R} : z ∈ center R ↔ ∀ g, g * z = z * g := Subsemigroup.mem_center_iff
instance decidableMemCenter [DecidableEq R] [Fintype R] : DecidablePred (· ∈ center R) := fun _ =>
decidable_of_iff' _ mem_center_iff
@[simp]
theorem center_eq_top (R) [NonUnitalCommRing R] : center R = ⊤ :=
SetLike.coe_injective (Set.center_eq_univ R)
end NonUnitalRing
end Center
/-! ## `NonUnitalSubring` closure of a subset -/
variable {F : Type w} {R : Type u} {S : Type v} {T : Type*}
[NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S] [NonUnitalNonAssocRing T]
[FunLike F R S] [NonUnitalRingHomClass F R S]
(g : S →ₙ+* T) (f : R →ₙ+* S)
/-- The `NonUnitalSubring` generated by a set. -/
def closure (s : Set R) : NonUnitalSubring R :=
sInf {S | s ⊆ S}
theorem mem_closure {x : R} {s : Set R} : x ∈ closure s ↔ ∀ S : NonUnitalSubring R, s ⊆ S → x ∈ S :=
mem_sInf
/-- The `NonUnitalSubring` generated by a set includes the set. -/
@[simp, aesop safe 20 apply (rule_sets := [SetLike])]
theorem subset_closure {s : Set R} : s ⊆ closure s := fun _x hx => mem_closure.2 fun _S hS => hS hx
theorem not_mem_of_not_mem_closure {s : Set R} {P : R} (hP : P ∉ closure s) : P ∉ s := fun h =>
hP (subset_closure h)
/-- A `NonUnitalSubring` `t` includes `closure s` if and only if it includes `s`. -/
@[simp]
theorem closure_le {s : Set R} {t : NonUnitalSubring R} : closure s ≤ t ↔ s ⊆ t :=
⟨Set.Subset.trans subset_closure, fun h => sInf_le h⟩
/-- `NonUnitalSubring` closure of a set is monotone in its argument: if `s ⊆ t`,
then `closure s ≤ closure t`. -/
theorem closure_mono ⦃s t : Set R⦄ (h : s ⊆ t) : closure s ≤ closure t :=
closure_le.2 <| Set.Subset.trans h subset_closure
theorem closure_eq_of_le {s : Set R} {t : NonUnitalSubring R} (h₁ : s ⊆ t) (h₂ : t ≤ closure s) :
closure s = t :=
le_antisymm (closure_le.2 h₁) h₂
/-- An induction principle for closure membership. If `p` holds for `0`, `1`, and all elements
of `s`, and is preserved under addition, negation, and multiplication, then `p` holds for all
elements of the closure of `s`. -/
@[elab_as_elim]
theorem closure_induction {s : Set R} {p : R → Prop} {x} (h : x ∈ closure s) (mem : ∀ x ∈ s, p x)
(zero : p 0) (add : ∀ x y, p x → p y → p (x + y)) (neg : ∀ x : R, p x → p (-x))
(mul : ∀ x y, p x → p y → p (x * y)) : p x :=
(@closure_le _ _ _ ⟨⟨⟨⟨p, add _ _⟩, zero⟩, mul _ _⟩, neg _⟩).2 mem h
/-- The difference with `NonUnitalSubring.closure_induction` is that this acts on the
subtype. -/
@[elab_as_elim]
theorem closure_induction' {s : Set R} {p : closure s → Prop} (a : closure s)
(mem : ∀ (x) (hx : x ∈ s), p ⟨x, subset_closure hx⟩) (zero : p 0)
(add : ∀ x y, p x → p y → p (x + y)) (neg : ∀ x, p x → p (-x))
(mul : ∀ x y, p x → p y → p (x * y)) : p a :=
Subtype.recOn a fun b hb => by
refine Exists.elim ?_ fun (hb : b ∈ closure s) (hc : p ⟨b, hb⟩) => hc
refine
closure_induction hb (fun x hx => ⟨subset_closure hx, mem x hx⟩)
⟨zero_mem (closure s), zero⟩ ?_ ?_ ?_
· rintro x y ⟨hx, hpx⟩ ⟨hy, hpy⟩
exact ⟨add_mem hx hy, add _ _ hpx hpy⟩
· rintro x ⟨hx, hpx⟩
exact ⟨neg_mem hx, neg _ hpx⟩
· rintro x y ⟨hx, hpx⟩ ⟨hy, hpy⟩
exact ⟨mul_mem hx hy, mul _ _ hpx hpy⟩
/-- An induction principle for closure membership, for predicates with two arguments. -/
@[elab_as_elim]
theorem closure_induction₂ {s : Set R} {p : R → R → Prop} {a b : R} (ha : a ∈ closure s)
(hb : b ∈ closure s) (Hs : ∀ x ∈ s, ∀ y ∈ s, p x y) (H0_left : ∀ x, p 0 x)
(H0_right : ∀ x, p x 0) (Hneg_left : ∀ x y, p x y → p (-x) y)
(Hneg_right : ∀ x y, p x y → p x (-y)) (Hadd_left : ∀ x₁ x₂ y, p x₁ y → p x₂ y → p (x₁ + x₂) y)
(Hadd_right : ∀ x y₁ y₂, p x y₁ → p x y₂ → p x (y₁ + y₂))
(Hmul_left : ∀ x₁ x₂ y, p x₁ y → p x₂ y → p (x₁ * x₂) y)
(Hmul_right : ∀ x y₁ y₂, p x y₁ → p x y₂ → p x (y₁ * y₂)) : p a b := by
refine closure_induction hb ?_ (H0_right _) (Hadd_right a) (Hneg_right a) (Hmul_right a)
refine closure_induction ha Hs (fun x _ => H0_left x) ?_ ?_ ?_
· exact fun x y H₁ H₂ z zs => Hadd_left x y z (H₁ z zs) (H₂ z zs)
· exact fun x hx z zs => Hneg_left x z (hx z zs)
· exact fun x y H₁ H₂ z zs => Hmul_left x y z (H₁ z zs) (H₂ z zs)
theorem mem_closure_iff {s : Set R} {x} :
x ∈ closure s ↔ x ∈ AddSubgroup.closure (Subsemigroup.closure s : Set R) :=
⟨fun h =>
closure_induction h (fun x hx => AddSubgroup.subset_closure <| Subsemigroup.subset_closure hx)
(AddSubgroup.zero_mem _) (fun x y hx hy => AddSubgroup.add_mem _ hx hy)
(fun x hx => AddSubgroup.neg_mem _ hx) fun x y hx hy =>
AddSubgroup.closure_induction hy
(fun q hq =>
AddSubgroup.closure_induction hx
(fun p hp => AddSubgroup.subset_closure ((Subsemigroup.closure s).mul_mem hp hq))
(by rw [zero_mul q]; apply AddSubgroup.zero_mem _)
(fun p₁ p₂ ihp₁ ihp₂ => by rw [add_mul p₁ p₂ q]; apply AddSubgroup.add_mem _ ihp₁ ihp₂)
fun x hx => by
have f : -x * q = -(x * q) := by simp
rw [f]; apply AddSubgroup.neg_mem _ hx)
(by rw [mul_zero x]; apply AddSubgroup.zero_mem _)
(fun q₁ q₂ ihq₁ ihq₂ => by rw [mul_add x q₁ q₂]; apply AddSubgroup.add_mem _ ihq₁ ihq₂)
fun z hz => by
have f : x * -z = -(x * z) := by simp
rw [f]; apply AddSubgroup.neg_mem _ hz,
fun h =>
AddSubgroup.closure_induction h
(fun x hx =>
Subsemigroup.closure_induction hx (fun x hx => subset_closure hx) fun x y hx hy =>
mul_mem hx hy)
(zero_mem _) (fun x y hx hy => add_mem hx hy) fun x hx => neg_mem hx⟩
/-- If all elements of `s : Set A` commute pairwise, then `closure s` is a commutative ring. -/
def closureNonUnitalCommRingOfComm {R : Type u} [NonUnitalRing R] {s : Set R}
(hcomm : ∀ a ∈ s, ∀ b ∈ s, a * b = b * a) : NonUnitalCommRing (closure s) :=
{ (closure s).toNonUnitalRing with
mul_comm := fun x y => by
ext
simp only [NonUnitalSubring.val_mul]
refine
closure_induction₂ x.prop y.prop hcomm
(fun x => by simp only [mul_zero, zero_mul])
(fun x => by simp only [mul_zero, zero_mul])
(fun x y hxy => by simp only [mul_neg, neg_mul, hxy])
(fun x y hxy => by simp only [mul_neg, neg_mul, hxy])
(fun x₁ x₂ y h₁ h₂ => by simp only [add_mul, mul_add, h₁, h₂])
(fun x₁ x₂ y h₁ h₂ => by simp only [add_mul, mul_add, h₁, h₂])
(fun x₁ x₂ y h₁ h₂ => by rw [← mul_assoc, ← h₁, mul_assoc x₁ y x₂, ← h₂, mul_assoc])
fun x₁ x₂ y h₁ h₂ => by rw [← mul_assoc, h₁, mul_assoc, h₂, ← mul_assoc] }
variable (R)
/-- `closure` forms a Galois insertion with the coercion to set. -/
protected def gi : GaloisInsertion (@closure R _) SetLike.coe where
choice s _ := closure s
gc _s _t := closure_le
le_l_u _s := subset_closure
choice_eq _s _h := rfl
variable {R}
/-- Closure of a `NonUnitalSubring` `S` equals `S`. -/
theorem closure_eq (s : NonUnitalSubring R) : closure (s : Set R) = s :=
(NonUnitalSubring.gi R).l_u_eq s
@[simp]
theorem closure_empty : closure (∅ : Set R) = ⊥ :=
(NonUnitalSubring.gi R).gc.l_bot
@[simp]
theorem closure_univ : closure (Set.univ : Set R) = ⊤ :=
@coe_top R _ ▸ closure_eq ⊤
theorem closure_union (s t : Set R) : closure (s ∪ t) = closure s ⊔ closure t :=
(NonUnitalSubring.gi R).gc.l_sup
theorem closure_iUnion {ι} (s : ι → Set R) : closure (⋃ i, s i) = ⨆ i, closure (s i) :=
(NonUnitalSubring.gi R).gc.l_iSup
theorem closure_sUnion (s : Set (Set R)) : closure (⋃₀ s) = ⨆ t ∈ s, closure t :=
(NonUnitalSubring.gi R).gc.l_sSup
theorem map_sup (s t : NonUnitalSubring R) (f : F) : (s ⊔ t).map f = s.map f ⊔ t.map f :=
(gc_map_comap f).l_sup
theorem map_iSup {ι : Sort*} (f : F) (s : ι → NonUnitalSubring R) :
(iSup s).map f = ⨆ i, (s i).map f :=
(gc_map_comap f).l_iSup
theorem comap_inf (s t : NonUnitalSubring S) (f : F) : (s ⊓ t).comap f = s.comap f ⊓ t.comap f :=
(gc_map_comap f).u_inf
theorem comap_iInf {ι : Sort*} (f : F) (s : ι → NonUnitalSubring S) :
(iInf s).comap f = ⨅ i, (s i).comap f :=
(gc_map_comap f).u_iInf
@[simp]
theorem map_bot (f : R →ₙ+* S) : (⊥ : NonUnitalSubring R).map f = ⊥ :=
(gc_map_comap f).l_bot
@[simp]
theorem comap_top (f : R →ₙ+* S) : (⊤ : NonUnitalSubring S).comap f = ⊤ :=
(gc_map_comap f).u_top
/-- Given `NonUnitalSubring`s `s`, `t` of rings `R`, `S` respectively, `s.prod t` is `s ×ˢ t`
as a `NonUnitalSubring` of `R × S`. -/
def prod (s : NonUnitalSubring R) (t : NonUnitalSubring S) : NonUnitalSubring (R × S) :=
{ s.toSubsemigroup.prod t.toSubsemigroup, s.toAddSubgroup.prod t.toAddSubgroup with
carrier := s ×ˢ t }
@[norm_cast]
theorem coe_prod (s : NonUnitalSubring R) (t : NonUnitalSubring S) :
(s.prod t : Set (R × S)) = (s : Set R) ×ˢ t :=
rfl
theorem mem_prod {s : NonUnitalSubring R} {t : NonUnitalSubring S} {p : R × S} :
p ∈ s.prod t ↔ p.1 ∈ s ∧ p.2 ∈ t :=
Iff.rfl
@[mono]
theorem prod_mono ⦃s₁ s₂ : NonUnitalSubring R⦄ (hs : s₁ ≤ s₂) ⦃t₁ t₂ : NonUnitalSubring S⦄
(ht : t₁ ≤ t₂) : s₁.prod t₁ ≤ s₂.prod t₂ :=
Set.prod_mono hs ht
theorem prod_mono_right (s : NonUnitalSubring R) :
Monotone fun t : NonUnitalSubring S => s.prod t :=
prod_mono (le_refl s)
theorem prod_mono_left (t : NonUnitalSubring S) : Monotone fun s : NonUnitalSubring R => s.prod t :=
fun _s₁ _s₂ hs => prod_mono hs (le_refl t)
theorem prod_top (s : NonUnitalSubring R) :
s.prod (⊤ : NonUnitalSubring S) = s.comap (NonUnitalRingHom.fst R S) :=
ext fun x => by simp [mem_prod, MonoidHom.coe_fst]
theorem top_prod (s : NonUnitalSubring S) :
(⊤ : NonUnitalSubring R).prod s = s.comap (NonUnitalRingHom.snd R S) :=
ext fun x => by simp [mem_prod, MonoidHom.coe_snd]
@[simp]
theorem top_prod_top : (⊤ : NonUnitalSubring R).prod (⊤ : NonUnitalSubring S) = ⊤ :=
(top_prod _).trans <| comap_top _
/-- Product of `NonUnitalSubring`s is isomorphic to their product as rings. -/
def prodEquiv (s : NonUnitalSubring R) (t : NonUnitalSubring S) : s.prod t ≃+* s × t :=
{ Equiv.Set.prod (s : Set R) (t : Set S) with
map_mul' := fun _ _ => rfl
map_add' := fun _ _ => rfl }
/-- The underlying set of a non-empty directed Sup of `NonUnitalSubring`s is just a union of the
`NonUnitalSubring`s. Note that this fails without the directedness assumption (the union of two
`NonUnitalSubring`s is typically not a `NonUnitalSubring`) -/
theorem mem_iSup_of_directed {ι} [hι : Nonempty ι] {S : ι → NonUnitalSubring R}
(hS : Directed (· ≤ ·) S) {x : R} : (x ∈ ⨆ i, S i) ↔ ∃ i, x ∈ S i := by
refine ⟨?_, fun ⟨i, hi⟩ ↦ le_iSup S i hi⟩
let U : NonUnitalSubring R :=
NonUnitalSubring.mk' (⋃ i, (S i : Set R)) (⨆ i, (S i).toSubsemigroup) (⨆ i, (S i).toAddSubgroup)
(Subsemigroup.coe_iSup_of_directed hS) (AddSubgroup.coe_iSup_of_directed hS)
suffices ⨆ i, S i ≤ U by simpa [U] using @this x
exact iSup_le fun i x hx ↦ Set.mem_iUnion.2 ⟨i, hx⟩
theorem coe_iSup_of_directed {ι} [Nonempty ι] {S : ι → NonUnitalSubring R}
(hS : Directed (· ≤ ·) S) : ((⨆ i, S i : NonUnitalSubring R) : Set R) = ⋃ i, S i :=
Set.ext fun x ↦ by simp [mem_iSup_of_directed hS]
theorem mem_sSup_of_directedOn {S : Set (NonUnitalSubring R)} (Sne : S.Nonempty)
(hS : DirectedOn (· ≤ ·) S) {x : R} : x ∈ sSup S ↔ ∃ s ∈ S, x ∈ s := by
haveI : Nonempty S := Sne.to_subtype
simp only [sSup_eq_iSup', mem_iSup_of_directed hS.directed_val, SetCoe.exists, Subtype.coe_mk,
exists_prop]
theorem coe_sSup_of_directedOn {S : Set (NonUnitalSubring R)} (Sne : S.Nonempty)
(hS : DirectedOn (· ≤ ·) S) : (↑(sSup S) : Set R) = ⋃ s ∈ S, ↑s :=
Set.ext fun x => by simp [mem_sSup_of_directedOn Sne hS]
theorem mem_map_equiv {f : R ≃+* S} {K : NonUnitalSubring R} {x : S} :
x ∈ K.map (f : R →ₙ+* S) ↔ f.symm x ∈ K :=
@Set.mem_image_equiv _ _ (K : Set R) f.toEquiv x
theorem map_equiv_eq_comap_symm (f : R ≃+* S) (K : NonUnitalSubring R) :
K.map (f : R →ₙ+* S) = K.comap f.symm :=
SetLike.coe_injective (f.toEquiv.image_eq_preimage K)
theorem comap_equiv_eq_map_symm (f : R ≃+* S) (K : NonUnitalSubring S) :
K.comap (f : R →ₙ+* S) = K.map f.symm :=
(map_equiv_eq_comap_symm f.symm K).symm
end NonUnitalSubring
namespace NonUnitalRingHom
variable {F : Type w} {R : Type u} {S : Type v} {T : Type*}
[NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S] [NonUnitalNonAssocRing T]
[FunLike F R S] [NonUnitalRingHomClass F R S]
(g : S →ₙ+* T) (f : R →ₙ+* S)
{s : NonUnitalSubring R}
open NonUnitalSubring
/-- Restriction of a ring homomorphism to its range interpreted as a `NonUnitalSubring`.
This is the bundled version of `Set.rangeFactorization`. -/
def rangeRestrict (f : R →ₙ+* S) : R →ₙ+* f.range :=
NonUnitalRingHom.codRestrict f f.range fun x => ⟨x, rfl⟩
@[simp]
theorem coe_rangeRestrict (f : R →ₙ+* S) (x : R) : (f.rangeRestrict x : S) = f x :=
rfl
theorem rangeRestrict_surjective (f : R →ₙ+* S) : Function.Surjective f.rangeRestrict :=
fun ⟨_y, hy⟩ =>
let ⟨x, hx⟩ := mem_range.mp hy
⟨x, Subtype.ext hx⟩
theorem range_top_iff_surjective {f : R →ₙ+* S} :
f.range = (⊤ : NonUnitalSubring S) ↔ Function.Surjective f :=
SetLike.ext'_iff.trans <| Iff.trans (by rw [coe_range, coe_top]) Set.range_iff_surjective
/-- The range of a surjective ring homomorphism is the whole of the codomain. -/
@[simp]
theorem range_top_of_surjective (f : R →ₙ+* S) (hf : Function.Surjective f) :
f.range = (⊤ : NonUnitalSubring S) :=
range_top_iff_surjective.2 hf
/-- The `NonUnitalSubring` of elements `x : R` such that `f x = g x`, i.e.,
the equalizer of f and g as a `NonUnitalSubring` of R -/
def eqLocus (f g : R →ₙ+* S) : NonUnitalSubring R :=
{ (f : R →ₙ* S).eqLocus g, (f : R →+ S).eqLocus g with carrier := {x | f x = g x} }
@[simp]
theorem eqLocus_same (f : R →ₙ+* S) : f.eqLocus f = ⊤ :=
SetLike.ext fun _ => eq_self_iff_true _
/-- If two ring homomorphisms are equal on a set, then they are equal on its
`NonUnitalSubring` closure. -/
theorem eqOn_set_closure {f g : R →ₙ+* S} {s : Set R} (h : Set.EqOn f g s) :
Set.EqOn f g (closure s) :=
show closure s ≤ f.eqLocus g from closure_le.2 h
theorem eq_of_eqOn_set_top {f g : R →ₙ+* S} (h : Set.EqOn f g (⊤ : NonUnitalSubring R)) : f = g :=
ext fun _x => h trivial
theorem eq_of_eqOn_set_dense {s : Set R} (hs : closure s = ⊤) {f g : R →ₙ+* S} (h : s.EqOn f g) :
f = g :=
eq_of_eqOn_set_top <| hs ▸ eqOn_set_closure h
theorem closure_preimage_le (f : R →ₙ+* S) (s : Set S) : closure (f ⁻¹' s) ≤ (closure s).comap f :=
closure_le.2 fun _x hx => SetLike.mem_coe.2 <| mem_comap.2 <| subset_closure hx
/-- The image under a ring homomorphism of the `NonUnitalSubring` generated by a set equals
the `NonUnitalSubring` generated by the image of the set. -/
theorem map_closure (f : R →ₙ+* S) (s : Set R) : (closure s).map f = closure (f '' s) :=
le_antisymm
(map_le_iff_le_comap.2 <|
le_trans (closure_mono <| Set.subset_preimage_image _ _) (closure_preimage_le _ _))
(closure_le.2 <| Set.image_subset _ subset_closure)
end NonUnitalRingHom
namespace NonUnitalSubring
variable {F : Type w} {R : Type u} {S : Type v} {T : Type*}
[NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S] [NonUnitalNonAssocRing T]
[FunLike F R S] [NonUnitalRingHomClass F R S]
(g : S →ₙ+* T) (f : R →ₙ+* S)
{s : NonUnitalSubring R}
open NonUnitalRingHom
/-- The ring homomorphism associated to an inclusion of `NonUnitalSubring`s. -/
def inclusion {S T : NonUnitalSubring R} (h : S ≤ T) : S →ₙ+* T :=
NonUnitalRingHom.codRestrict (NonUnitalSubringClass.subtype S) _ fun x => h x.2
@[simp]
theorem range_subtype (s : NonUnitalSubring R) : (NonUnitalSubringClass.subtype s).range = s :=
SetLike.coe_injective <| (coe_srange _).trans Subtype.range_coe
theorem range_fst : NonUnitalRingHom.srange (fst R S) = ⊤ :=
NonUnitalSubsemiring.range_fst
theorem range_snd : NonUnitalRingHom.srange (snd R S) = ⊤ :=
NonUnitalSubsemiring.range_snd
end NonUnitalSubring
namespace RingEquiv
variable {F : Type w} {R : Type u} {S : Type v} {T : Type*}
[NonUnitalRing R] [NonUnitalRing S] [NonUnitalRing T]
[FunLike F R S] [NonUnitalRingHomClass F R S]
(g : S →ₙ+* T) (f : R →ₙ+* S)
{s t : NonUnitalSubring R}
/-- Makes the identity isomorphism from a proof two `NonUnitalSubring`s of a multiplicative
monoid are equal. -/
def nonUnitalSubringCongr (h : s = t) : s ≃+* t :=
{
Equiv.setCongr <| congr_arg _ h with
map_mul' := fun _ _ => rfl
map_add' := fun _ _ => rfl }
/-- Restrict a ring homomorphism with a left inverse to a ring isomorphism to its
`RingHom.range`. -/
def ofLeftInverse' {g : S → R} {f : R →ₙ+* S} (h : Function.LeftInverse g f) : R ≃+* f.range :=
{ f.rangeRestrict with
toFun := fun x => f.rangeRestrict x
invFun := fun x => (g ∘ NonUnitalSubringClass.subtype f.range) x
left_inv := h
right_inv := fun x =>
Subtype.ext <|
let ⟨x', hx'⟩ := NonUnitalRingHom.mem_range.mp x.prop
show f (g x) = x by rw [← hx', h x'] }
@[simp]
theorem ofLeftInverse'_apply {g : S → R} {f : R →ₙ+* S} (h : Function.LeftInverse g f) (x : R) :
↑(ofLeftInverse' h x) = f x :=
rfl
@[simp]
theorem ofLeftInverse'_symm_apply {g : S → R} {f : R →ₙ+* S} (h : Function.LeftInverse g f)
(x : f.range) : (ofLeftInverse' h).symm x = g x :=
rfl
end RingEquiv
namespace NonUnitalSubring
variable {F : Type w} {R : Type u} {S : Type v}
[NonUnitalNonAssocRing R] [NonUnitalNonAssocRing S]
[FunLike F R S] [NonUnitalRingHomClass F R S]
theorem closure_preimage_le (f : F) (s : Set S) :
closure ((f : R → S) ⁻¹' s) ≤ (closure s).comap f :=
closure_le.2 fun _x hx => SetLike.mem_coe.2 <| mem_comap.2 <| subset_closure hx
end NonUnitalSubring
end Hom
|
RingTheory\NonUnitalSubsemiring\Basic.lean | /-
Copyright (c) 2022 Jireh Loreaux. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jireh Loreaux
-/
import Mathlib.Algebra.Group.Submonoid.Membership
import Mathlib.Algebra.Group.Subsemigroup.Membership
import Mathlib.Algebra.GroupWithZero.Center
import Mathlib.Algebra.Ring.Center
import Mathlib.Algebra.Ring.Centralizer
import Mathlib.Algebra.Ring.Equiv
import Mathlib.Algebra.Ring.Prod
import Mathlib.Algebra.Group.Hom.End
import Mathlib.Data.Set.Finite
import Mathlib.GroupTheory.Subsemigroup.Centralizer
/-!
# Bundled non-unital subsemirings
We define bundled non-unital subsemirings and some standard constructions:
`CompleteLattice` structure, `subtype` and `inclusion` ring homomorphisms, non-unital subsemiring
`map`, `comap` and range (`srange`) of a `NonUnitalRingHom` etc.
-/
universe u v w
variable {R : Type u} {S : Type v} {T : Type w} [NonUnitalNonAssocSemiring R] (M : Subsemigroup R)
/-- `NonUnitalSubsemiringClass S R` states that `S` is a type of subsets `s ⊆ R` that
are both an additive submonoid and also a multiplicative subsemigroup. -/
class NonUnitalSubsemiringClass (S : Type*) (R : Type u) [NonUnitalNonAssocSemiring R]
[SetLike S R] extends AddSubmonoidClass S R : Prop where
mul_mem : ∀ {s : S} {a b : R}, a ∈ s → b ∈ s → a * b ∈ s
-- See note [lower instance priority]
instance (priority := 100) NonUnitalSubsemiringClass.mulMemClass (S : Type*) (R : Type u)
[NonUnitalNonAssocSemiring R] [SetLike S R] [h : NonUnitalSubsemiringClass S R] :
MulMemClass S R :=
{ h with }
namespace NonUnitalSubsemiringClass
variable [SetLike S R] [NonUnitalSubsemiringClass S R] (s : S)
open AddSubmonoidClass
/- Prefer subclasses of `NonUnitalNonAssocSemiring` over subclasses of
`NonUnitalSubsemiringClass`. -/
/-- A non-unital subsemiring of a `NonUnitalNonAssocSemiring` inherits a
`NonUnitalNonAssocSemiring` structure -/
instance (priority := 75) toNonUnitalNonAssocSemiring : NonUnitalNonAssocSemiring s :=
Subtype.coe_injective.nonUnitalNonAssocSemiring (↑) rfl (by simp) (fun _ _ => rfl) fun _ _ => rfl
instance noZeroDivisors [NoZeroDivisors R] : NoZeroDivisors s :=
Subtype.coe_injective.noZeroDivisors (↑) rfl fun _ _ => rfl
/-- The natural non-unital ring hom from a non-unital subsemiring of a non-unital semiring `R` to
`R`. -/
def subtype : s →ₙ+* R :=
{ AddSubmonoidClass.subtype s, MulMemClass.subtype s with toFun := (↑) }
@[simp]
theorem coeSubtype : (subtype s : s → R) = ((↑) : s → R) :=
rfl
/-- A non-unital subsemiring of a `NonUnitalSemiring` is a `NonUnitalSemiring`. -/
instance toNonUnitalSemiring {R} [NonUnitalSemiring R] [SetLike S R]
[NonUnitalSubsemiringClass S R] : NonUnitalSemiring s :=
Subtype.coe_injective.nonUnitalSemiring (↑) rfl (by simp) (fun _ _ => rfl) fun _ _ => rfl
/-- A non-unital subsemiring of a `NonUnitalCommSemiring` is a `NonUnitalCommSemiring`. -/
instance toNonUnitalCommSemiring {R} [NonUnitalCommSemiring R] [SetLike S R]
[NonUnitalSubsemiringClass S R] : NonUnitalCommSemiring s :=
Subtype.coe_injective.nonUnitalCommSemiring (↑) rfl (by simp) (fun _ _ => rfl) fun _ _ => rfl
/-! Note: currently, there are no ordered versions of non-unital rings. -/
end NonUnitalSubsemiringClass
/-- A non-unital subsemiring of a non-unital semiring `R` is a subset `s` that is both an additive
submonoid and a semigroup. -/
structure NonUnitalSubsemiring (R : Type u) [NonUnitalNonAssocSemiring R] extends AddSubmonoid R,
Subsemigroup R
/-- Reinterpret a `NonUnitalSubsemiring` as a `Subsemigroup`. -/
add_decl_doc NonUnitalSubsemiring.toSubsemigroup
/-- Reinterpret a `NonUnitalSubsemiring` as an `AddSubmonoid`. -/
add_decl_doc NonUnitalSubsemiring.toAddSubmonoid
namespace NonUnitalSubsemiring
instance : SetLike (NonUnitalSubsemiring R) R where
coe s := s.carrier
coe_injective' p q h := by cases p; cases q; congr; exact SetLike.coe_injective' h
instance : NonUnitalSubsemiringClass (NonUnitalSubsemiring R) R where
zero_mem {s} := AddSubmonoid.zero_mem' s.toAddSubmonoid
add_mem {s} := AddSubsemigroup.add_mem' s.toAddSubmonoid.toAddSubsemigroup
mul_mem {s} := mul_mem' s
theorem mem_carrier {s : NonUnitalSubsemiring R} {x : R} : x ∈ s.carrier ↔ x ∈ s :=
Iff.rfl
/-- Two non-unital subsemirings are equal if they have the same elements. -/
@[ext]
theorem ext {S T : NonUnitalSubsemiring R} (h : ∀ x, x ∈ S ↔ x ∈ T) : S = T :=
SetLike.ext h
/-- Copy of a non-unital subsemiring with a new `carrier` equal to the old one. Useful to fix
definitional equalities. -/
protected def copy (S : NonUnitalSubsemiring R) (s : Set R) (hs : s = ↑S) :
NonUnitalSubsemiring R :=
{ S.toAddSubmonoid.copy s hs, S.toSubsemigroup.copy s hs with carrier := s }
@[simp]
theorem coe_copy (S : NonUnitalSubsemiring R) (s : Set R) (hs : s = ↑S) :
(S.copy s hs : Set R) = s :=
rfl
theorem copy_eq (S : NonUnitalSubsemiring R) (s : Set R) (hs : s = ↑S) : S.copy s hs = S :=
SetLike.coe_injective hs
theorem toSubsemigroup_injective :
Function.Injective (toSubsemigroup : NonUnitalSubsemiring R → Subsemigroup R)
| _, _, h => ext (SetLike.ext_iff.mp h : _)
@[mono]
theorem toSubsemigroup_strictMono :
StrictMono (toSubsemigroup : NonUnitalSubsemiring R → Subsemigroup R) := fun _ _ => id
@[mono]
theorem toSubsemigroup_mono : Monotone (toSubsemigroup : NonUnitalSubsemiring R → Subsemigroup R) :=
toSubsemigroup_strictMono.monotone
theorem toAddSubmonoid_injective :
Function.Injective (toAddSubmonoid : NonUnitalSubsemiring R → AddSubmonoid R)
| _, _, h => ext (SetLike.ext_iff.mp h : _)
@[mono]
theorem toAddSubmonoid_strictMono :
StrictMono (toAddSubmonoid : NonUnitalSubsemiring R → AddSubmonoid R) := fun _ _ => id
@[mono]
theorem toAddSubmonoid_mono : Monotone (toAddSubmonoid : NonUnitalSubsemiring R → AddSubmonoid R) :=
toAddSubmonoid_strictMono.monotone
/-- Construct a `NonUnitalSubsemiring R` from a set `s`, a subsemigroup `sg`, and an additive
submonoid `sa` such that `x ∈ s ↔ x ∈ sg ↔ x ∈ sa`. -/
protected def mk' (s : Set R) (sg : Subsemigroup R) (hg : ↑sg = s) (sa : AddSubmonoid R)
(ha : ↑sa = s) : NonUnitalSubsemiring R where
carrier := s
zero_mem' := by subst ha; exact sa.zero_mem
add_mem' := by subst ha; exact sa.add_mem
mul_mem' := by subst hg; exact sg.mul_mem
@[simp]
theorem coe_mk' {s : Set R} {sg : Subsemigroup R} (hg : ↑sg = s) {sa : AddSubmonoid R}
(ha : ↑sa = s) : (NonUnitalSubsemiring.mk' s sg hg sa ha : Set R) = s :=
rfl
@[simp]
theorem mem_mk' {s : Set R} {sg : Subsemigroup R} (hg : ↑sg = s) {sa : AddSubmonoid R}
(ha : ↑sa = s) {x : R} : x ∈ NonUnitalSubsemiring.mk' s sg hg sa ha ↔ x ∈ s :=
Iff.rfl
@[simp]
theorem mk'_toSubsemigroup {s : Set R} {sg : Subsemigroup R} (hg : ↑sg = s) {sa : AddSubmonoid R}
(ha : ↑sa = s) : (NonUnitalSubsemiring.mk' s sg hg sa ha).toSubsemigroup = sg :=
SetLike.coe_injective hg.symm
@[simp]
theorem mk'_toAddSubmonoid {s : Set R} {sg : Subsemigroup R} (hg : ↑sg = s) {sa : AddSubmonoid R}
(ha : ↑sa = s) : (NonUnitalSubsemiring.mk' s sg hg sa ha).toAddSubmonoid = sa :=
SetLike.coe_injective ha.symm
end NonUnitalSubsemiring
namespace NonUnitalSubsemiring
variable [NonUnitalNonAssocSemiring S] [NonUnitalNonAssocSemiring T]
variable {F G : Type*} [FunLike F R S] [NonUnitalRingHomClass F R S]
[FunLike G S T] [NonUnitalRingHomClass G S T]
(s : NonUnitalSubsemiring R)
@[simp, norm_cast]
theorem coe_zero : ((0 : s) : R) = (0 : R) :=
rfl
@[simp, norm_cast]
theorem coe_add (x y : s) : ((x + y : s) : R) = (x + y : R) :=
rfl
@[simp, norm_cast]
theorem coe_mul (x y : s) : ((x * y : s) : R) = (x * y : R) :=
rfl
/-! Note: currently, there are no ordered versions of non-unital rings. -/
@[simp high]
theorem mem_toSubsemigroup {s : NonUnitalSubsemiring R} {x : R} : x ∈ s.toSubsemigroup ↔ x ∈ s :=
Iff.rfl
@[simp high]
theorem coe_toSubsemigroup (s : NonUnitalSubsemiring R) : (s.toSubsemigroup : Set R) = s :=
rfl
@[simp]
theorem mem_toAddSubmonoid {s : NonUnitalSubsemiring R} {x : R} : x ∈ s.toAddSubmonoid ↔ x ∈ s :=
Iff.rfl
@[simp]
theorem coe_toAddSubmonoid (s : NonUnitalSubsemiring R) : (s.toAddSubmonoid : Set R) = s :=
rfl
/-- The non-unital subsemiring `R` of the non-unital semiring `R`. -/
instance : Top (NonUnitalSubsemiring R) :=
⟨{ (⊤ : Subsemigroup R), (⊤ : AddSubmonoid R) with }⟩
@[simp]
theorem mem_top (x : R) : x ∈ (⊤ : NonUnitalSubsemiring R) :=
Set.mem_univ x
@[simp]
theorem coe_top : ((⊤ : NonUnitalSubsemiring R) : Set R) = Set.univ :=
rfl
/-- The ring equiv between the top element of `NonUnitalSubsemiring R` and `R`. -/
@[simps!]
def topEquiv : (⊤ : NonUnitalSubsemiring R) ≃+* R :=
{ Subsemigroup.topEquiv, AddSubmonoid.topEquiv with }
/-- The preimage of a non-unital subsemiring along a non-unital ring homomorphism is a
non-unital subsemiring. -/
def comap (f : F) (s : NonUnitalSubsemiring S) : NonUnitalSubsemiring R :=
{ s.toSubsemigroup.comap (f : MulHom R S), s.toAddSubmonoid.comap (f : R →+ S) with
carrier := f ⁻¹' s }
@[simp]
theorem coe_comap (s : NonUnitalSubsemiring S) (f : F) : (s.comap f : Set R) = f ⁻¹' s :=
rfl
@[simp]
theorem mem_comap {s : NonUnitalSubsemiring S} {f : F} {x : R} : x ∈ s.comap f ↔ f x ∈ s :=
Iff.rfl
-- this has some nasty coercions, how to deal with it?
theorem comap_comap (s : NonUnitalSubsemiring T) (g : G) (f : F) :
((s.comap g : NonUnitalSubsemiring S).comap f : NonUnitalSubsemiring R) =
s.comap ((g : S →ₙ+* T).comp (f : R →ₙ+* S)) :=
rfl
/-- The image of a non-unital subsemiring along a ring homomorphism is a non-unital subsemiring. -/
def map (f : F) (s : NonUnitalSubsemiring R) : NonUnitalSubsemiring S :=
{ s.toSubsemigroup.map (f : R →ₙ* S), s.toAddSubmonoid.map (f : R →+ S) with carrier := f '' s }
@[simp]
theorem coe_map (f : F) (s : NonUnitalSubsemiring R) : (s.map f : Set S) = f '' s :=
rfl
@[simp]
theorem mem_map {f : F} {s : NonUnitalSubsemiring R} {y : S} : y ∈ s.map f ↔ ∃ x ∈ s, f x = y :=
Iff.rfl
@[simp]
theorem map_id : s.map (NonUnitalRingHom.id R) = s :=
SetLike.coe_injective <| Set.image_id _
-- unavoidable coercions?
theorem map_map (g : G) (f : F) :
(s.map (f : R →ₙ+* S)).map (g : S →ₙ+* T) = s.map ((g : S →ₙ+* T).comp (f : R →ₙ+* S)) :=
SetLike.coe_injective <| Set.image_image _ _ _
theorem map_le_iff_le_comap {f : F} {s : NonUnitalSubsemiring R} {t : NonUnitalSubsemiring S} :
s.map f ≤ t ↔ s ≤ t.comap f :=
Set.image_subset_iff
theorem gc_map_comap (f : F) :
@GaloisConnection (NonUnitalSubsemiring R) (NonUnitalSubsemiring S) _ _ (map f) (comap f) :=
fun _ _ => map_le_iff_le_comap
/-- A non-unital subsemiring is isomorphic to its image under an injective function -/
noncomputable def equivMapOfInjective (f : F) (hf : Function.Injective (f : R → S)) :
s ≃+* s.map f :=
{ Equiv.Set.image f s hf with
map_mul' := fun _ _ => Subtype.ext (map_mul f _ _)
map_add' := fun _ _ => Subtype.ext (map_add f _ _) }
@[simp]
theorem coe_equivMapOfInjective_apply (f : F) (hf : Function.Injective f) (x : s) :
(equivMapOfInjective s f hf x : S) = f x :=
rfl
end NonUnitalSubsemiring
namespace NonUnitalRingHom
open NonUnitalSubsemiring
variable [NonUnitalNonAssocSemiring S] [NonUnitalNonAssocSemiring T]
variable {F G : Type*} [FunLike F R S] [NonUnitalRingHomClass F R S]
variable [FunLike G S T] [NonUnitalRingHomClass G S T] (f : F) (g : G)
/-- The range of a non-unital ring homomorphism is a non-unital subsemiring.
See note [range copy pattern]. -/
def srange : NonUnitalSubsemiring S :=
((⊤ : NonUnitalSubsemiring R).map (f : R →ₙ+* S)).copy (Set.range f) Set.image_univ.symm
@[simp]
theorem coe_srange : (srange f : Set S) = Set.range f :=
rfl
@[simp]
theorem mem_srange {f : F} {y : S} : y ∈ srange f ↔ ∃ x, f x = y :=
Iff.rfl
theorem srange_eq_map : srange f = (⊤ : NonUnitalSubsemiring R).map f := by
ext
simp
theorem mem_srange_self (f : F) (x : R) : f x ∈ srange f :=
mem_srange.mpr ⟨x, rfl⟩
theorem map_srange (g : S →ₙ+* T) (f : R →ₙ+* S) : map g (srange f) = srange (g.comp f) := by
simpa only [srange_eq_map] using (⊤ : NonUnitalSubsemiring R).map_map g f
/-- The range of a morphism of non-unital semirings is finite if the domain is a finite. -/
instance finite_srange [Finite R] (f : F) : Finite (srange f : NonUnitalSubsemiring S) :=
(Set.finite_range f).to_subtype
end NonUnitalRingHom
namespace NonUnitalSubsemiring
-- should we define this as the range of the zero homomorphism?
instance : Bot (NonUnitalSubsemiring R) :=
⟨{ carrier := {0}
add_mem' := fun _ _ => by simp_all
zero_mem' := Set.mem_singleton 0
mul_mem' := fun _ _ => by simp_all }⟩
instance : Inhabited (NonUnitalSubsemiring R) :=
⟨⊥⟩
theorem coe_bot : ((⊥ : NonUnitalSubsemiring R) : Set R) = {0} :=
rfl
theorem mem_bot {x : R} : x ∈ (⊥ : NonUnitalSubsemiring R) ↔ x = 0 :=
Set.mem_singleton_iff
/-- The inf of two non-unital subsemirings is their intersection. -/
instance : Inf (NonUnitalSubsemiring R) :=
⟨fun s t =>
{ s.toSubsemigroup ⊓ t.toSubsemigroup, s.toAddSubmonoid ⊓ t.toAddSubmonoid with
carrier := s ∩ t }⟩
@[simp]
theorem coe_inf (p p' : NonUnitalSubsemiring R) :
((p ⊓ p' : NonUnitalSubsemiring R) : Set R) = (p : Set R) ∩ p' :=
rfl
@[simp]
theorem mem_inf {p p' : NonUnitalSubsemiring R} {x : R} : x ∈ p ⊓ p' ↔ x ∈ p ∧ x ∈ p' :=
Iff.rfl
instance : InfSet (NonUnitalSubsemiring R) :=
⟨fun s =>
NonUnitalSubsemiring.mk' (⋂ t ∈ s, ↑t) (⨅ t ∈ s, NonUnitalSubsemiring.toSubsemigroup t)
(by simp) (⨅ t ∈ s, NonUnitalSubsemiring.toAddSubmonoid t) (by simp)⟩
@[simp, norm_cast]
theorem coe_sInf (S : Set (NonUnitalSubsemiring R)) :
((sInf S : NonUnitalSubsemiring R) : Set R) = ⋂ s ∈ S, ↑s :=
rfl
theorem mem_sInf {S : Set (NonUnitalSubsemiring R)} {x : R} : x ∈ sInf S ↔ ∀ p ∈ S, x ∈ p :=
Set.mem_iInter₂
@[simp]
theorem sInf_toSubsemigroup (s : Set (NonUnitalSubsemiring R)) :
(sInf s).toSubsemigroup = ⨅ t ∈ s, NonUnitalSubsemiring.toSubsemigroup t :=
mk'_toSubsemigroup _ _
@[simp]
theorem sInf_toAddSubmonoid (s : Set (NonUnitalSubsemiring R)) :
(sInf s).toAddSubmonoid = ⨅ t ∈ s, NonUnitalSubsemiring.toAddSubmonoid t :=
mk'_toAddSubmonoid _ _
/-- Non-unital subsemirings of a non-unital semiring form a complete lattice. -/
instance : CompleteLattice (NonUnitalSubsemiring R) :=
{ completeLatticeOfInf (NonUnitalSubsemiring R)
fun _ => IsGLB.of_image SetLike.coe_subset_coe isGLB_biInf with
bot := ⊥
bot_le := fun s _ hx => (mem_bot.mp hx).symm ▸ zero_mem s
top := ⊤
le_top := fun _ _ _ => trivial
inf := (· ⊓ ·)
inf_le_left := fun _ _ _ => And.left
inf_le_right := fun _ _ _ => And.right
le_inf := fun _ _ _ h₁ h₂ _ hx => ⟨h₁ hx, h₂ hx⟩ }
theorem eq_top_iff' (A : NonUnitalSubsemiring R) : A = ⊤ ↔ ∀ x : R, x ∈ A :=
eq_top_iff.trans ⟨fun h m => h <| mem_top m, fun h m _ => h m⟩
section NonUnitalNonAssocSemiring
variable (R)
/-- The center of a semiring `R` is the set of elements that commute and associate with everything
in `R` -/
def center : NonUnitalSubsemiring R :=
{ Subsemigroup.center R with
zero_mem' := Set.zero_mem_center
add_mem' := Set.add_mem_center }
theorem coe_center : ↑(center R) = Set.center R :=
rfl
@[simp]
theorem center_toSubsemigroup :
(center R).toSubsemigroup = Subsemigroup.center R :=
rfl
/-- The center is commutative and associative. -/
instance center.instNonUnitalCommSemiring : NonUnitalCommSemiring (center R) :=
{ Subsemigroup.center.commSemigroup,
NonUnitalSubsemiringClass.toNonUnitalNonAssocSemiring (center R) with }
/-- A point-free means of proving membership in the center, for a non-associative ring.
This can be helpful when working with types that have ext lemmas for `R →+ R`. -/
lemma _root_.Set.mem_center_iff_addMonoidHom (a : R) :
a ∈ Set.center R ↔
AddMonoidHom.mulLeft a = .mulRight a ∧
AddMonoidHom.compr₂ .mul (.mulLeft a) = .comp .mul (.mulLeft a) ∧
AddMonoidHom.comp .mul (.mulRight a) = .compl₂ .mul (.mulLeft a) ∧
AddMonoidHom.compr₂ .mul (.mulRight a) = .compl₂ .mul (.mulRight a) := by
rw [Set.mem_center_iff, isMulCentral_iff]
simp [DFunLike.ext_iff]
end NonUnitalNonAssocSemiring
section NonUnitalSemiring
-- no instance diamond, unlike the unital version
example {R} [NonUnitalSemiring R] :
(center.instNonUnitalCommSemiring _).toNonUnitalSemiring =
NonUnitalSubsemiringClass.toNonUnitalSemiring (center R) := by
with_reducible_and_instances rfl
theorem mem_center_iff {R} [NonUnitalSemiring R] {z : R} : z ∈ center R ↔ ∀ g, g * z = z * g := by
rw [← Semigroup.mem_center_iff]
exact Iff.rfl
instance decidableMemCenter {R} [NonUnitalSemiring R] [DecidableEq R] [Fintype R] :
DecidablePred (· ∈ center R) := fun _ => decidable_of_iff' _ mem_center_iff
@[simp]
theorem center_eq_top (R) [NonUnitalCommSemiring R] : center R = ⊤ :=
SetLike.coe_injective (Set.center_eq_univ R)
end NonUnitalSemiring
section Centralizer
/-- The centralizer of a set as non-unital subsemiring. -/
def centralizer {R} [NonUnitalSemiring R] (s : Set R) : NonUnitalSubsemiring R :=
{ Subsemigroup.centralizer s with
carrier := s.centralizer
zero_mem' := Set.zero_mem_centralizer
add_mem' := Set.add_mem_centralizer }
@[simp, norm_cast]
theorem coe_centralizer {R} [NonUnitalSemiring R] (s : Set R) :
(centralizer s : Set R) = s.centralizer :=
rfl
theorem centralizer_toSubsemigroup {R} [NonUnitalSemiring R] (s : Set R) :
(centralizer s).toSubsemigroup = Subsemigroup.centralizer s :=
rfl
theorem mem_centralizer_iff {R} [NonUnitalSemiring R] {s : Set R} {z : R} :
z ∈ centralizer s ↔ ∀ g ∈ s, g * z = z * g :=
Iff.rfl
theorem center_le_centralizer {R} [NonUnitalSemiring R] (s) : center R ≤ centralizer s :=
s.center_subset_centralizer
theorem centralizer_le {R} [NonUnitalSemiring R] (s t : Set R) (h : s ⊆ t) :
centralizer t ≤ centralizer s :=
Set.centralizer_subset h
@[simp]
theorem centralizer_eq_top_iff_subset {R} [NonUnitalSemiring R] {s : Set R} :
centralizer s = ⊤ ↔ s ⊆ center R :=
SetLike.ext'_iff.trans Set.centralizer_eq_top_iff_subset
@[simp]
theorem centralizer_univ {R} [NonUnitalSemiring R] : centralizer Set.univ = center R :=
SetLike.ext' (Set.centralizer_univ R)
end Centralizer
/-- The `NonUnitalSubsemiring` generated by a set. -/
def closure (s : Set R) : NonUnitalSubsemiring R :=
sInf { S | s ⊆ S }
theorem mem_closure {x : R} {s : Set R} :
x ∈ closure s ↔ ∀ S : NonUnitalSubsemiring R, s ⊆ S → x ∈ S :=
mem_sInf
/-- The non-unital subsemiring generated by a set includes the set. -/
@[simp, aesop safe 20 apply (rule_sets := [SetLike])]
theorem subset_closure {s : Set R} : s ⊆ closure s := fun _ hx => mem_closure.2 fun _ hS => hS hx
theorem not_mem_of_not_mem_closure {s : Set R} {P : R} (hP : P ∉ closure s) : P ∉ s := fun h =>
hP (subset_closure h)
/-- A non-unital subsemiring `S` includes `closure s` if and only if it includes `s`. -/
@[simp]
theorem closure_le {s : Set R} {t : NonUnitalSubsemiring R} : closure s ≤ t ↔ s ⊆ t :=
⟨Set.Subset.trans subset_closure, fun h => sInf_le h⟩
/-- Subsemiring closure of a set is monotone in its argument: if `s ⊆ t`,
then `closure s ≤ closure t`. -/
theorem closure_mono ⦃s t : Set R⦄ (h : s ⊆ t) : closure s ≤ closure t :=
closure_le.2 <| Set.Subset.trans h subset_closure
theorem closure_eq_of_le {s : Set R} {t : NonUnitalSubsemiring R} (h₁ : s ⊆ t)
(h₂ : t ≤ closure s) : closure s = t :=
le_antisymm (closure_le.2 h₁) h₂
variable [NonUnitalNonAssocSemiring S]
theorem mem_map_equiv {f : R ≃+* S} {K : NonUnitalSubsemiring R} {x : S} :
x ∈ K.map (f : R →ₙ+* S) ↔ f.symm x ∈ K := by
convert @Set.mem_image_equiv _ _ (↑K) f.toEquiv x
theorem map_equiv_eq_comap_symm (f : R ≃+* S) (K : NonUnitalSubsemiring R) :
K.map (f : R →ₙ+* S) = K.comap f.symm :=
SetLike.coe_injective (f.toEquiv.image_eq_preimage K)
theorem comap_equiv_eq_map_symm (f : R ≃+* S) (K : NonUnitalSubsemiring S) :
K.comap (f : R →ₙ+* S) = K.map f.symm :=
(map_equiv_eq_comap_symm f.symm K).symm
end NonUnitalSubsemiring
namespace Subsemigroup
/-- The additive closure of a non-unital subsemigroup is a non-unital subsemiring. -/
def nonUnitalSubsemiringClosure (M : Subsemigroup R) : NonUnitalSubsemiring R :=
{ AddSubmonoid.closure (M : Set R) with mul_mem' := MulMemClass.mul_mem_add_closure }
theorem nonUnitalSubsemiringClosure_coe :
(M.nonUnitalSubsemiringClosure : Set R) = AddSubmonoid.closure (M : Set R) :=
rfl
theorem nonUnitalSubsemiringClosure_toAddSubmonoid :
M.nonUnitalSubsemiringClosure.toAddSubmonoid = AddSubmonoid.closure (M : Set R) :=
rfl
/-- The `NonUnitalSubsemiring` generated by a multiplicative subsemigroup coincides with the
`NonUnitalSubsemiring.closure` of the subsemigroup itself . -/
theorem nonUnitalSubsemiringClosure_eq_closure :
M.nonUnitalSubsemiringClosure = NonUnitalSubsemiring.closure (M : Set R) := by
ext
refine ⟨fun hx => ?_,
fun hx => (NonUnitalSubsemiring.mem_closure.mp hx) M.nonUnitalSubsemiringClosure fun s sM => ?_⟩
<;> rintro - ⟨H1, rfl⟩
<;> rintro - ⟨H2, rfl⟩
· exact AddSubmonoid.mem_closure.mp hx H1.toAddSubmonoid H2
· exact H2 sM
end Subsemigroup
namespace NonUnitalSubsemiring
@[simp]
theorem closure_subsemigroup_closure (s : Set R) : closure ↑(Subsemigroup.closure s) = closure s :=
le_antisymm
(closure_le.mpr fun _ hy =>
(Subsemigroup.mem_closure.mp hy) (closure s).toSubsemigroup subset_closure)
(closure_mono Subsemigroup.subset_closure)
/-- The elements of the non-unital subsemiring closure of `M` are exactly the elements of the
additive closure of a multiplicative subsemigroup `M`. -/
theorem coe_closure_eq (s : Set R) :
(closure s : Set R) = AddSubmonoid.closure (Subsemigroup.closure s : Set R) := by
simp [← Subsemigroup.nonUnitalSubsemiringClosure_toAddSubmonoid,
Subsemigroup.nonUnitalSubsemiringClosure_eq_closure]
theorem mem_closure_iff {s : Set R} {x} :
x ∈ closure s ↔ x ∈ AddSubmonoid.closure (Subsemigroup.closure s : Set R) :=
Set.ext_iff.mp (coe_closure_eq s) x
@[simp]
theorem closure_addSubmonoid_closure {s : Set R} :
closure ↑(AddSubmonoid.closure s) = closure s := by
ext x
refine ⟨fun hx => ?_, fun hx => closure_mono AddSubmonoid.subset_closure hx⟩
rintro - ⟨H, rfl⟩
rintro - ⟨J, rfl⟩
refine (AddSubmonoid.mem_closure.mp (mem_closure_iff.mp hx)) H.toAddSubmonoid fun y hy => ?_
refine (Subsemigroup.mem_closure.mp hy) H.toSubsemigroup fun z hz => ?_
exact (AddSubmonoid.mem_closure.mp hz) H.toAddSubmonoid fun w hw => J hw
/-- An induction principle for closure membership. If `p` holds for `0`, `1`, and all elements
of `s`, and is preserved under addition and multiplication, then `p` holds for all elements
of the closure of `s`. -/
@[elab_as_elim]
theorem closure_induction {s : Set R} {p : R → Prop} {x} (h : x ∈ closure s) (mem : ∀ x ∈ s, p x)
(zero : p 0) (add : ∀ x y, p x → p y → p (x + y)) (mul : ∀ x y, p x → p y → p (x * y)) : p x :=
(@closure_le _ _ _ ⟨⟨⟨p, fun {a b} => add a b⟩, zero⟩, fun {a b} => mul a b⟩).2 mem h
/-- An induction principle for closure membership for predicates with two arguments. -/
@[elab_as_elim]
theorem closure_induction₂ {s : Set R} {p : R → R → Prop} {x} {y : R} (hx : x ∈ closure s)
(hy : y ∈ closure s) (Hs : ∀ x ∈ s, ∀ y ∈ s, p x y) (H0_left : ∀ x, p 0 x)
(H0_right : ∀ x, p x 0) (Hadd_left : ∀ x₁ x₂ y, p x₁ y → p x₂ y → p (x₁ + x₂) y)
(Hadd_right : ∀ x y₁ y₂, p x y₁ → p x y₂ → p x (y₁ + y₂))
(Hmul_left : ∀ x₁ x₂ y, p x₁ y → p x₂ y → p (x₁ * x₂) y)
(Hmul_right : ∀ x y₁ y₂, p x y₁ → p x y₂ → p x (y₁ * y₂)) : p x y :=
closure_induction hx
(fun x₁ x₁s => closure_induction hy (Hs x₁ x₁s) (H0_right x₁) (Hadd_right x₁) (Hmul_right x₁))
(H0_left y) (fun z z' => Hadd_left z z' y) fun z z' => Hmul_left z z' y
variable (R)
/-- `closure` forms a Galois insertion with the coercion to set. -/
protected def gi : GaloisInsertion (@closure R _) (↑) where
choice s _ := closure s
gc _ _ := closure_le
le_l_u _ := subset_closure
choice_eq _ _ := rfl
variable {R}
variable [NonUnitalNonAssocSemiring S]
variable {F : Type*} [FunLike F R S] [NonUnitalRingHomClass F R S]
/-- Closure of a non-unital subsemiring `S` equals `S`. -/
theorem closure_eq (s : NonUnitalSubsemiring R) : closure (s : Set R) = s :=
(NonUnitalSubsemiring.gi R).l_u_eq s
@[simp]
theorem closure_empty : closure (∅ : Set R) = ⊥ :=
(NonUnitalSubsemiring.gi R).gc.l_bot
@[simp]
theorem closure_univ : closure (Set.univ : Set R) = ⊤ :=
@coe_top R _ ▸ closure_eq ⊤
theorem closure_union (s t : Set R) : closure (s ∪ t) = closure s ⊔ closure t :=
(NonUnitalSubsemiring.gi R).gc.l_sup
theorem closure_iUnion {ι} (s : ι → Set R) : closure (⋃ i, s i) = ⨆ i, closure (s i) :=
(NonUnitalSubsemiring.gi R).gc.l_iSup
theorem closure_sUnion (s : Set (Set R)) : closure (⋃₀ s) = ⨆ t ∈ s, closure t :=
(NonUnitalSubsemiring.gi R).gc.l_sSup
theorem map_sup (s t : NonUnitalSubsemiring R) (f : F) :
(map f (s ⊔ t) : NonUnitalSubsemiring S) = map f s ⊔ map f t :=
@GaloisConnection.l_sup _ _ s t _ _ _ _ (gc_map_comap f)
theorem map_iSup {ι : Sort*} (f : F) (s : ι → NonUnitalSubsemiring R) :
(map f (iSup s) : NonUnitalSubsemiring S) = ⨆ i, map f (s i) :=
@GaloisConnection.l_iSup _ _ _ _ _ _ _ (gc_map_comap f) s
theorem comap_inf (s t : NonUnitalSubsemiring S) (f : F) :
(comap f (s ⊓ t) : NonUnitalSubsemiring R) = comap f s ⊓ comap f t :=
@GaloisConnection.u_inf _ _ s t _ _ _ _ (gc_map_comap f)
theorem comap_iInf {ι : Sort*} (f : F) (s : ι → NonUnitalSubsemiring S) :
(comap f (iInf s) : NonUnitalSubsemiring R) = ⨅ i, comap f (s i) :=
@GaloisConnection.u_iInf _ _ _ _ _ _ _ (gc_map_comap f) s
@[simp]
theorem map_bot (f : F) : map f (⊥ : NonUnitalSubsemiring R) = (⊥ : NonUnitalSubsemiring S) :=
(gc_map_comap f).l_bot
@[simp]
theorem comap_top (f : F) : comap f (⊤ : NonUnitalSubsemiring S) = (⊤ : NonUnitalSubsemiring R) :=
(gc_map_comap f).u_top
/-- Given `NonUnitalSubsemiring`s `s`, `t` of semirings `R`, `S` respectively, `s.prod t` is
`s × t` as a non-unital subsemiring of `R × S`. -/
def prod (s : NonUnitalSubsemiring R) (t : NonUnitalSubsemiring S) : NonUnitalSubsemiring (R × S) :=
{ s.toSubsemigroup.prod t.toSubsemigroup, s.toAddSubmonoid.prod t.toAddSubmonoid with
carrier := (s : Set R) ×ˢ (t : Set S) }
@[norm_cast]
theorem coe_prod (s : NonUnitalSubsemiring R) (t : NonUnitalSubsemiring S) :
(s.prod t : Set (R × S)) = (s : Set R) ×ˢ (t : Set S) :=
rfl
theorem mem_prod {s : NonUnitalSubsemiring R} {t : NonUnitalSubsemiring S} {p : R × S} :
p ∈ s.prod t ↔ p.1 ∈ s ∧ p.2 ∈ t :=
Iff.rfl
@[mono]
theorem prod_mono ⦃s₁ s₂ : NonUnitalSubsemiring R⦄ (hs : s₁ ≤ s₂) ⦃t₁ t₂ : NonUnitalSubsemiring S⦄
(ht : t₁ ≤ t₂) : s₁.prod t₁ ≤ s₂.prod t₂ :=
Set.prod_mono hs ht
theorem prod_mono_right (s : NonUnitalSubsemiring R) :
Monotone fun t : NonUnitalSubsemiring S => s.prod t :=
prod_mono (le_refl s)
theorem prod_mono_left (t : NonUnitalSubsemiring S) :
Monotone fun s : NonUnitalSubsemiring R => s.prod t := fun _ _ hs => prod_mono hs (le_refl t)
theorem prod_top (s : NonUnitalSubsemiring R) :
s.prod (⊤ : NonUnitalSubsemiring S) = s.comap (NonUnitalRingHom.fst R S) :=
ext fun x => by simp [mem_prod, MonoidHom.coe_fst]
theorem top_prod (s : NonUnitalSubsemiring S) :
(⊤ : NonUnitalSubsemiring R).prod s = s.comap (NonUnitalRingHom.snd R S) :=
ext fun x => by simp [mem_prod, MonoidHom.coe_snd]
@[simp]
theorem top_prod_top : (⊤ : NonUnitalSubsemiring R).prod (⊤ : NonUnitalSubsemiring S) = ⊤ :=
(top_prod _).trans <| comap_top _
/-- Product of non-unital subsemirings is isomorphic to their product as semigroups. -/
def prodEquiv (s : NonUnitalSubsemiring R) (t : NonUnitalSubsemiring S) : s.prod t ≃+* s × t :=
{ Equiv.Set.prod (s : Set R) (t : Set S) with
map_mul' := fun _ _ => rfl
map_add' := fun _ _ => rfl }
theorem mem_iSup_of_directed {ι} [hι : Nonempty ι] {S : ι → NonUnitalSubsemiring R}
(hS : Directed (· ≤ ·) S) {x : R} : (x ∈ ⨆ i, S i) ↔ ∃ i, x ∈ S i := by
refine ⟨?_, fun ⟨i, hi⟩ ↦ le_iSup S i hi⟩
let U : NonUnitalSubsemiring R :=
NonUnitalSubsemiring.mk' (⋃ i, (S i : Set R))
(⨆ i, (S i).toSubsemigroup) (Subsemigroup.coe_iSup_of_directed hS)
(⨆ i, (S i).toAddSubmonoid) (AddSubmonoid.coe_iSup_of_directed hS)
-- Porting note `@this` doesn't work
suffices H : ⨆ i, S i ≤ U by simpa [U] using @H x
exact iSup_le fun i x hx => Set.mem_iUnion.2 ⟨i, hx⟩
theorem coe_iSup_of_directed {ι} [hι : Nonempty ι] {S : ι → NonUnitalSubsemiring R}
(hS : Directed (· ≤ ·) S) : ((⨆ i, S i : NonUnitalSubsemiring R) : Set R) = ⋃ i, S i :=
Set.ext fun x ↦ by simp [mem_iSup_of_directed hS]
theorem mem_sSup_of_directedOn {S : Set (NonUnitalSubsemiring R)} (Sne : S.Nonempty)
(hS : DirectedOn (· ≤ ·) S) {x : R} : x ∈ sSup S ↔ ∃ s ∈ S, x ∈ s := by
haveI : Nonempty S := Sne.to_subtype
simp only [sSup_eq_iSup', mem_iSup_of_directed hS.directed_val, Subtype.exists, exists_prop]
theorem coe_sSup_of_directedOn {S : Set (NonUnitalSubsemiring R)} (Sne : S.Nonempty)
(hS : DirectedOn (· ≤ ·) S) : (↑(sSup S) : Set R) = ⋃ s ∈ S, ↑s :=
Set.ext fun x => by simp [mem_sSup_of_directedOn Sne hS]
end NonUnitalSubsemiring
namespace NonUnitalRingHom
variable {F : Type*} [FunLike F R S]
theorem eq_of_eqOn_stop {f g : F}
(h : Set.EqOn (f : R → S) (g : R → S) (⊤ : NonUnitalSubsemiring R)) : f = g :=
DFunLike.ext _ _ fun _ => h trivial
variable [NonUnitalNonAssocSemiring S] [NonUnitalNonAssocSemiring T]
[NonUnitalRingHomClass F R S]
{S' : Type*} [SetLike S' S] [NonUnitalSubsemiringClass S' S]
{s : NonUnitalSubsemiring R}
open NonUnitalSubsemiringClass NonUnitalSubsemiring
/-- Restriction of a non-unital ring homomorphism to a non-unital subsemiring of the codomain. -/
def codRestrict (f : F) (s : S') (h : ∀ x, f x ∈ s) : R →ₙ+* s where
toFun n := ⟨f n, h n⟩
map_mul' x y := Subtype.eq (map_mul f x y)
map_add' x y := Subtype.eq (map_add f x y)
map_zero' := Subtype.eq (map_zero f)
/-- Restriction of a non-unital ring homomorphism to its range interpreted as a
non-unital subsemiring.
This is the bundled version of `Set.rangeFactorization`. -/
def srangeRestrict (f : F) : R →ₙ+* (srange f : NonUnitalSubsemiring S) :=
codRestrict f (srange f) (mem_srange_self f)
@[simp]
theorem coe_srangeRestrict (f : F) (x : R) : (srangeRestrict f x : S) = f x :=
rfl
theorem srangeRestrict_surjective (f : F) :
Function.Surjective (srangeRestrict f : R → (srange f : NonUnitalSubsemiring S)) :=
fun ⟨_, hy⟩ =>
let ⟨x, hx⟩ := mem_srange.mp hy
⟨x, Subtype.ext hx⟩
theorem srange_top_iff_surjective {f : F} :
srange f = (⊤ : NonUnitalSubsemiring S) ↔ Function.Surjective (f : R → S) :=
SetLike.ext'_iff.trans <| Iff.trans (by rw [coe_srange, coe_top]) Set.range_iff_surjective
/-- The range of a surjective non-unital ring homomorphism is the whole of the codomain. -/
@[simp]
theorem srange_top_of_surjective (f : F) (hf : Function.Surjective (f : R → S)) :
srange f = (⊤ : NonUnitalSubsemiring S) :=
srange_top_iff_surjective.2 hf
/-- The non-unital subsemiring of elements `x : R` such that `f x = g x` -/
def eqSlocus (f g : F) : NonUnitalSubsemiring R :=
{ (f : R →ₙ* S).eqLocus (g : R →ₙ* S), (f : R →+ S).eqLocusM g with
carrier := { x | f x = g x } }
/-- If two non-unital ring homomorphisms are equal on a set, then they are equal on its
non-unital subsemiring closure. -/
theorem eqOn_sclosure {f g : F} {s : Set R} (h : Set.EqOn (f : R → S) (g : R → S) s) :
Set.EqOn f g (closure s) :=
show closure s ≤ eqSlocus f g from closure_le.2 h
theorem eq_of_eqOn_sdense {s : Set R} (hs : closure s = ⊤) {f g : F}
(h : s.EqOn (f : R → S) (g : R → S)) : f = g :=
eq_of_eqOn_stop <| hs ▸ eqOn_sclosure h
theorem sclosure_preimage_le (f : F) (s : Set S) :
closure ((f : R → S) ⁻¹' s) ≤ (closure s).comap f :=
closure_le.2 fun _ hx => SetLike.mem_coe.2 <| mem_comap.2 <| subset_closure hx
/-- The image under a ring homomorphism of the subsemiring generated by a set equals
the subsemiring generated by the image of the set. -/
theorem map_sclosure (f : F) (s : Set R) : (closure s).map f = closure ((f : R → S) '' s) :=
le_antisymm
(map_le_iff_le_comap.2 <|
le_trans (closure_mono <| Set.subset_preimage_image _ _) (sclosure_preimage_le _ _))
(closure_le.2 <| Set.image_subset _ subset_closure)
end NonUnitalRingHom
namespace NonUnitalSubsemiring
open NonUnitalRingHom NonUnitalSubsemiringClass
/-- The non-unital ring homomorphism associated to an inclusion of
non-unital subsemirings. -/
def inclusion {S T : NonUnitalSubsemiring R} (h : S ≤ T) : S →ₙ+* T :=
codRestrict (subtype S) _ fun x => h x.2
@[simp]
theorem srange_subtype (s : NonUnitalSubsemiring R) : NonUnitalRingHom.srange (subtype s) = s :=
SetLike.coe_injective <| (coe_srange _).trans Subtype.range_coe
variable [NonUnitalNonAssocSemiring S]
@[simp]
theorem range_fst : NonUnitalRingHom.srange (fst R S) = ⊤ :=
NonUnitalRingHom.srange_top_of_surjective (fst R S) Prod.fst_surjective
@[simp]
theorem range_snd : NonUnitalRingHom.srange (snd R S) = ⊤ :=
NonUnitalRingHom.srange_top_of_surjective (snd R S) <| Prod.snd_surjective
end NonUnitalSubsemiring
namespace RingEquiv
open NonUnitalRingHom NonUnitalSubsemiringClass
variable {s t : NonUnitalSubsemiring R}
variable [NonUnitalNonAssocSemiring S] {F : Type*} [FunLike F R S] [NonUnitalRingHomClass F R S]
/-- Makes the identity isomorphism from a proof two non-unital subsemirings of a multiplicative
monoid are equal. -/
def nonUnitalSubsemiringCongr (h : s = t) : s ≃+* t :=
{ Equiv.setCongr <| congr_arg _ h with
map_mul' := fun _ _ => rfl
map_add' := fun _ _ => rfl }
/-- Restrict a non-unital ring homomorphism with a left inverse to a ring isomorphism to its
`NonUnitalRingHom.srange`. -/
def sofLeftInverse' {g : S → R} {f : F} (h : Function.LeftInverse g f) : R ≃+* srange f :=
{ srangeRestrict f with
toFun := srangeRestrict f
invFun := fun x => g (subtype (srange f) x)
left_inv := h
right_inv := fun x =>
Subtype.ext <|
let ⟨x', hx'⟩ := NonUnitalRingHom.mem_srange.mp x.prop
show f (g x) = x by rw [← hx', h x'] }
@[simp]
theorem sofLeftInverse'_apply {g : S → R} {f : F} (h : Function.LeftInverse g f) (x : R) :
↑(sofLeftInverse' h x) = f x :=
rfl
@[simp]
theorem sofLeftInverse'_symm_apply {g : S → R} {f : F} (h : Function.LeftInverse g f)
(x : srange f) : (sofLeftInverse' h).symm x = g x :=
rfl
/-- Given an equivalence `e : R ≃+* S` of non-unital semirings and a non-unital subsemiring
`s` of `R`, `non_unital_subsemiring_map e s` is the induced equivalence between `s` and
`s.map e` -/
@[simps!]
def nonUnitalSubsemiringMap (e : R ≃+* S) (s : NonUnitalSubsemiring R) :
s ≃+* NonUnitalSubsemiring.map e.toNonUnitalRingHom s :=
{ e.toAddEquiv.addSubmonoidMap s.toAddSubmonoid,
e.toMulEquiv.subsemigroupMap s.toSubsemigroup with }
end RingEquiv
|
RingTheory\Norm\Basic.lean | /-
Copyright (c) 2021 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen
-/
import Mathlib.RingTheory.Norm.Defs
import Mathlib.FieldTheory.PrimitiveElement
import Mathlib.LinearAlgebra.Matrix.Charpoly.Minpoly
import Mathlib.LinearAlgebra.Matrix.ToLinearEquiv
import Mathlib.FieldTheory.IsAlgClosed.AlgebraicClosure
import Mathlib.FieldTheory.Galois
/-!
# Norm for (finite) ring extensions
Suppose we have an `R`-algebra `S` with a finite basis. For each `s : S`,
the determinant of the linear map given by multiplying by `s` gives information
about the roots of the minimal polynomial of `s` over `R`.
## Implementation notes
Typically, the norm is defined specifically for finite field extensions.
The current definition is as general as possible and the assumption that we have
fields or that the extension is finite is added to the lemmas as needed.
We only define the norm for left multiplication (`Algebra.leftMulMatrix`,
i.e. `LinearMap.mulLeft`).
For now, the definitions assume `S` is commutative, so the choice doesn't
matter anyway.
See also `Algebra.trace`, which is defined similarly as the trace of
`Algebra.leftMulMatrix`.
## References
* https://en.wikipedia.org/wiki/Field_norm
-/
universe u v w
variable {R S T : Type*} [CommRing R] [Ring S]
variable [Algebra R S]
variable {K L F : Type*} [Field K] [Field L] [Field F]
variable [Algebra K L] [Algebra K F]
variable {ι : Type w}
open FiniteDimensional
open LinearMap
open Matrix Polynomial
open scoped Matrix
namespace Algebra
section EqProdRoots
/-- Given `pb : PowerBasis K S`, then the norm of `pb.gen` is
`(-1) ^ pb.dim * coeff (minpoly K pb.gen) 0`. -/
theorem PowerBasis.norm_gen_eq_coeff_zero_minpoly (pb : PowerBasis R S) :
norm R pb.gen = (-1) ^ pb.dim * coeff (minpoly R pb.gen) 0 := by
rw [norm_eq_matrix_det pb.basis, det_eq_sign_charpoly_coeff, charpoly_leftMulMatrix,
Fintype.card_fin]
/-- Given `pb : PowerBasis R S`, then the norm of `pb.gen` is
`((minpoly R pb.gen).aroots F).prod`. -/
theorem PowerBasis.norm_gen_eq_prod_roots [Algebra R F] (pb : PowerBasis R S)
(hf : (minpoly R pb.gen).Splits (algebraMap R F)) :
algebraMap R F (norm R pb.gen) = ((minpoly R pb.gen).aroots F).prod := by
haveI := Module.nontrivial R F
have := minpoly.monic pb.isIntegral_gen
rw [PowerBasis.norm_gen_eq_coeff_zero_minpoly, ← pb.natDegree_minpoly, RingHom.map_mul,
← coeff_map,
prod_roots_eq_coeff_zero_of_monic_of_split (this.map _) ((splits_id_iff_splits _).2 hf),
this.natDegree_map, map_pow, ← mul_assoc, ← mul_pow]
simp only [map_neg, _root_.map_one, neg_mul, neg_neg, one_pow, one_mul]
end EqProdRoots
section EqZeroIff
variable [Finite ι]
@[simp]
theorem norm_zero [Nontrivial S] [Module.Free R S] [Module.Finite R S] : norm R (0 : S) = 0 := by
nontriviality
rw [norm_apply, coe_lmul_eq_mul, map_zero, LinearMap.det_zero' (Module.Free.chooseBasis R S)]
@[simp]
theorem norm_eq_zero_iff [IsDomain R] [IsDomain S] [Module.Free R S] [Module.Finite R S] {x : S} :
norm R x = 0 ↔ x = 0 := by
constructor
on_goal 1 => let b := Module.Free.chooseBasis R S
swap
· rintro rfl; exact norm_zero
· letI := Classical.decEq (Module.Free.ChooseBasisIndex R S)
rw [norm_eq_matrix_det b, ← Matrix.exists_mulVec_eq_zero_iff]
rintro ⟨v, v_ne, hv⟩
rw [← b.equivFun.apply_symm_apply v, b.equivFun_symm_apply, b.equivFun_apply,
leftMulMatrix_mulVec_repr] at hv
refine (mul_eq_zero.mp (b.ext_elem fun i => ?_)).resolve_right (show ∑ i, v i • b i ≠ 0 from ?_)
· simpa only [LinearEquiv.map_zero, Pi.zero_apply] using congr_fun hv i
· contrapose! v_ne with sum_eq
apply b.equivFun.symm.injective
rw [b.equivFun_symm_apply, sum_eq, LinearEquiv.map_zero]
theorem norm_ne_zero_iff [IsDomain R] [IsDomain S] [Module.Free R S] [Module.Finite R S] {x : S} :
norm R x ≠ 0 ↔ x ≠ 0 := not_iff_not.mpr norm_eq_zero_iff
/-- This is `Algebra.norm_eq_zero_iff` composed with `Algebra.norm_apply`. -/
@[simp]
theorem norm_eq_zero_iff' [IsDomain R] [IsDomain S] [Module.Free R S] [Module.Finite R S] {x : S} :
LinearMap.det (LinearMap.mul R S x) = 0 ↔ x = 0 := norm_eq_zero_iff
theorem norm_eq_zero_iff_of_basis [IsDomain R] [IsDomain S] (b : Basis ι R S) {x : S} :
Algebra.norm R x = 0 ↔ x = 0 := by
haveI : Module.Free R S := Module.Free.of_basis b
haveI : Module.Finite R S := Module.Finite.of_basis b
exact norm_eq_zero_iff
theorem norm_ne_zero_iff_of_basis [IsDomain R] [IsDomain S] (b : Basis ι R S) {x : S} :
Algebra.norm R x ≠ 0 ↔ x ≠ 0 :=
not_iff_not.mpr (norm_eq_zero_iff_of_basis b)
end EqZeroIff
open IntermediateField
variable (K)
theorem norm_eq_norm_adjoin [FiniteDimensional K L] [Algebra.IsSeparable K L] (x : L) :
norm K x = norm K (AdjoinSimple.gen K x) ^ finrank K⟮x⟯ L := by
letI := Algebra.isSeparable_tower_top_of_isSeparable K K⟮x⟯ L
let pbL := Field.powerBasisOfFiniteOfSeparable K⟮x⟯ L
let pbx := IntermediateField.adjoin.powerBasis (Algebra.IsSeparable.isIntegral K x)
-- This used to be `rw`, but we need `erw` after leanprover/lean4#2644
erw [← AdjoinSimple.algebraMap_gen K x, norm_eq_matrix_det (pbx.basis.smulTower pbL.basis) _,
smulTower_leftMulMatrix_algebraMap, det_blockDiagonal, norm_eq_matrix_det pbx.basis]
simp only [Finset.card_fin, Finset.prod_const]
congr
rw [← PowerBasis.finrank, AdjoinSimple.algebraMap_gen K x]
variable {K}
section IntermediateField
theorem _root_.IntermediateField.AdjoinSimple.norm_gen_eq_one {x : L} (hx : ¬IsIntegral K x) :
norm K (AdjoinSimple.gen K x) = 1 := by
rw [norm_eq_one_of_not_exists_basis]
contrapose! hx
obtain ⟨s, ⟨b⟩⟩ := hx
refine .of_mem_of_fg K⟮x⟯.toSubalgebra ?_ x ?_
· exact (Submodule.fg_iff_finiteDimensional _).mpr (of_fintype_basis b)
· exact IntermediateField.subset_adjoin K _ (Set.mem_singleton x)
theorem _root_.IntermediateField.AdjoinSimple.norm_gen_eq_prod_roots (x : L)
(hf : (minpoly K x).Splits (algebraMap K F)) :
(algebraMap K F) (norm K (AdjoinSimple.gen K x)) =
((minpoly K x).aroots F).prod := by
have injKxL := (algebraMap K⟮x⟯ L).injective
by_cases hx : IsIntegral K x; swap
· simp [minpoly.eq_zero hx, IntermediateField.AdjoinSimple.norm_gen_eq_one hx, aroots_def]
rw [← adjoin.powerBasis_gen hx, PowerBasis.norm_gen_eq_prod_roots] <;>
rw [adjoin.powerBasis_gen hx, ← minpoly.algebraMap_eq injKxL] <;>
try simp only [AdjoinSimple.algebraMap_gen _ _]
exact hf
end IntermediateField
section EqProdEmbeddings
open IntermediateField IntermediateField.AdjoinSimple Polynomial
variable (F) (E : Type*) [Field E] [Algebra K E]
theorem norm_eq_prod_embeddings_gen [Algebra R F] (pb : PowerBasis R S)
(hE : (minpoly R pb.gen).Splits (algebraMap R F)) (hfx : IsSeparable R pb.gen) :
algebraMap R F (norm R pb.gen) =
(@Finset.univ _ (PowerBasis.AlgHom.fintype pb)).prod fun σ => σ pb.gen := by
letI := Classical.decEq F
rw [PowerBasis.norm_gen_eq_prod_roots pb hE]
rw [@Fintype.prod_equiv (S →ₐ[R] F) _ _ (PowerBasis.AlgHom.fintype pb) _ _ pb.liftEquiv'
(fun σ => σ pb.gen) (fun x => x) ?_]
· rw [Finset.prod_mem_multiset, Finset.prod_eq_multiset_prod, Multiset.toFinset_val,
Multiset.dedup_eq_self.mpr, Multiset.map_id]
· exact nodup_roots hfx.map
· intro x; rfl
· intro σ; simp only [PowerBasis.liftEquiv'_apply_coe]
theorem norm_eq_prod_roots [Algebra.IsSeparable K L] [FiniteDimensional K L] {x : L}
(hF : (minpoly K x).Splits (algebraMap K F)) :
algebraMap K F (norm K x) =
((minpoly K x).aroots F).prod ^ finrank K⟮x⟯ L := by
rw [norm_eq_norm_adjoin K x, map_pow, IntermediateField.AdjoinSimple.norm_gen_eq_prod_roots _ hF]
theorem prod_embeddings_eq_finrank_pow [Algebra L F] [IsScalarTower K L F] [IsAlgClosed E]
[Algebra.IsSeparable K F] [FiniteDimensional K F] (pb : PowerBasis K L) :
∏ σ : F →ₐ[K] E, σ (algebraMap L F pb.gen) =
((@Finset.univ _ (PowerBasis.AlgHom.fintype pb)).prod
fun σ : L →ₐ[K] E => σ pb.gen) ^ finrank L F := by
haveI : FiniteDimensional L F := FiniteDimensional.right K L F
haveI : Algebra.IsSeparable L F := Algebra.isSeparable_tower_top_of_isSeparable K L F
letI : Fintype (L →ₐ[K] E) := PowerBasis.AlgHom.fintype pb
rw [Fintype.prod_equiv algHomEquivSigma (fun σ : F →ₐ[K] E => _) fun σ => σ.1 pb.gen,
← Finset.univ_sigma_univ, Finset.prod_sigma, ← Finset.prod_pow]
· refine Finset.prod_congr rfl fun σ _ => ?_
letI : Algebra L E := σ.toRingHom.toAlgebra
simp_rw [Finset.prod_const]
congr
exact AlgHom.card L F E
· intro σ
simp only [algHomEquivSigma, Equiv.coe_fn_mk, AlgHom.restrictDomain, AlgHom.comp_apply,
IsScalarTower.coe_toAlgHom']
variable (K)
/-- For `L/K` a finite separable extension of fields and `E` an algebraically closed extension
of `K`, the norm (down to `K`) of an element `x` of `L` is equal to the product of the images
of `x` over all the `K`-embeddings `σ` of `L` into `E`. -/
theorem norm_eq_prod_embeddings [FiniteDimensional K L] [Algebra.IsSeparable K L] [IsAlgClosed E]
(x : L) : algebraMap K E (norm K x) = ∏ σ : L →ₐ[K] E, σ x := by
have hx := Algebra.IsSeparable.isIntegral K x
rw [norm_eq_norm_adjoin K x, RingHom.map_pow, ← adjoin.powerBasis_gen hx,
norm_eq_prod_embeddings_gen E (adjoin.powerBasis hx) (IsAlgClosed.splits_codomain _)]
· exact (prod_embeddings_eq_finrank_pow L (L := K⟮x⟯) E (adjoin.powerBasis hx)).symm
· haveI := Algebra.isSeparable_tower_bot_of_isSeparable K K⟮x⟯ L
exact Algebra.IsSeparable.isSeparable K _
theorem norm_eq_prod_automorphisms [FiniteDimensional K L] [IsGalois K L] (x : L) :
algebraMap K L (norm K x) = ∏ σ : L ≃ₐ[K] L, σ x := by
apply NoZeroSMulDivisors.algebraMap_injective L (AlgebraicClosure L)
rw [map_prod (algebraMap L (AlgebraicClosure L))]
rw [← Fintype.prod_equiv (Normal.algHomEquivAut K (AlgebraicClosure L) L)]
· rw [← norm_eq_prod_embeddings _ _ x, ← IsScalarTower.algebraMap_apply]
· intro σ
simp only [Normal.algHomEquivAut, AlgHom.restrictNormal', Equiv.coe_fn_mk,
AlgEquiv.coe_ofBijective, AlgHom.restrictNormal_commutes, id.map_eq_id, RingHom.id_apply]
theorem isIntegral_norm [Algebra R L] [Algebra R K] [IsScalarTower R K L] [Algebra.IsSeparable K L]
[FiniteDimensional K L] {x : L} (hx : IsIntegral R x) : IsIntegral R (norm K x) := by
have hx' : IsIntegral K x := hx.tower_top
rw [← isIntegral_algebraMap_iff (algebraMap K (AlgebraicClosure L)).injective, norm_eq_prod_roots]
· refine (IsIntegral.multiset_prod fun y hy => ?_).pow _
rw [mem_roots_map (minpoly.ne_zero hx')] at hy
use minpoly R x, minpoly.monic hx
rw [← aeval_def] at hy ⊢
exact minpoly.aeval_of_isScalarTower R x y hy
· apply IsAlgClosed.splits_codomain
lemma norm_eq_of_algEquiv [Ring T] [Algebra R T] (e : S ≃ₐ[R] T) (x) :
Algebra.norm R (e x) = Algebra.norm R x := by
simp_rw [Algebra.norm_apply, ← LinearMap.det_conj _ e.toLinearEquiv]; congr; ext; simp
lemma norm_eq_of_ringEquiv {A B C : Type*} [CommRing A] [CommRing B] [Ring C]
[Algebra A C] [Algebra B C] (e : A ≃+* B) (he : (algebraMap B C).comp e = algebraMap A C)
(x : C) :
e (Algebra.norm A x) = Algebra.norm B x := by
classical
by_cases h : ∃ s : Finset C, Nonempty (Basis s B C)
· obtain ⟨s, ⟨b⟩⟩ := h
letI : Algebra A B := RingHom.toAlgebra e
letI : IsScalarTower A B C := IsScalarTower.of_algebraMap_eq' he.symm
rw [Algebra.norm_eq_matrix_det b,
Algebra.norm_eq_matrix_det (b.mapCoeffs e.symm (by simp [Algebra.smul_def, ← he])),
e.map_det]
congr
ext i j
simp [leftMulMatrix_apply, LinearMap.toMatrix_apply]
rw [norm_eq_one_of_not_exists_basis _ h, norm_eq_one_of_not_exists_basis, _root_.map_one]
intro ⟨s, ⟨b⟩⟩
exact h ⟨s, ⟨b.mapCoeffs e (by simp [Algebra.smul_def, ← he])⟩⟩
lemma norm_eq_of_equiv_equiv {A₁ B₁ A₂ B₂ : Type*} [CommRing A₁] [Ring B₁]
[CommRing A₂] [Ring B₂] [Algebra A₁ B₁] [Algebra A₂ B₂] (e₁ : A₁ ≃+* A₂) (e₂ : B₁ ≃+* B₂)
(he : RingHom.comp (algebraMap A₂ B₂) ↑e₁ = RingHom.comp ↑e₂ (algebraMap A₁ B₁)) (x) :
Algebra.norm A₁ x = e₁.symm (Algebra.norm A₂ (e₂ x)) := by
letI := (RingHom.comp (e₂ : B₁ →+* B₂) (algebraMap A₁ B₁)).toAlgebra' ?_
· let e' : B₁ ≃ₐ[A₁] B₂ := { e₂ with commutes' := fun _ ↦ rfl }
rw [← Algebra.norm_eq_of_ringEquiv e₁ he, ← Algebra.norm_eq_of_algEquiv e']
simp [e']
intros c x
apply e₂.symm.injective
simp only [RingHom.coe_comp, RingHom.coe_coe, Function.comp_apply, _root_.map_mul,
RingEquiv.symm_apply_apply, commutes]
variable {F} (L)
-- TODO. Generalize this proof to rings
theorem norm_norm [Algebra L F] [IsScalarTower K L F] [Algebra.IsSeparable K F] (x : F) :
norm K (norm L x) = norm K x := by
by_cases hKF : FiniteDimensional K F
· let A := AlgebraicClosure K
apply (algebraMap K A).injective
haveI : FiniteDimensional L F := FiniteDimensional.right K L F
haveI : FiniteDimensional K L := FiniteDimensional.left K L F
haveI : Algebra.IsSeparable K L := Algebra.isSeparable_tower_bot_of_isSeparable K L F
haveI : Algebra.IsSeparable L F := Algebra.isSeparable_tower_top_of_isSeparable K L F
letI : ∀ σ : L →ₐ[K] A,
haveI := σ.toRingHom.toAlgebra
Fintype (F →ₐ[L] A) := fun _ => inferInstance
rw [norm_eq_prod_embeddings K A (_ : F),
Fintype.prod_equiv algHomEquivSigma (fun σ : F →ₐ[K] A => σ x)
(fun π : Σ f : L →ₐ[K] A, _ => (π.2 : F → A) x) fun _ => rfl]
suffices ∀ σ : L →ₐ[K] A,
haveI := σ.toRingHom.toAlgebra
∏ π : F →ₐ[L] A, π x = σ (norm L x)
by simp_rw [← Finset.univ_sigma_univ, Finset.prod_sigma, this, norm_eq_prod_embeddings]
intro σ
letI : Algebra L A := σ.toRingHom.toAlgebra
rw [← norm_eq_prod_embeddings L A (_ : F)]
simp [RingHom.algebraMap_toAlgebra]
· rw [norm_eq_one_of_not_module_finite hKF]
by_cases hKL : FiniteDimensional K L
· have hLF : ¬FiniteDimensional L F := by
refine (mt ?_) hKF
intro hKF
exact FiniteDimensional.trans K L F
rw [norm_eq_one_of_not_module_finite hLF, _root_.map_one]
· rw [norm_eq_one_of_not_module_finite hKL]
end EqProdEmbeddings
end Algebra
|
RingTheory\Norm\Defs.lean | /-
Copyright (c) 2021 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen
-/
import Mathlib.LinearAlgebra.Determinant
/-!
# Norm for (finite) ring extensions
Suppose we have an `R`-algebra `S` with a finite basis. For each `s : S`,
the determinant of the linear map given by multiplying by `s` gives information
about the roots of the minimal polynomial of `s` over `R`.
## Implementation notes
Typically, the norm is defined specifically for finite field extensions.
The current definition is as general as possible and the assumption that we have
fields or that the extension is finite is added to the lemmas as needed.
We only define the norm for left multiplication (`Algebra.leftMulMatrix`,
i.e. `LinearMap.mulLeft`).
For now, the definitions assume `S` is commutative, so the choice doesn't
matter anyway.
See also `Algebra.trace`, which is defined similarly as the trace of
`Algebra.leftMulMatrix`.
## References
* https://en.wikipedia.org/wiki/Field_norm
-/
universe u v w
variable {R S T : Type*} [CommRing R] [Ring S]
variable [Algebra R S]
variable {K L F : Type*} [Field K] [Field L] [Field F]
variable [Algebra K L] [Algebra K F]
variable {ι : Type w}
open FiniteDimensional
open LinearMap
open Matrix Polynomial
open scoped Matrix
namespace Algebra
variable (R)
/-- The norm of an element `s` of an `R`-algebra is the determinant of `(*) s`. -/
noncomputable def norm : S →* R :=
LinearMap.det.comp (lmul R S).toRingHom.toMonoidHom
theorem norm_apply (x : S) : norm R x = LinearMap.det (lmul R S x) := rfl
theorem norm_eq_one_of_not_exists_basis (h : ¬∃ s : Finset S, Nonempty (Basis s R S)) (x : S) :
norm R x = 1 := by rw [norm_apply, LinearMap.det]; split_ifs <;> trivial
variable {R}
theorem norm_eq_one_of_not_module_finite (h : ¬Module.Finite R S) (x : S) : norm R x = 1 := by
refine norm_eq_one_of_not_exists_basis _ (mt ?_ h) _
rintro ⟨s, ⟨b⟩⟩
exact Module.Finite.of_basis b
-- Can't be a `simp` lemma because it depends on a choice of basis
theorem norm_eq_matrix_det [Fintype ι] [DecidableEq ι] (b : Basis ι R S) (s : S) :
norm R s = Matrix.det (Algebra.leftMulMatrix b s) := by
rw [norm_apply, ← LinearMap.det_toMatrix b, ← toMatrix_lmul_eq]; rfl
/-- If `x` is in the base ring `K`, then the norm is `x ^ [L : K]`. -/
theorem norm_algebraMap_of_basis [Fintype ι] (b : Basis ι R S) (x : R) :
norm R (algebraMap R S x) = x ^ Fintype.card ι := by
haveI := Classical.decEq ι
rw [norm_apply, ← det_toMatrix b, lmul_algebraMap]
convert @det_diagonal _ _ _ _ _ fun _ : ι => x
· ext (i j); rw [toMatrix_lsmul]
· rw [Finset.prod_const, Finset.card_univ]
/-- If `x` is in the base field `K`, then the norm is `x ^ [L : K]`.
(If `L` is not finite-dimensional over `K`, then `norm = 1 = x ^ 0 = x ^ (finrank L K)`.)
-/
@[simp]
protected theorem norm_algebraMap {L : Type*} [Ring L] [Algebra K L] (x : K) :
norm K (algebraMap K L x) = x ^ finrank K L := by
by_cases H : ∃ s : Finset L, Nonempty (Basis s K L)
· rw [norm_algebraMap_of_basis H.choose_spec.some, finrank_eq_card_basis H.choose_spec.some]
· rw [norm_eq_one_of_not_exists_basis K H, finrank_eq_zero_of_not_exists_basis, pow_zero]
rintro ⟨s, ⟨b⟩⟩
exact H ⟨s, ⟨b⟩⟩
end Algebra
|
RingTheory\OreLocalization\Basic.lean | /-
Copyright (c) 2022 Jakob von Raumer. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jakob von Raumer, Kevin Klinge, Andrew Yang
-/
import Mathlib.Algebra.Group.Submonoid.DistribMulAction
import Mathlib.RingTheory.OreLocalization.OreSet
/-!
# Localization over left Ore sets.
This file defines the localization of a monoid over a left Ore set and proves its universal
mapping property.
## Notations
Introduces the notation `R[S⁻¹]` for the Ore localization of a monoid `R` at a right Ore
subset `S`. Also defines a new heterogeneous division notation `r /ₒ s` for a numerator `r : R` and
a denominator `s : S`.
## References
* <https://ncatlab.org/nlab/show/Ore+localization>
* [Zoran Škoda, *Noncommutative localization in noncommutative geometry*][skoda2006]
## Tags
localization, Ore, non-commutative
-/
universe u
open OreLocalization
namespace OreLocalization
variable {R : Type*} [Monoid R] (S : Submonoid R) [OreSet S] (X) [MulAction R X]
/-- The setoid on `R × S` used for the Ore localization. -/
@[to_additive AddOreLocalization.oreEqv "The setoid on `R × S` used for the Ore localization."]
def oreEqv : Setoid (X × S) where
r rs rs' := ∃ (u : S) (v : R), u • rs'.1 = v • rs.1 ∧ u * rs'.2 = v * rs.2
iseqv := by
refine ⟨fun _ => ⟨1, 1, by simp⟩, ?_, ?_⟩
· rintro ⟨r, s⟩ ⟨r', s'⟩ ⟨u, v, hru, hsu⟩; dsimp only at *
rcases oreCondition (s : R) s' with ⟨r₂, s₂, h₁⟩
rcases oreCondition r₂ u with ⟨r₃, s₃, h₂⟩
have : r₃ * v * s = s₃ * s₂ * s := by
-- Porting note: the proof used `assoc_rw`
rw [mul_assoc _ (s₂ : R), h₁, ← mul_assoc, h₂, mul_assoc, ← hsu, ← mul_assoc]
rcases ore_right_cancel (r₃ * v) (s₃ * s₂) s this with ⟨w, hw⟩
refine ⟨w * (s₃ * s₂), w * (r₃ * u), ?_, ?_⟩ <;>
simp only [Submonoid.coe_mul, Submonoid.smul_def, ← hw]
· simp only [mul_smul, hru, ← Submonoid.smul_def]
· simp only [mul_assoc, hsu]
· rintro ⟨r₁, s₁⟩ ⟨r₂, s₂⟩ ⟨r₃, s₃⟩ ⟨u, v, hur₁, hs₁u⟩ ⟨u', v', hur₂, hs₂u⟩
rcases oreCondition v' u with ⟨r', s', h⟩; dsimp only at *
refine ⟨s' * u', r' * v, ?_, ?_⟩ <;>
simp only [Submonoid.smul_def, Submonoid.coe_mul, mul_smul, mul_assoc] at *
· rw [hur₂, smul_smul, h, mul_smul, hur₁]
· rw [hs₂u, ← mul_assoc, h, mul_assoc, hs₁u]
end OreLocalization
/-- The Ore localization of a monoid and a submonoid fulfilling the Ore condition. -/
@[to_additive AddOreLocalization "The Ore localization of an additive monoid and a submonoid
fulfilling the Ore condition."]
def OreLocalization {R : Type*} [Monoid R] (S : Submonoid R) [OreSet S]
(X : Type*) [MulAction R X] :=
Quotient (OreLocalization.oreEqv S X)
namespace OreLocalization
section Monoid
variable (R : Type*) [Monoid R] (S : Submonoid R) [OreSet S]
@[inherit_doc OreLocalization]
scoped syntax:1075 term noWs atomic("[" term "⁻¹" noWs "]") : term
macro_rules | `($R[$S⁻¹]) => ``(OreLocalization $S $R)
attribute [local instance] oreEqv
variable {R S}
variable {X} [MulAction R X]
/-- The division in the Ore localization `X[S⁻¹]`, as a fraction of an element of `X` and `S`. -/
@[to_additive "The subtraction in the Ore localization,
as a difference of an element of `X` and `S`."]
def oreDiv (r : X) (s : S) : X[S⁻¹] :=
Quotient.mk' (r, s)
@[inherit_doc]
infixl:70 " /ₒ " => oreDiv
@[inherit_doc]
infixl:65 " -ₒ " => _root_.AddOreLocalization.oreSub
@[to_additive (attr := elab_as_elim, cases_eliminator, induction_eliminator)]
protected theorem ind {β : X[S⁻¹] → Prop}
(c : ∀ (r : X) (s : S), β (r /ₒ s)) : ∀ q, β q := by
apply Quotient.ind
rintro ⟨r, s⟩
exact c r s
@[to_additive]
theorem oreDiv_eq_iff {r₁ r₂ : X} {s₁ s₂ : S} :
r₁ /ₒ s₁ = r₂ /ₒ s₂ ↔ ∃ (u : S) (v : R), u • r₂ = v • r₁ ∧ u * s₂ = v * s₁ :=
Quotient.eq''
/-- A fraction `r /ₒ s` is equal to its expansion by an arbitrary factor `t` if `t * s ∈ S`. -/
@[to_additive "A difference `r -ₒ s` is equal to its expansion by an
arbitrary translation `t` if `t + s ∈ S`."]
protected theorem expand (r : X) (s : S) (t : R) (hst : t * (s : R) ∈ S) :
r /ₒ s = t • r /ₒ ⟨t * s, hst⟩ := by
apply Quotient.sound
exact ⟨s, s * t, by rw [mul_smul, Submonoid.smul_def], by rw [← mul_assoc]⟩
/-- A fraction is equal to its expansion by a factor from `S`. -/
@[to_additive "A difference is equal to its expansion by a summand from `S`."]
protected theorem expand' (r : X) (s s' : S) : r /ₒ s = s' • r /ₒ (s' * s) :=
OreLocalization.expand r s s' (by norm_cast; apply SetLike.coe_mem)
/-- Fractions which differ by a factor of the numerator can be proven equal if
those factors expand to equal elements of `R`. -/
@[to_additive "Differences whose minuends differ by a common summand can be proven equal if
those summands expand to equal elements of `R`."]
protected theorem eq_of_num_factor_eq {r r' r₁ r₂ : R} {s t : S} (h : t * r = t * r') :
r₁ * r * r₂ /ₒ s = r₁ * r' * r₂ /ₒ s := by
rcases oreCondition r₁ t with ⟨r₁', t', hr₁⟩
rw [OreLocalization.expand' _ s t', OreLocalization.expand' _ s t']
congr 1
-- Porting note (#11215): TODO: use `assoc_rw`?
calc (t' : R) * (r₁ * r * r₂)
= t' * r₁ * r * r₂ := by simp [← mul_assoc]
_ = r₁' * t * r * r₂ := by rw [hr₁]
_ = r₁' * (t * r) * r₂ := by simp [← mul_assoc]
_ = r₁' * (t * r') * r₂ := by rw [h]
_ = r₁' * t * r' * r₂ := by simp [← mul_assoc]
_ = t' * r₁ * r' * r₂ := by rw [hr₁]
_ = t' * (r₁ * r' * r₂) := by simp [← mul_assoc]
/-- A function or predicate over `X` and `S` can be lifted to `X[S⁻¹]` if it is invariant
under expansion on the left. -/
@[to_additive "A function or predicate over `X` and `S` can be lifted to the localizaton if it is
invariant under expansion on the left."]
def liftExpand {C : Sort*} (P : X → S → C)
(hP : ∀ (r : X) (t : R) (s : S) (ht : t * s ∈ S), P r s = P (t • r) ⟨t * s, ht⟩) :
X[S⁻¹] → C :=
Quotient.lift (fun p : X × S => P p.1 p.2) fun (r₁, s₁) (r₂, s₂) ⟨u, v, hr₂, hs₂⟩ => by
dsimp at *
have s₁vS : v * s₁ ∈ S := by
rw [← hs₂, ← S.coe_mul]
exact SetLike.coe_mem (u * s₂)
replace hs₂ : u * s₂ = ⟨_, s₁vS⟩ := by ext; simp [hs₂]
rw [hP r₁ v s₁ s₁vS, hP r₂ u s₂ (by norm_cast; rwa [hs₂]), ← hr₂]
simp only [← hs₂]; rfl
@[to_additive (attr := simp)]
theorem liftExpand_of {C : Sort*} {P : X → S → C}
{hP : ∀ (r : X) (t : R) (s : S) (ht : t * s ∈ S), P r s = P (t • r) ⟨t * s, ht⟩} (r : X)
(s : S) : liftExpand P hP (r /ₒ s) = P r s :=
rfl
/-- A version of `liftExpand` used to simultaneously lift functions with two arguments
in `X[S⁻¹]`. -/
@[to_additive "A version of `liftExpand` used to simultaneously lift functions with two arguments"]
def lift₂Expand {C : Sort*} (P : X → S → X → S → C)
(hP :
∀ (r₁ : X) (t₁ : R) (s₁ : S) (ht₁ : t₁ * s₁ ∈ S) (r₂ : X) (t₂ : R) (s₂ : S)
(ht₂ : t₂ * s₂ ∈ S),
P r₁ s₁ r₂ s₂ = P (t₁ • r₁) ⟨t₁ * s₁, ht₁⟩ (t₂ • r₂) ⟨t₂ * s₂, ht₂⟩) :
X[S⁻¹] → X[S⁻¹] → C :=
liftExpand
(fun r₁ s₁ => liftExpand (P r₁ s₁) fun r₂ t₂ s₂ ht₂ => by
have := hP r₁ 1 s₁ (by simp) r₂ t₂ s₂ ht₂
simp [this])
fun r₁ t₁ s₁ ht₁ => by
ext x; induction' x with r₂ s₂
dsimp only
rw [liftExpand_of, liftExpand_of, hP r₁ t₁ s₁ ht₁ r₂ 1 s₂ (by simp)]; simp
@[to_additive (attr := simp)]
theorem lift₂Expand_of {C : Sort*} {P : X → S → X → S → C}
{hP :
∀ (r₁ : X) (t₁ : R) (s₁ : S) (ht₁ : t₁ * s₁ ∈ S) (r₂ : X) (t₂ : R) (s₂ : S)
(ht₂ : t₂ * s₂ ∈ S),
P r₁ s₁ r₂ s₂ = P (t₁ • r₁) ⟨t₁ * s₁, ht₁⟩ (t₂ • r₂) ⟨t₂ * s₂, ht₂⟩}
(r₁ : X) (s₁ : S) (r₂ : X) (s₂ : S) : lift₂Expand P hP (r₁ /ₒ s₁) (r₂ /ₒ s₂) = P r₁ s₁ r₂ s₂ :=
rfl
@[to_additive]
private def smul' (r₁ : R) (s₁ : S) (r₂ : X) (s₂ : S) : X[S⁻¹] :=
oreNum r₁ s₂ • r₂ /ₒ (oreDenom r₁ s₂ * s₁)
@[to_additive]
private theorem smul'_char (r₁ : R) (r₂ : X) (s₁ s₂ : S) (u : S) (v : R) (huv : u * r₁ = v * s₂) :
OreLocalization.smul' r₁ s₁ r₂ s₂ = v • r₂ /ₒ (u * s₁) := by
-- Porting note: `assoc_rw` was not ported yet
simp only [smul']
have h₀ := ore_eq r₁ s₂; set v₀ := oreNum r₁ s₂; set u₀ := oreDenom r₁ s₂
rcases oreCondition (u₀ : R) u with ⟨r₃, s₃, h₃⟩
have :=
calc
r₃ * v * s₂ = r₃ * (u * r₁) := by rw [mul_assoc, ← huv]
_ = s₃ * (u₀ * r₁) := by rw [← mul_assoc, ← mul_assoc, h₃]
_ = s₃ * v₀ * s₂ := by rw [mul_assoc, h₀]
rcases ore_right_cancel _ _ _ this with ⟨s₄, hs₄⟩
symm; rw [oreDiv_eq_iff]
use s₄ * s₃
use s₄ * r₃
simp only [Submonoid.coe_mul, Submonoid.smul_def, smul_eq_mul]
constructor
· rw [smul_smul, mul_assoc (c := v₀), ← hs₄]
simp only [smul_smul, mul_assoc]
· rw [← mul_assoc (b := (u₀ : R)), mul_assoc (c := (u₀ : R)), h₃]
simp only [mul_assoc]
/-- The multiplication on the Ore localization of monoids. -/
@[to_additive]
private def smul'' (r : R) (s : S) : X[S⁻¹] → X[S⁻¹] :=
liftExpand (smul' r s) fun r₁ r₂ s' hs => by
rcases oreCondition r s' with ⟨r₁', s₁', h₁⟩
rw [smul'_char _ _ _ _ _ _ h₁]
rcases oreCondition r ⟨_, hs⟩ with ⟨r₂', s₂', h₂⟩
rw [smul'_char _ _ _ _ _ _ h₂]
rcases oreCondition (s₁' : R) (s₂') with ⟨r₃', s₃', h₃⟩
have : s₃' * r₁' * s' = (r₃' * r₂' * r₂) * s' := by
rw [mul_assoc, ← h₁, ← mul_assoc, h₃, mul_assoc, h₂]
simp [mul_assoc]
rcases ore_right_cancel _ _ _ this with ⟨s₄', h₄⟩
have : (s₄' * r₃') * (s₂' * s) ∈ S := by
rw [mul_assoc, ← mul_assoc r₃', ← h₃]
exact (s₄' * (s₃' * s₁' * s)).2
rw [OreLocalization.expand' _ _ (s₄' * s₃'), OreLocalization.expand _ (s₂' * s) _ this]
simp only [Submonoid.smul_def, Submonoid.coe_mul, smul_smul, mul_assoc, h₄]
congr 1
ext; simp only [Submonoid.coe_mul, ← mul_assoc]
rw [mul_assoc (s₄' : R), h₃, ← mul_assoc]
/-- The scalar multiplication on the Ore localization of monoids. -/
@[to_additive (attr := irreducible)
"the vector addition on the Ore localization of additive monoids."]
protected def smul : R[S⁻¹] → X[S⁻¹] → X[S⁻¹] :=
liftExpand smul'' fun r₁ r₂ s hs => by
ext x
induction' x with x s₂
show OreLocalization.smul' r₁ s x s₂ = OreLocalization.smul' (r₂ * r₁) ⟨_, hs⟩ x s₂
rcases oreCondition r₁ s₂ with ⟨r₁', s₁', h₁⟩
rw [smul'_char _ _ _ _ _ _ h₁]
rcases oreCondition (r₂ * r₁) s₂ with ⟨r₂', s₂', h₂⟩
rw [smul'_char _ _ _ _ _ _ h₂]
rcases oreCondition (s₂' * r₂) (s₁') with ⟨r₃', s₃', h₃⟩
have : s₃' * r₂' * s₂ = r₃' * r₁' * s₂ := by
rw [mul_assoc, ← h₂, ← mul_assoc _ r₂, ← mul_assoc, h₃, mul_assoc, h₁, mul_assoc]
rcases ore_right_cancel _ _ _ this with ⟨s₄', h₄⟩
have : (s₄' * r₃') * (s₁' * s) ∈ S := by
rw [← mul_assoc, mul_assoc _ r₃', ← h₃, ← mul_assoc, ← mul_assoc, mul_assoc]
exact mul_mem (s₄' * s₃' * s₂').2 hs
rw [OreLocalization.expand' (r₂' • x) _ (s₄' * s₃'), OreLocalization.expand _ _ _ this]
simp only [Submonoid.smul_def, Submonoid.coe_mul, smul_smul, mul_assoc, h₄]
congr 1
ext; simp only [Submonoid.coe_mul, ← mul_assoc]
rw [mul_assoc _ r₃', ← h₃, ← mul_assoc, ← mul_assoc]
@[to_additive]
instance : SMul R[S⁻¹] X[S⁻¹] :=
⟨OreLocalization.smul⟩
@[to_additive]
instance : Mul R[S⁻¹] :=
⟨OreLocalization.smul⟩
@[to_additive]
theorem oreDiv_smul_oreDiv {r₁ : R} {r₂ : X} {s₁ s₂ : S} :
(r₁ /ₒ s₁) • (r₂ /ₒ s₂) = oreNum r₁ s₂ • r₂ /ₒ (oreDenom r₁ s₂ * s₁) := by
with_unfolding_all rfl
@[to_additive]
theorem oreDiv_mul_oreDiv {r₁ : R} {r₂ : R} {s₁ s₂ : S} :
(r₁ /ₒ s₁) * (r₂ /ₒ s₂) = oreNum r₁ s₂ * r₂ /ₒ (oreDenom r₁ s₂ * s₁) := by
with_unfolding_all rfl
/-- A characterization lemma for the scalar multiplication on the Ore localization,
allowing for a choice of Ore numerator and Ore denominator. -/
@[to_additive "A characterization lemma for the vector addition on the Ore localization,
allowing for a choice of Ore minuend and Ore subtrahend."]
theorem oreDiv_smul_char (r₁ : R) (r₂ : X) (s₁ s₂ : S) (r' : R) (s' : S) (huv : s' * r₁ = r' * s₂) :
(r₁ /ₒ s₁) • (r₂ /ₒ s₂) = r' • r₂ /ₒ (s' * s₁) := by
with_unfolding_all exact smul'_char r₁ r₂ s₁ s₂ s' r' huv
/-- A characterization lemma for the multiplication on the Ore localization, allowing for a choice
of Ore numerator and Ore denominator. -/
@[to_additive "A characterization lemma for the addition on the Ore localization,
allowing for a choice of Ore minuend and Ore subtrahend."]
theorem oreDiv_mul_char (r₁ r₂ : R) (s₁ s₂ : S) (r' : R) (s' : S) (huv : s' * r₁ = r' * s₂) :
r₁ /ₒ s₁ * (r₂ /ₒ s₂) = r' * r₂ /ₒ (s' * s₁) := by
with_unfolding_all exact smul'_char r₁ r₂ s₁ s₂ s' r' huv
/-- Another characterization lemma for the scalar multiplication on the Ore localizaion delivering
Ore witnesses and conditions bundled in a sigma type. -/
@[to_additive "Another characterization lemma for the vector addition on the
Ore localizaion delivering Ore witnesses and conditions bundled in a sigma type."]
def oreDivSMulChar' (r₁ : R) (r₂ : X) (s₁ s₂ : S) :
Σ'r' : R, Σ's' : S, s' * r₁ = r' * s₂ ∧ (r₁ /ₒ s₁) • (r₂ /ₒ s₂) = r' • r₂ /ₒ (s' * s₁) :=
⟨oreNum r₁ s₂, oreDenom r₁ s₂, ore_eq r₁ s₂, oreDiv_smul_oreDiv⟩
/-- Another characterization lemma for the multiplication on the Ore localizaion delivering
Ore witnesses and conditions bundled in a sigma type. -/
@[to_additive "Another characterization lemma for the addition on the Ore localizaion delivering
Ore witnesses and conditions bundled in a sigma type."]
def oreDivMulChar' (r₁ r₂ : R) (s₁ s₂ : S) :
Σ'r' : R, Σ's' : S, s' * r₁ = r' * s₂ ∧ r₁ /ₒ s₁ * (r₂ /ₒ s₂) = r' * r₂ /ₒ (s' * s₁) :=
⟨oreNum r₁ s₂, oreDenom r₁ s₂, ore_eq r₁ s₂, oreDiv_mul_oreDiv⟩
/-- `1` in the localization, defined as `1 /ₒ 1`. -/
@[to_additive (attr := irreducible) "`0` in the additive localization, defined as `0 -ₒ 0`."]
protected def one : R[S⁻¹] := 1 /ₒ 1
@[to_additive]
instance : One R[S⁻¹] :=
⟨OreLocalization.one⟩
@[to_additive]
protected theorem one_def : (1 : R[S⁻¹]) = 1 /ₒ 1 := by
with_unfolding_all rfl
@[to_additive]
instance : Inhabited R[S⁻¹] :=
⟨1⟩
@[to_additive (attr := simp)]
protected theorem div_eq_one' {r : R} (hr : r ∈ S) : r /ₒ ⟨r, hr⟩ = 1 := by
rw [OreLocalization.one_def, oreDiv_eq_iff]
exact ⟨⟨r, hr⟩, 1, by simp, by simp⟩
@[to_additive (attr := simp)]
protected theorem div_eq_one {s : S} : (s : R) /ₒ s = 1 :=
OreLocalization.div_eq_one' _
@[to_additive]
protected theorem one_smul (x : X[S⁻¹]) : (1 : R[S⁻¹]) • x = x := by
induction' x with r s
simp [OreLocalization.one_def, oreDiv_smul_char 1 r 1 s 1 s (by simp)]
@[to_additive]
protected theorem one_mul (x : R[S⁻¹]) : 1 * x = x :=
OreLocalization.one_smul x
@[to_additive]
protected theorem mul_one (x : R[S⁻¹]) : x * 1 = x := by
induction' x with r s
simp [OreLocalization.one_def, oreDiv_mul_char r (1 : R) s (1 : S) r 1 (by simp)]
@[to_additive]
protected theorem mul_smul (x y : R[S⁻¹]) (z : X[S⁻¹]) : (x * y) • z = x • y • z := by
-- Porting note: `assoc_rw` was not ported yet
induction' x with r₁ s₁
induction' y with r₂ s₂
induction' z with r₃ s₃
rcases oreDivMulChar' r₁ r₂ s₁ s₂ with ⟨ra, sa, ha, ha'⟩; rw [ha']; clear ha'
rcases oreDivSMulChar' r₂ r₃ s₂ s₃ with ⟨rb, sb, hb, hb'⟩; rw [hb']; clear hb'
rcases oreCondition ra sb with ⟨rc, sc, hc⟩
rw [oreDiv_smul_char (ra * r₂) r₃ (sa * s₁) s₃ (rc * rb) sc]; swap
· rw [← mul_assoc _ ra, hc, mul_assoc, hb, ← mul_assoc]
rw [← mul_assoc, mul_smul]
symm; apply oreDiv_smul_char
rw [Submonoid.coe_mul, Submonoid.coe_mul, ← mul_assoc, ← hc, mul_assoc _ ra, ← ha, mul_assoc]
@[to_additive]
protected theorem mul_assoc (x y z : R[S⁻¹]) : x * y * z = x * (y * z) :=
OreLocalization.mul_smul x y z
/-- `npow` of `OreLocalization` -/
@[to_additive (attr := irreducible) "`nsmul` of `AddOreLocalization`"]
protected def npow : ℕ → R[S⁻¹] → R[S⁻¹] := npowRec
unseal OreLocalization.npow in
@[to_additive]
instance : Monoid R[S⁻¹] where
one_mul := OreLocalization.one_mul
mul_one := OreLocalization.mul_one
mul_assoc := OreLocalization.mul_assoc
npow := OreLocalization.npow
@[to_additive]
instance instMulActionOreLocalization : MulAction R[S⁻¹] X[S⁻¹] where
one_smul := OreLocalization.one_smul
mul_smul := OreLocalization.mul_smul
@[to_additive]
protected theorem mul_inv (s s' : S) : ((s : R) /ₒ s') * ((s' : R) /ₒ s) = 1 := by
simp [oreDiv_mul_char (s : R) s' s' s 1 1 (by simp)]
@[to_additive (attr := simp)]
protected theorem one_div_smul {r : X} {s t : S} : ((1 : R) /ₒ t) • (r /ₒ s) = r /ₒ (s * t) := by
simp [oreDiv_smul_char 1 r t s 1 s (by simp)]
@[to_additive (attr := simp)]
protected theorem one_div_mul {r : R} {s t : S} : (1 /ₒ t) * (r /ₒ s) = r /ₒ (s * t) := by
simp [oreDiv_mul_char 1 r t s 1 s (by simp)]
@[to_additive (attr := simp)]
protected theorem smul_cancel {r : X} {s t : S} : ((s : R) /ₒ t) • (r /ₒ s) = r /ₒ t := by
simp [oreDiv_smul_char s.1 r t s 1 1 (by simp)]
@[to_additive (attr := simp)]
protected theorem mul_cancel {r : R} {s t : S} : ((s : R) /ₒ t) * (r /ₒ s) = r /ₒ t := by
simp [oreDiv_mul_char s.1 r t s 1 1 (by simp)]
@[to_additive (attr := simp)]
protected theorem smul_cancel' {r₁ : R} {r₂ : X} {s t : S} :
((r₁ * s) /ₒ t) • (r₂ /ₒ s) = (r₁ • r₂) /ₒ t := by
simp [oreDiv_smul_char (r₁ * s) r₂ t s r₁ 1 (by simp)]
@[to_additive (attr := simp)]
protected theorem mul_cancel' {r₁ r₂ : R} {s t : S} :
((r₁ * s) /ₒ t) * (r₂ /ₒ s) = (r₁ * r₂) /ₒ t := by
simp [oreDiv_mul_char (r₁ * s) r₂ t s r₁ 1 (by simp)]
@[to_additive (attr := simp)]
theorem smul_div_one {p : R} {r : X} {s : S} : (p /ₒ s) • (r /ₒ 1) = (p • r) /ₒ s := by
simp [oreDiv_smul_char p r s 1 p 1 (by simp)]
@[to_additive (attr := simp)]
theorem mul_div_one {p r : R} {s : S} : (p /ₒ s) * (r /ₒ 1) = (p * r) /ₒ s := by
--TODO use coercion r ↦ r /ₒ 1
simp [oreDiv_mul_char p r s 1 p 1 (by simp)]
/-- The fraction `s /ₒ 1` as a unit in `R[S⁻¹]`, where `s : S`. -/
@[to_additive "The difference `s -ₒ 0` as a an additive unit."]
def numeratorUnit (s : S) : Units R[S⁻¹] where
val := (s : R) /ₒ 1
inv := (1 : R) /ₒ s
val_inv := OreLocalization.mul_inv s 1
inv_val := OreLocalization.mul_inv 1 s
/-- The multiplicative homomorphism from `R` to `R[S⁻¹]`, mapping `r : R` to the
fraction `r /ₒ 1`. -/
@[to_additive "The additive homomorphism from `R` to `AddOreLocalization R S`,
mapping `r : R` to the difference `r -ₒ 0`."]
def numeratorHom : R →* R[S⁻¹] where
toFun r := r /ₒ 1
map_one' := by with_unfolding_all rfl
map_mul' _ _ := mul_div_one.symm
@[to_additive]
theorem numeratorHom_apply {r : R} : numeratorHom r = r /ₒ (1 : S) :=
rfl
@[to_additive]
theorem numerator_isUnit (s : S) : IsUnit (numeratorHom (s : R) : R[S⁻¹]) :=
⟨numeratorUnit s, rfl⟩
section UMP
variable {T : Type*} [Monoid T]
variable (f : R →* T) (fS : S →* Units T)
/-- The universal lift from a morphism `R →* T`, which maps elements of `S` to units of `T`,
to a morphism `R[S⁻¹] →* T`. -/
@[to_additive "The universal lift from a morphism `R →+ T`, which maps elements of `S` to
additive-units of `T`, to a morphism `AddOreLocalization R S →+ T`."]
def universalMulHom (hf : ∀ s : S, f s = fS s) : R[S⁻¹] →* T where
-- Porting note(#12129): additional beta reduction needed
toFun x :=
x.liftExpand (fun r s => ((fS s)⁻¹ : Units T) * f r) fun r t s ht => by
simp only [smul_eq_mul]
have : (fS ⟨t * s, ht⟩ : T) = f t * fS s := by
simp only [← hf, MonoidHom.map_mul]
conv_rhs =>
rw [MonoidHom.map_mul, ← one_mul (f r), ← Units.val_one, ← mul_right_inv (fS s)]
rw [Units.val_mul, mul_assoc, ← mul_assoc _ (fS s : T), ← this, ← mul_assoc]
simp only [one_mul, Units.inv_mul]
map_one' := by beta_reduce; rw [OreLocalization.one_def, liftExpand_of]; simp
map_mul' x y := by
-- Porting note: `simp only []` required, not just for beta reductions
beta_reduce
simp only [] -- TODO more!
induction' x with r₁ s₁
induction' y with r₂ s₂
rcases oreDivMulChar' r₁ r₂ s₁ s₂ with ⟨ra, sa, ha, ha'⟩; rw [ha']; clear ha'
rw [liftExpand_of, liftExpand_of, liftExpand_of, Units.inv_mul_eq_iff_eq_mul, map_mul, map_mul,
Units.val_mul, mul_assoc, ← mul_assoc (fS s₁ : T), ← mul_assoc (fS s₁ : T), Units.mul_inv,
one_mul, ← hf, ← mul_assoc, ← map_mul _ _ r₁, ha, map_mul, hf s₂, mul_assoc,
← mul_assoc (fS s₂ : T), (fS s₂).mul_inv, one_mul]
variable (hf : ∀ s : S, f s = fS s)
@[to_additive]
theorem universalMulHom_apply {r : R} {s : S} :
universalMulHom f fS hf (r /ₒ s) = ((fS s)⁻¹ : Units T) * f r :=
rfl
@[to_additive]
theorem universalMulHom_commutes {r : R} : universalMulHom f fS hf (numeratorHom r) = f r := by
simp [numeratorHom_apply, universalMulHom_apply]
/-- The universal morphism `universalMulHom` is unique. -/
@[to_additive "The universal morphism `universalAddHom` is unique."]
theorem universalMulHom_unique (φ : R[S⁻¹] →* T) (huniv : ∀ r : R, φ (numeratorHom r) = f r) :
φ = universalMulHom f fS hf := by
ext x; induction' x with r s
rw [universalMulHom_apply, ← huniv r, numeratorHom_apply, ← one_mul (φ (r /ₒ s)), ←
Units.val_one, ← mul_left_inv (fS s), Units.val_mul, mul_assoc, ← hf, ← huniv, ← φ.map_mul,
numeratorHom_apply, OreLocalization.mul_cancel]
end UMP
end Monoid
section SMul
variable {R R' M X : Type*} [Monoid M] {S : Submonoid M} [OreSet S] [MulAction M X]
variable [SMul R X] [SMul R M] [IsScalarTower R M M] [IsScalarTower R M X]
variable [SMul R' X] [SMul R' M] [IsScalarTower R' M M] [IsScalarTower R' M X]
variable [SMul R R'] [IsScalarTower R R' M]
/-- Scalar multiplication in a monoid localization. -/
@[to_additive (attr := irreducible) "Vector addition in an additive monoid localization."]
protected def hsmul (c : R) :
X[S⁻¹] → X[S⁻¹] :=
liftExpand (fun m s ↦ oreNum (c • 1) s • m /ₒ oreDenom (c • 1) s) (fun r t s ht ↦ by
dsimp only
rw [← mul_one (oreDenom (c • 1) s), ← oreDiv_smul_oreDiv, ← mul_one (oreDenom (c • 1) _),
← oreDiv_smul_oreDiv, ← OreLocalization.expand])
/- Warning: This gives an diamond on `SMul R[S⁻¹] M[S⁻¹][S⁻¹]`, but we will almost never localize
at the same monoid twice. -/
/- Although the definition does not require `IsScalarTower R M X`,
it does not make sense without it. -/
@[to_additive (attr := nolint unusedArguments)]
instance [SMul R X] [SMul R M] [IsScalarTower R M X] [IsScalarTower R M M] : SMul R (X[S⁻¹]) where
smul := OreLocalization.hsmul
@[to_additive]
theorem smul_oreDiv (r : R) (x : X) (s : S) :
r • (x /ₒ s) = oreNum (r • 1) s • x /ₒ oreDenom (r • 1) s := by with_unfolding_all rfl
@[to_additive (attr := simp)]
theorem oreDiv_one_smul (r : M) (x : X[S⁻¹]) : (r /ₒ (1 : S)) • x = r • x := by
induction' x using OreLocalization.ind with r' s
rw [smul_oreDiv, oreDiv_smul_oreDiv, mul_one, smul_eq_mul, mul_one]
@[to_additive]
theorem smul_one_smul (r : R) (x : X[S⁻¹]) : (r • 1 : M) • x = r • x := by
induction' x using OreLocalization.ind with r' s
simp only [smul_oreDiv, smul_eq_mul, mul_one]
@[to_additive]
theorem smul_one_oreDiv_one_smul (r : R) (x : X[S⁻¹]) :
((r • 1 : M) /ₒ (1 : S)) • x = r • x := by
rw [oreDiv_one_smul, smul_one_smul]
@[to_additive]
instance : IsScalarTower R R' X[S⁻¹] where
smul_assoc r m x := by
rw [← smul_one_oreDiv_one_smul, ← smul_one_oreDiv_one_smul, ← smul_one_oreDiv_one_smul,
← mul_smul, mul_div_one]
simp only [smul_eq_mul, mul_one, smul_mul_assoc, smul_assoc, one_mul]
@[to_additive]
instance [SMulCommClass R R' M] : SMulCommClass R R' X[S⁻¹] where
smul_comm r m x := by
rw [← smul_one_smul m, ← smul_assoc, smul_comm, smul_assoc, smul_one_smul]
@[to_additive]
instance : IsScalarTower R M[S⁻¹] X[S⁻¹] where
smul_assoc r m x := by
rw [← smul_one_oreDiv_one_smul, ← smul_one_oreDiv_one_smul, ← mul_smul, smul_eq_mul]
@[to_additive]
instance [SMulCommClass R M M] : SMulCommClass R M[S⁻¹] X[S⁻¹] where
smul_comm r x y := by
induction' x using OreLocalization.ind with r₁ s₁
induction' y using OreLocalization.ind with r₂ s₂
rw [← smul_one_oreDiv_one_smul, ← smul_one_oreDiv_one_smul, smul_smul, smul_smul,
mul_div_one, oreDiv_mul_char _ _ _ _ (r • 1) s₁ (by simp), mul_one]
simp
@[to_additive]
instance [SMul Rᵐᵒᵖ M] [SMul Rᵐᵒᵖ X] [IsScalarTower Rᵐᵒᵖ M M] [IsScalarTower Rᵐᵒᵖ M X]
[IsCentralScalar R M] : IsCentralScalar R X[S⁻¹] where
op_smul_eq_smul r x := by
rw [← smul_one_oreDiv_one_smul, ← smul_one_oreDiv_one_smul, op_smul_eq_smul]
@[to_additive]
instance {R} [Monoid R] [MulAction R M] [IsScalarTower R M M]
[MulAction R X] [IsScalarTower R M X] : MulAction R X[S⁻¹] where
one_smul := OreLocalization.ind fun x s ↦ by
rw [← smul_one_oreDiv_one_smul, one_smul, ← OreLocalization.one_def, one_smul]
mul_smul s₁ s₂ x := by rw [← smul_eq_mul, smul_assoc]
@[to_additive]
theorem smul_oreDiv_one (r : R) (x : X) : r • (x /ₒ (1 : S)) = (r • x) /ₒ (1 : S) := by
rw [← smul_one_oreDiv_one_smul, smul_div_one, smul_assoc, one_smul]
end SMul
section CommMonoid
variable {R : Type*} [CommMonoid R] {S : Submonoid R} [OreSet S]
@[to_additive]
theorem oreDiv_mul_oreDiv_comm {r₁ r₂ : R} {s₁ s₂ : S} :
r₁ /ₒ s₁ * (r₂ /ₒ s₂) = r₁ * r₂ /ₒ (s₁ * s₂) := by
rw [oreDiv_mul_char r₁ r₂ s₁ s₂ r₁ s₂ (by simp [mul_comm]), mul_comm s₂]
@[to_additive]
instance : CommMonoid R[S⁻¹] where
mul_comm := fun x y => by
induction' x with r₁ s₁
induction' y with r₂ s₂
rw [oreDiv_mul_oreDiv_comm, oreDiv_mul_oreDiv_comm, mul_comm r₁, mul_comm s₁]
end CommMonoid
section Zero
variable {R : Type*} [Monoid R] {S : Submonoid R} [OreSet S] {X : Type*} [Zero X]
variable [MulAction R X]
/-- `0` in the localization, defined as `0 /ₒ 1`. -/
@[irreducible]
protected def zero : X[S⁻¹] := 0 /ₒ 1
instance : Zero X[S⁻¹] :=
⟨OreLocalization.zero⟩
protected theorem zero_def : (0 : X[S⁻¹]) = 0 /ₒ 1 := by
with_unfolding_all rfl
end Zero
section MonoidWithZero
variable {R : Type*} [MonoidWithZero R] {S : Submonoid R} [OreSet S]
@[simp]
theorem zero_oreDiv' (s : S) : (0 : R) /ₒ s = 0 := by
rw [OreLocalization.zero_def, oreDiv_eq_iff]
exact ⟨s, 1, by simp [Submonoid.smul_def]⟩
instance : MonoidWithZero R[S⁻¹] where
zero_mul x := by
induction' x using OreLocalization.ind with r s
rw [OreLocalization.zero_def, oreDiv_mul_char 0 r 1 s 0 1 (by simp), zero_mul, one_mul]
mul_zero x := by
induction' x using OreLocalization.ind with r s
rw [OreLocalization.zero_def, mul_div_one, mul_zero, zero_oreDiv', zero_oreDiv']
end MonoidWithZero
section CommMonoidWithZero
variable {R : Type*} [CommMonoidWithZero R] {S : Submonoid R} [OreSet S]
instance : CommMonoidWithZero R[S⁻¹] where
__ := inferInstanceAs (MonoidWithZero R[S⁻¹])
__ := inferInstanceAs (CommMonoid R[S⁻¹])
end CommMonoidWithZero
section DistribMulAction
variable {R : Type*} [Monoid R] {S : Submonoid R} [OreSet S] {X : Type*} [AddMonoid X]
variable [DistribMulAction R X]
private def add'' (r₁ : X) (s₁ : S) (r₂ : X) (s₂ : S) : X[S⁻¹] :=
(oreDenom (s₁ : R) s₂ • r₁ + oreNum (s₁ : R) s₂ • r₂) /ₒ (oreDenom (s₁ : R) s₂ * s₁)
private theorem add''_char (r₁ : X) (s₁ : S) (r₂ : X) (s₂ : S) (rb : R) (sb : R)
(hb : sb * s₁ = rb * s₂) (h : sb * s₁ ∈ S) :
add'' r₁ s₁ r₂ s₂ = (sb • r₁ + rb • r₂) /ₒ ⟨sb * s₁, h⟩ := by
simp only [add'']
have ha := ore_eq (s₁ : R) s₂
generalize oreNum (s₁ : R) s₂ = ra at *
generalize oreDenom (s₁ : R) s₂ = sa at *
rw [oreDiv_eq_iff]
rcases oreCondition sb sa with ⟨rc, sc, hc⟩
have : sc * rb * s₂ = rc * ra * s₂ := by
rw [mul_assoc rc, ← ha, ← mul_assoc, ← hc, mul_assoc, mul_assoc, hb]
rcases ore_right_cancel _ _ s₂ this with ⟨sd, hd⟩
use sd * sc
use sd * rc
simp only [smul_add, smul_smul, Submonoid.smul_def, Submonoid.coe_mul]
constructor
· rw [mul_assoc _ _ rb, hd, mul_assoc, hc, mul_assoc, mul_assoc]
· rw [mul_assoc, ← mul_assoc (sc : R), hc, mul_assoc, mul_assoc]
attribute [local instance] OreLocalization.oreEqv
private def add' (r₂ : X) (s₂ : S) : X[S⁻¹] → X[S⁻¹] :=
(--plus tilde
Quotient.lift
fun r₁s₁ : X × S => add'' r₁s₁.1 r₁s₁.2 r₂ s₂) <| by
-- Porting note: `assoc_rw` & `noncomm_ring` were not ported yet
rintro ⟨r₁', s₁'⟩ ⟨r₁, s₁⟩ ⟨sb, rb, hb, hb'⟩
-- s*, r*
rcases oreCondition (s₁' : R) s₂ with ⟨rc, sc, hc⟩
--s~~, r~~
rcases oreCondition rb sc with ⟨rd, sd, hd⟩
-- s#, r#
dsimp at *
rw [add''_char _ _ _ _ rc sc hc (sc * s₁').2]
have : sd * sb * s₁ = rd * rc * s₂ := by
rw [mul_assoc, hb', ← mul_assoc, hd, mul_assoc, hc, ← mul_assoc]
rw [add''_char _ _ _ _ (rd * rc : R) (sd * sb) this (sd * sb * s₁).2]
rw [mul_smul, ← Submonoid.smul_def sb, hb, smul_smul, hd, oreDiv_eq_iff]
use 1
use rd
simp only [mul_smul, smul_add, one_smul, OneMemClass.coe_one, one_mul, true_and]
rw [this, hc, mul_assoc]
/-- The addition on the Ore localization. -/
@[irreducible]
private def add : X[S⁻¹] → X[S⁻¹] → X[S⁻¹] := fun x =>
Quotient.lift (fun rs : X × S => add' rs.1 rs.2 x)
(by
rintro ⟨r₁, s₁⟩ ⟨r₂, s₂⟩ ⟨sb, rb, hb, hb'⟩
induction' x with r₃ s₃
show add'' _ _ _ _ = add'' _ _ _ _
dsimp only at *
rcases oreCondition (s₃ : R) s₂ with ⟨rc, sc, hc⟩
rcases oreCondition rc sb with ⟨rd, sd, hd⟩
have : rd * rb * s₁ = sd * sc * s₃ := by
rw [mul_assoc, ← hb', ← mul_assoc, ← hd, mul_assoc, ← hc, mul_assoc]
rw [add''_char _ _ _ _ rc sc hc (sc * s₃).2]
rw [add''_char _ _ _ _ _ _ this.symm (sd * sc * s₃).2]
refine oreDiv_eq_iff.mpr ?_
simp only [Submonoid.mk_smul, smul_add]
use sd, 1
simp only [one_smul, one_mul, mul_smul, ← hb, Submonoid.smul_def, ← mul_assoc, and_true]
simp only [smul_smul, hd])
instance : Add X[S⁻¹] :=
⟨add⟩
theorem oreDiv_add_oreDiv {r r' : X} {s s' : S} :
r /ₒ s + r' /ₒ s' =
(oreDenom (s : R) s' • r + oreNum (s : R) s' • r') /ₒ (oreDenom (s : R) s' * s) := by
with_unfolding_all rfl
theorem oreDiv_add_char' {r r' : X} (s s' : S) (rb : R) (sb : R)
(h : sb * s = rb * s') (h' : sb * s ∈ S) :
r /ₒ s + r' /ₒ s' = (sb • r + rb • r') /ₒ ⟨sb * s, h'⟩ := by
with_unfolding_all exact add''_char r s r' s' rb sb h h'
/-- A characterization of the addition on the Ore localizaion, allowing for arbitrary Ore
numerator and Ore denominator. -/
theorem oreDiv_add_char {r r' : X} (s s' : S) (rb : R) (sb : S) (h : sb * s = rb * s') :
r /ₒ s + r' /ₒ s' = (sb • r + rb • r') /ₒ (sb * s) :=
oreDiv_add_char' s s' rb sb h (sb * s).2
/-- Another characterization of the addition on the Ore localization, bundling up all witnesses
and conditions into a sigma type. -/
def oreDivAddChar' (r r' : X) (s s' : S) :
Σ'r'' : R,
Σ's'' : S, s'' * s = r'' * s' ∧ r /ₒ s + r' /ₒ s' = (s'' • r + r'' • r') /ₒ (s'' * s) :=
⟨oreNum (s : R) s', oreDenom (s : R) s', ore_eq (s : R) s', oreDiv_add_oreDiv⟩
@[simp]
theorem add_oreDiv {r r' : X} {s : S} : r /ₒ s + r' /ₒ s = (r + r') /ₒ s := by
simp [oreDiv_add_char s s 1 1 (by simp)]
protected theorem add_assoc (x y z : X[S⁻¹]) : x + y + z = x + (y + z) := by
induction' x with r₁ s₁
induction' y with r₂ s₂
induction' z with r₃ s₃
rcases oreDivAddChar' r₁ r₂ s₁ s₂ with ⟨ra, sa, ha, ha'⟩; rw [ha']; clear ha'
rcases oreDivAddChar' (sa • r₁ + ra • r₂) r₃ (sa * s₁) s₃ with ⟨rc, sc, hc, q⟩; rw [q]; clear q
simp only [smul_add, mul_assoc, add_assoc]
simp_rw [← add_oreDiv, ← OreLocalization.expand']
congr 2
· rw [OreLocalization.expand r₂ s₂ ra (ha.symm ▸ (sa * s₁).2)]; congr; ext; exact ha
· rw [OreLocalization.expand r₃ s₃ rc (hc.symm ▸ (sc * (sa * s₁)).2)]; congr; ext; exact hc
@[simp]
theorem zero_oreDiv (s : S) : (0 : X) /ₒ s = 0 := by
rw [OreLocalization.zero_def, oreDiv_eq_iff]
exact ⟨s, 1, by simp⟩
protected theorem zero_add (x : X[S⁻¹]) : 0 + x = x := by
induction x
rw [← zero_oreDiv, add_oreDiv]; simp
protected theorem add_zero (x : X[S⁻¹]) : x + 0 = x := by
induction x
rw [← zero_oreDiv, add_oreDiv]; simp
@[irreducible]
private def nsmul : ℕ → X[S⁻¹] → X[S⁻¹] := nsmulRec
instance : AddMonoid X[S⁻¹] where
add_assoc := OreLocalization.add_assoc
zero_add := OreLocalization.zero_add
add_zero := OreLocalization.add_zero
nsmul := nsmul
nsmul_zero _ := by with_unfolding_all rfl
nsmul_succ _ _ := by with_unfolding_all rfl
protected theorem smul_zero (x : R[S⁻¹]) : x • (0 : X[S⁻¹]) = 0 := by
induction' x with r s
rw [OreLocalization.zero_def, smul_div_one, smul_zero, zero_oreDiv, zero_oreDiv]
protected theorem smul_add (z : R[S⁻¹]) (x y : X[S⁻¹]) :
z • (x + y) = z • x + z • y := by
induction' x with r₁ s₁
induction' y with r₂ s₂
induction' z with r₃ s₃
rcases oreDivAddChar' r₁ r₂ s₁ s₂ with ⟨ra, sa, ha, ha'⟩; rw [ha']; clear ha'; norm_cast at ha
rw [OreLocalization.expand' r₁ s₁ sa]
rw [OreLocalization.expand r₂ s₂ ra (by rw [← ha]; apply SetLike.coe_mem)]
rw [← Subtype.coe_eq_of_eq_mk ha]
repeat rw [oreDiv_smul_oreDiv]
simp only [smul_add, add_oreDiv]
instance : DistribMulAction R[S⁻¹] X[S⁻¹] where
smul_zero := OreLocalization.smul_zero
smul_add := OreLocalization.smul_add
instance {R₀} [Monoid R₀] [MulAction R₀ X] [MulAction R₀ R]
[IsScalarTower R₀ R X] [IsScalarTower R₀ R R] :
DistribMulAction R₀ X[S⁻¹] where
smul_zero _ := by rw [← smul_one_oreDiv_one_smul, smul_zero]
smul_add _ _ _ := by simp only [← smul_one_oreDiv_one_smul, smul_add]
end DistribMulAction
section AddCommMonoid
variable {R : Type*} [Monoid R] {S : Submonoid R} [OreSet S]
variable {X : Type*} [AddCommMonoid X] [DistribMulAction R X]
protected theorem add_comm (x y : X[S⁻¹]) : x + y = y + x := by
induction' x with r s
induction' y with r' s'
rcases oreDivAddChar' r r' s s' with ⟨ra, sa, ha, ha'⟩
rw [ha', oreDiv_add_char' s' s _ _ ha.symm (ha ▸ (sa * s).2), add_comm]
congr; ext; exact ha
instance instAddCommMonoidOreLocalization : AddCommMonoid X[S⁻¹] where
add_comm := OreLocalization.add_comm
end AddCommMonoid
section AddGroup
variable {R : Type*} [Monoid R] {S : Submonoid R} [OreSet S]
variable {X : Type*} [AddGroup X] [DistribMulAction R X]
/-- Negation on the Ore localization is defined via negation on the numerator. -/
@[irreducible]
protected def neg : X[S⁻¹] → X[S⁻¹] :=
liftExpand (fun (r : X) (s : S) => -r /ₒ s) fun r t s ht => by
-- Porting note(#12129): additional beta reduction needed
beta_reduce
rw [← smul_neg, ← OreLocalization.expand]
instance instNegOreLocalization : Neg X[S⁻¹] :=
⟨OreLocalization.neg⟩
@[simp]
protected theorem neg_def (r : X) (s : S) : -(r /ₒ s) = -r /ₒ s := by
with_unfolding_all rfl
protected theorem add_left_neg (x : X[S⁻¹]) : -x + x = 0 := by
induction' x with r s; simp
/-- `zsmul` of `OreLocalization` -/
@[irreducible]
protected def zsmul : ℤ → X[S⁻¹] → X[S⁻¹] := zsmulRec
unseal OreLocalization.zsmul in
instance instAddGroupOreLocalization : AddGroup X[S⁻¹] where
add_left_neg := OreLocalization.add_left_neg
zsmul := OreLocalization.zsmul
end AddGroup
section AddCommGroup
variable {R : Type*} [Monoid R] {S : Submonoid R} [OreSet S]
variable {X : Type*} [AddCommGroup X] [DistribMulAction R X]
instance : AddCommGroup X[S⁻¹] where
__ := inferInstanceAs (AddGroup X[S⁻¹])
__ := inferInstanceAs (AddCommMonoid X[S⁻¹])
end AddCommGroup
end OreLocalization
|
RingTheory\OreLocalization\OreSet.lean | /-
Copyright (c) 2022 Jakob von Raumer. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jakob von Raumer, Kevin Klinge
-/
import Mathlib.Algebra.Group.Submonoid.Basic
import Mathlib.Algebra.GroupWithZero.Basic
import Mathlib.Algebra.Ring.Regular
/-!
# (Left) Ore sets
This defines left Ore sets on arbitrary monoids.
## References
* https://ncatlab.org/nlab/show/Ore+set
-/
namespace AddOreLocalization
/-- A submonoid `S` of an additive monoid `R` is (left) Ore if common summands on the right can be
turned into common summands on the left, and if each pair of `r : R` and `s : S` admits an Ore
minuend `v : R` and an Ore subtrahend `u : S` such that `u + r = v + s`. -/
class AddOreSet {R : Type*} [AddMonoid R] (S : AddSubmonoid R) where
/-- Common summands on the right can be turned into common summands on the left, a weak form of
cancellability. -/
ore_right_cancel : ∀ (r₁ r₂ : R) (s : S), r₁ + s = r₂ + s → ∃ s' : S, s' + r₁ = s' + r₂
/-- The Ore minuend of a difference. -/
oreMin : R → S → R
/-- The Ore subtrahend of a difference. -/
oreSubtra : R → S → S
/-- The Ore condition of a difference, expressed in terms of `oreMin` and `oreSubtra`. -/
ore_eq : ∀ (r : R) (s : S), oreSubtra r s + r = oreMin r s + s
end AddOreLocalization
namespace OreLocalization
section Monoid
/-- A submonoid `S` of a monoid `R` is (left) Ore if common factors on the right can be turned
into common factors on the left, and if each pair of `r : R` and `s : S` admits an Ore numerator
`v : R` and an Ore denominator `u : S` such that `u * r = v * s`. -/
@[to_additive AddOreLocalization.AddOreSet]
class OreSet {R : Type*} [Monoid R] (S : Submonoid R) where
/-- Common factors on the right can be turned into common factors on the left, a weak form of
cancellability. -/
ore_right_cancel : ∀ (r₁ r₂ : R) (s : S), r₁ * s = r₂ * s → ∃ s' : S, s' * r₁ = s' * r₂
/-- The Ore numerator of a fraction. -/
oreNum : R → S → R
/-- The Ore denominator of a fraction. -/
oreDenom : R → S → S
/-- The Ore condition of a fraction, expressed in terms of `oreNum` and `oreDenom`. -/
ore_eq : ∀ (r : R) (s : S), oreDenom r s * r = oreNum r s * s
-- TODO: use this once it's available.
-- run_cmd to_additive.map_namespace `OreLocalization `AddOreLocalization
variable {R : Type*} [Monoid R] {S : Submonoid R} [OreSet S]
/-- Common factors on the right can be turned into common factors on the left, a weak form of
cancellability. -/
@[to_additive AddOreLocalization.ore_right_cancel]
theorem ore_right_cancel (r₁ r₂ : R) (s : S) (h : r₁ * s = r₂ * s) : ∃ s' : S, s' * r₁ = s' * r₂ :=
OreSet.ore_right_cancel r₁ r₂ s h
/-- The Ore numerator of a fraction. -/
@[to_additive AddOreLocalization.oreMin "The Ore minuend of a difference."]
def oreNum (r : R) (s : S) : R :=
OreSet.oreNum r s
/-- The Ore denominator of a fraction. -/
@[to_additive AddOreLocalization.oreSubtra "The Ore subtrahend of a difference."]
def oreDenom (r : R) (s : S) : S :=
OreSet.oreDenom r s
/-- The Ore condition of a fraction, expressed in terms of `oreNum` and `oreDenom`. -/
@[to_additive AddOreLocalization.add_ore_eq
"The Ore condition of a difference, expressed in terms of `oreMin` and `oreSubtra`."]
theorem ore_eq (r : R) (s : S) : oreDenom r s * r = oreNum r s * s :=
OreSet.ore_eq r s
/-- The Ore condition bundled in a sigma type. This is useful in situations where we want to obtain
both witnesses and the condition for a given fraction. -/
@[to_additive AddOreLocalization.addOreCondition
"The Ore condition bundled in a sigma type. This is useful in situations where we want to obtain
both witnesses and the condition for a given difference."]
def oreCondition (r : R) (s : S) : Σ'r' : R, Σ's' : S, s' * r = r' * s :=
⟨oreNum r s, oreDenom r s, ore_eq r s⟩
/-- The trivial submonoid is an Ore set. -/
@[to_additive AddOreLocalization.addOreSetBot]
instance oreSetBot : OreSet (⊥ : Submonoid R) where
ore_right_cancel _ _ s h :=
⟨s, by
rcases s with ⟨s, hs⟩
rw [Submonoid.mem_bot] at hs
subst hs
rw [mul_one, mul_one] at h
subst h
rfl⟩
oreNum r _ := r
oreDenom _ s := s
ore_eq _ s := by
rcases s with ⟨s, hs⟩
rw [Submonoid.mem_bot] at hs
simp [hs]
/-- Every submonoid of a commutative monoid is an Ore set. -/
@[to_additive AddOreLocalization.addOreSetComm]
instance (priority := 100) oreSetComm {R} [CommMonoid R] (S : Submonoid R) : OreSet S where
ore_right_cancel m n s h := ⟨s, by rw [mul_comm (s : R) n, mul_comm (s : R) m, h]⟩
oreNum r _ := r
oreDenom _ s := s
ore_eq r s := by rw [mul_comm]
end Monoid
/-- Cancellability in monoids with zeros can act as a replacement for the `ore_right_cancel`
condition of an ore set. -/
def oreSetOfCancelMonoidWithZero {R : Type*} [CancelMonoidWithZero R] {S : Submonoid R}
(oreNum : R → S → R) (oreDenom : R → S → S)
(ore_eq : ∀ (r : R) (s : S), oreDenom r s * r = oreNum r s * s) : OreSet S :=
{ ore_right_cancel := fun _ _ s h => ⟨s, mul_eq_mul_left_iff.mpr (mul_eq_mul_right_iff.mp h)⟩
oreNum
oreDenom
ore_eq }
/-- In rings without zero divisors, the first (cancellability) condition is always fulfilled,
it suffices to give a proof for the Ore condition itself. -/
def oreSetOfNoZeroDivisors {R : Type*} [Ring R] [NoZeroDivisors R] {S : Submonoid R}
(oreNum : R → S → R) (oreDenom : R → S → S)
(ore_eq : ∀ (r : R) (s : S), oreDenom r s * r = oreNum r s * s) : OreSet S :=
letI : CancelMonoidWithZero R := NoZeroDivisors.toCancelMonoidWithZero
oreSetOfCancelMonoidWithZero oreNum oreDenom ore_eq
lemma nonempty_oreSet_iff {R : Type*} [Ring R] {S : Submonoid R} :
Nonempty (OreSet S) ↔ (∀ (r₁ r₂ : R) (s : S), r₁ * s = r₂ * s → ∃ s' : S, s' * r₁ = s' * r₂) ∧
(∀ (r : R) (s : S), ∃ (r' : R) (s' : S), s' * r = r' * s) := by
constructor
· exact fun ⟨_⟩ ↦ ⟨ore_right_cancel, fun r s ↦ ⟨oreNum r s, oreDenom r s, ore_eq r s⟩⟩
· intro ⟨H, H'⟩
choose r' s' h using H'
exact ⟨H, r', s', h⟩
lemma nonempty_oreSet_iff_of_noZeroDivisors {R : Type*} [Ring R] [NoZeroDivisors R]
{S : Submonoid R} :
Nonempty (OreSet S) ↔ ∀ (r : R) (s : S), ∃ (r' : R) (s' : S), s' * r = r' * s := by
constructor
· exact fun ⟨_⟩ ↦ fun r s ↦ ⟨oreNum r s, oreDenom r s, ore_eq r s⟩
· intro H
choose r' s' h using H
exact ⟨oreSetOfNoZeroDivisors r' s' h⟩
end OreLocalization
|
RingTheory\OreLocalization\Ring.lean | /-
Copyright (c) 2022 Jakob von Raumer. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jakob von Raumer, Kevin Klinge, Andrew Yang
-/
import Mathlib.Algebra.GroupWithZero.NonZeroDivisors
import Mathlib.Algebra.Algebra.Defs
import Mathlib.Algebra.Field.Defs
import Mathlib.RingTheory.OreLocalization.Basic
/-!
# Module and Ring instances of Ore Localizations
The `Monoid` and `DistribMulAction` instances and additive versions are provided in
`Mathlib/RingTheory/OreLocalization/Basic.lean`.
-/
universe u
namespace OreLocalization
section Module
variable {R : Type*} [Semiring R] {S : Submonoid R} [OreSet S]
variable {X : Type*} [AddCommMonoid X] [Module R X]
protected theorem zero_smul (x : X[S⁻¹]) : (0 : R[S⁻¹]) • x = 0 := by
induction' x with r s
rw [OreLocalization.zero_def, oreDiv_smul_char 0 r 1 s 0 1 (by simp)]; simp
protected theorem add_smul (y z : R[S⁻¹]) (x : X[S⁻¹]) :
(y + z) • x = y • x + z • x := by
induction' x with r₁ s₁
induction' y with r₂ s₂
induction' z with r₃ s₃
rcases oreDivAddChar' r₂ r₃ s₂ s₃ with ⟨ra, sa, ha, q⟩
rw [q]
clear q
rw [OreLocalization.expand' r₂ s₂ sa]
rcases oreDivSMulChar' (sa • r₂) r₁ (sa * s₂) s₁ with ⟨rb, sb, hb, q⟩
rw [q]
clear q
have hs₃rasb : sb * ra * s₃ ∈ S := by
rw [mul_assoc, ← ha]
norm_cast
apply SetLike.coe_mem
rw [OreLocalization.expand _ _ _ hs₃rasb]
have ha' : ↑((sb * sa) * s₂) = sb * ra * s₃ := by simp [ha, mul_assoc]
rw [← Subtype.coe_eq_of_eq_mk ha']
rcases oreDivSMulChar' ((sb * ra) • r₃) r₁ (sb * sa * s₂) s₁ with ⟨rc, sc, hc, hc'⟩
rw [hc']
rw [oreDiv_add_char _ _ 1 sc (by simp [mul_assoc])]
rw [OreLocalization.expand' (sa • r₂ + ra • r₃) (sa * s₂) (sc * sb)]
simp only [smul_eq_mul, one_smul, Submonoid.smul_def, mul_add, Submonoid.coe_mul] at hb hc ⊢
rw [mul_assoc, hb, mul_assoc, ← mul_assoc _ ra, hc, ← mul_assoc, ← add_mul]
rw [OreLocalization.smul_cancel']
simp only [add_smul, ← mul_assoc, smul_smul]
end Module
section Semiring
variable {R : Type*} [Semiring R] {S : Submonoid R} [OreSet S]
attribute [local instance] OreLocalization.oreEqv
protected theorem zero_mul (x : R[S⁻¹]) : 0 * x = 0 :=
OreLocalization.zero_smul x
protected theorem mul_zero (x : R[S⁻¹]) : x * 0 = 0 :=
OreLocalization.smul_zero x
protected theorem left_distrib (x y z : R[S⁻¹]) : x * (y + z) = x * y + x * z :=
OreLocalization.smul_add _ _ _
theorem right_distrib (x y z : R[S⁻¹]) : (x + y) * z = x * z + y * z :=
OreLocalization.add_smul _ _ _
instance : Semiring R[S⁻¹] where
__ := inferInstanceAs (MonoidWithZero (R[S⁻¹]))
__ := inferInstanceAs (AddCommMonoid (R[S⁻¹]))
left_distrib := OreLocalization.left_distrib
right_distrib := right_distrib
variable {X : Type*} [AddCommMonoid X] [Module R X]
instance : Module R[S⁻¹] X[S⁻¹] where
add_smul := OreLocalization.add_smul
zero_smul := OreLocalization.zero_smul
instance {R₀} [Semiring R₀] [Module R₀ X] [Module R₀ R]
[IsScalarTower R₀ R X] [IsScalarTower R₀ R R] :
Module R₀ X[S⁻¹] where
add_smul r s x := by simp only [← smul_one_oreDiv_one_smul, add_smul, ← add_oreDiv]
zero_smul x := by rw [← smul_one_oreDiv_one_smul, zero_smul, zero_oreDiv, zero_smul]
@[simp]
lemma nsmul_eq_nsmul (n : ℕ) (x : X[S⁻¹]) :
letI inst := OreLocalization.instModuleOfIsScalarTower (R₀ := ℕ) (R := R) (X := X) (S := S)
HSMul.hSMul (self := @instHSMul _ _ inst.toSMul) n x = n • x := by
letI inst := OreLocalization.instModuleOfIsScalarTower (R₀ := ℕ) (R := R) (X := X) (S := S)
exact congr($(AddCommMonoid.uniqueNatModule.2 inst).smul n x)
/-- The ring homomorphism from `R` to `R[S⁻¹]`, mapping `r : R` to the fraction `r /ₒ 1`. -/
@[simps!]
def numeratorRingHom : R →+* R[S⁻¹] where
__ := numeratorHom
map_zero' := by with_unfolding_all exact OreLocalization.zero_def
map_add' _ _ := add_oreDiv.symm
instance {R₀} [CommSemiring R₀] [Algebra R₀ R] : Algebra R₀ R[S⁻¹] where
__ := inferInstanceAs (Module R₀ R[S⁻¹])
__ := numeratorRingHom.comp (algebraMap R₀ R)
commutes' r x := by
induction' x using OreLocalization.ind with r₁ s₁
dsimp
rw [mul_div_one, oreDiv_mul_char _ _ _ _ (algebraMap R₀ R r) s₁ (Algebra.commutes _ _).symm,
Algebra.commutes, mul_one]
smul_def' r x := by
dsimp
rw [Algebra.algebraMap_eq_smul_one, ← smul_eq_mul, smul_one_oreDiv_one_smul]
section UMP
variable {T : Type*} [Semiring T]
variable (f : R →+* T) (fS : S →* Units T)
variable (hf : ∀ s : S, f s = fS s)
/-- The universal lift from a ring homomorphism `f : R →+* T`, which maps elements in `S` to
units of `T`, to a ring homomorphism `R[S⁻¹] →+* T`. This extends the construction on
monoids. -/
def universalHom : R[S⁻¹] →+* T :=
{ universalMulHom f.toMonoidHom fS hf with
map_zero' := by
simp only [RingHom.toMonoidHom_eq_coe, OneHom.toFun_eq_coe, MonoidHom.toOneHom_coe]
rw [OreLocalization.zero_def, universalMulHom_apply]
simp
map_add' := fun x y => by
simp only [RingHom.toMonoidHom_eq_coe, OneHom.toFun_eq_coe, MonoidHom.toOneHom_coe]
induction' x with r₁ s₁
induction' y with r₂ s₂
rcases oreDivAddChar' r₁ r₂ s₁ s₂ with ⟨r₃, s₃, h₃, h₃'⟩
rw [h₃']
clear h₃'
simp only [RingHom.toMonoidHom_eq_coe, smul_eq_mul, universalMulHom_apply, MonoidHom.coe_coe,
Submonoid.smul_def]
simp only [mul_inv_rev, MonoidHom.map_mul, RingHom.map_add, RingHom.map_mul, Units.val_mul]
rw [mul_add, mul_assoc, ← mul_assoc _ (f s₃), hf, ← Units.val_mul]
simp only [one_mul, mul_left_inv, Units.val_one]
congr 1
rw [← mul_assoc]
congr 1
norm_cast at h₃
have h₃' := Subtype.coe_eq_of_eq_mk h₃
rw [← Units.val_mul, ← mul_inv_rev, ← fS.map_mul, h₃']
rw [Units.inv_mul_eq_iff_eq_mul, Units.eq_mul_inv_iff_mul_eq, ← hf, ← hf]
simp only [map_mul] }
theorem universalHom_apply {r : R} {s : S} :
universalHom f fS hf (r /ₒ s) = ((fS s)⁻¹ : Units T) * f r :=
rfl
theorem universalHom_commutes {r : R} : universalHom f fS hf (numeratorHom r) = f r := by
simp [numeratorHom_apply, universalHom_apply]
theorem universalHom_unique (φ : R[S⁻¹] →+* T) (huniv : ∀ r : R, φ (numeratorHom r) = f r) :
φ = universalHom f fS hf :=
RingHom.coe_monoidHom_injective <| universalMulHom_unique (RingHom.toMonoidHom f) fS hf (↑φ) huniv
end UMP
end Semiring
section Ring
variable {R : Type*} [Ring R] {S : Submonoid R} [OreSet S]
variable {X : Type*} [AddCommGroup X] [Module R X]
instance : Ring R[S⁻¹] where
__ := inferInstanceAs (Semiring R[S⁻¹])
__ := inferInstanceAs (AddGroup R[S⁻¹])
@[simp]
lemma zsmul_eq_zsmul (n : ℤ) (x : X[S⁻¹]) :
letI inst := OreLocalization.instModuleOfIsScalarTower (R₀ := ℤ) (R := R) (X := X) (S := S)
HSMul.hSMul (self := @instHSMul _ _ inst.toSMul) n x = n • x := by
letI inst := OreLocalization.instModuleOfIsScalarTower (R₀ := ℤ) (R := R) (X := X) (S := S)
exact congr($(AddCommGroup.uniqueIntModule.2 inst).smul n x)
open nonZeroDivisors
theorem numeratorHom_inj (hS : S ≤ nonZeroDivisorsRight R) :
Function.Injective (numeratorHom : R → R[S⁻¹]) :=
fun r₁ r₂ h => by
rw [numeratorHom_apply, numeratorHom_apply, oreDiv_eq_iff] at h
rcases h with ⟨u, v, h₁, h₂⟩
simp only [S.coe_one, mul_one, Submonoid.smul_def, smul_eq_mul] at h₁ h₂
rw [← h₂, ← sub_eq_zero, ← mul_sub] at h₁
exact (sub_eq_zero.mp (hS u.2 _ h₁)).symm
theorem subsingleton_iff :
Subsingleton R[S⁻¹] ↔ 0 ∈ S := by
rw [← subsingleton_iff_zero_eq_one, OreLocalization.one_def,
OreLocalization.zero_def, oreDiv_eq_iff]
simp
theorem nontrivial_iff :
Nontrivial R[S⁻¹] ↔ 0 ∉ S := by
rw [← not_subsingleton_iff_nontrivial, subsingleton_iff]
theorem nontrivial_of_nonZeroDivisors [Nontrivial R] (hS : S ≤ R⁰) :
Nontrivial R[S⁻¹] :=
nontrivial_iff.mpr (fun e ↦ one_ne_zero <| hS e 1 (mul_zero _))
end Ring
noncomputable section DivisionRing
open nonZeroDivisors
variable {R : Type*} [Ring R] [Nontrivial R] [OreSet R⁰]
instance nontrivial : Nontrivial R[R⁰⁻¹] :=
nontrivial_of_nonZeroDivisors (refl R⁰)
variable [NoZeroDivisors R]
open Classical in
/-- The inversion of Ore fractions for a ring without zero divisors, satisying `0⁻¹ = 0` and
`(r /ₒ r')⁻¹ = r' /ₒ r` for `r ≠ 0`. -/
@[irreducible]
protected def inv : R[R⁰⁻¹] → R[R⁰⁻¹] :=
liftExpand
(fun r s =>
if hr : r = (0 : R) then (0 : R[R⁰⁻¹])
else s /ₒ ⟨r, fun _ => eq_zero_of_ne_zero_of_mul_right_eq_zero hr⟩)
(by
intro r t s hst
by_cases hr : r = 0
· simp [hr]
· by_cases ht : t = 0
· exfalso
apply nonZeroDivisors.coe_ne_zero ⟨_, hst⟩
simp [ht, mul_zero]
· simp only [hr, ht, dif_neg, not_false_iff, or_self_iff, mul_eq_zero, smul_eq_mul]
apply OreLocalization.expand)
instance inv' : Inv R[R⁰⁻¹] :=
⟨OreLocalization.inv⟩
open Classical in
protected theorem inv_def {r : R} {s : R⁰} :
(r /ₒ s)⁻¹ =
if hr : r = (0 : R) then (0 : R[R⁰⁻¹])
else s /ₒ ⟨r, fun _ => eq_zero_of_ne_zero_of_mul_right_eq_zero hr⟩ := by
with_unfolding_all rfl
protected theorem mul_inv_cancel (x : R[R⁰⁻¹]) (h : x ≠ 0) : x * x⁻¹ = 1 := by
induction' x with r s
rw [OreLocalization.inv_def, OreLocalization.one_def]
have hr : r ≠ 0 := by
rintro rfl
simp at h
simp only [hr]
with_unfolding_all apply OreLocalization.mul_inv ⟨r, _⟩
protected theorem inv_zero : (0 : R[R⁰⁻¹])⁻¹ = 0 := by
rw [OreLocalization.zero_def, OreLocalization.inv_def]
simp
instance : DivisionRing R[R⁰⁻¹] where
mul_inv_cancel := OreLocalization.mul_inv_cancel
inv_zero := OreLocalization.inv_zero
nnqsmul := _
nnqsmul_def := fun q a => rfl
qsmul := _
qsmul_def := fun q a => rfl
end DivisionRing
section CommSemiring
variable {R : Type*} [CommSemiring R] {S : Submonoid R} [OreSet S]
instance : CommSemiring R[S⁻¹] where
__ := inferInstanceAs (Semiring R[S⁻¹])
__ := inferInstanceAs (CommMonoid R[S⁻¹])
end CommSemiring
section CommRing
variable {R : Type*} [CommRing R] {S : Submonoid R} [OreSet S]
instance : CommRing R[S⁻¹] where
__ := inferInstanceAs (Ring R[S⁻¹])
__ := inferInstanceAs (CommMonoid R[S⁻¹])
end CommRing
section Field
open nonZeroDivisors
variable {R : Type*} [CommRing R] [Nontrivial R] [NoZeroDivisors R] [OreSet R⁰]
noncomputable
instance : Field R[R⁰⁻¹] where
__ := inferInstanceAs (DivisionRing R[R⁰⁻¹])
__ := inferInstanceAs (CommMonoid R[R⁰⁻¹])
end Field
end OreLocalization
|
RingTheory\Polynomial\Basic.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import Mathlib.Algebra.CharP.ExpChar
import Mathlib.Algebra.MvPolynomial.CommRing
import Mathlib.Algebra.MvPolynomial.Equiv
import Mathlib.RingTheory.Polynomial.Content
/-!
# Ring-theoretic supplement of Algebra.Polynomial.
## Main results
* `MvPolynomial.isDomain`:
If a ring is an integral domain, then so is its polynomial ring over finitely many variables.
* `Polynomial.isNoetherianRing`:
Hilbert basis theorem, that if a ring is noetherian then so is its polynomial ring.
* `Polynomial.wfDvdMonoid`:
If an integral domain is a `WFDvdMonoid`, then so is its polynomial ring.
* `Polynomial.uniqueFactorizationMonoid`, `MvPolynomial.uniqueFactorizationMonoid`:
If an integral domain is a `UniqueFactorizationMonoid`, then so is its polynomial ring (of any
number of variables).
-/
noncomputable section
open Polynomial
open Finset
universe u v w
variable {R : Type u} {S : Type*}
namespace Polynomial
section Semiring
variable [Semiring R]
instance instCharP (p : ℕ) [h : CharP R p] : CharP R[X] p :=
let ⟨h⟩ := h
⟨fun n => by rw [← map_natCast C, ← C_0, C_inj, h]⟩
instance instExpChar (p : ℕ) [h : ExpChar R p] : ExpChar R[X] p := by
cases h; exacts [ExpChar.zero, ExpChar.prime ‹_›]
variable (R)
/-- The `R`-submodule of `R[X]` consisting of polynomials of degree ≤ `n`. -/
def degreeLE (n : WithBot ℕ) : Submodule R R[X] :=
⨅ k : ℕ, ⨅ _ : ↑k > n, LinearMap.ker (lcoeff R k)
/-- The `R`-submodule of `R[X]` consisting of polynomials of degree < `n`. -/
def degreeLT (n : ℕ) : Submodule R R[X] :=
⨅ k : ℕ, ⨅ (_ : k ≥ n), LinearMap.ker (lcoeff R k)
variable {R}
theorem mem_degreeLE {n : WithBot ℕ} {f : R[X]} : f ∈ degreeLE R n ↔ degree f ≤ n := by
simp only [degreeLE, Submodule.mem_iInf, degree_le_iff_coeff_zero, LinearMap.mem_ker]; rfl
@[mono]
theorem degreeLE_mono {m n : WithBot ℕ} (H : m ≤ n) : degreeLE R m ≤ degreeLE R n := fun _ hf =>
mem_degreeLE.2 (le_trans (mem_degreeLE.1 hf) H)
theorem degreeLE_eq_span_X_pow [DecidableEq R] {n : ℕ} :
degreeLE R n = Submodule.span R ↑((Finset.range (n + 1)).image fun n => (X : R[X]) ^ n) := by
apply le_antisymm
· intro p hp
replace hp := mem_degreeLE.1 hp
rw [← Polynomial.sum_monomial_eq p, Polynomial.sum]
refine Submodule.sum_mem _ fun k hk => ?_
have := WithBot.coe_le_coe.1 (Finset.sup_le_iff.1 hp k hk)
rw [← C_mul_X_pow_eq_monomial, C_mul']
refine
Submodule.smul_mem _ _
(Submodule.subset_span <|
Finset.mem_coe.2 <|
Finset.mem_image.2 ⟨_, Finset.mem_range.2 (Nat.lt_succ_of_le this), rfl⟩)
rw [Submodule.span_le, Finset.coe_image, Set.image_subset_iff]
intro k hk
apply mem_degreeLE.2
exact
(degree_X_pow_le _).trans (WithBot.coe_le_coe.2 <| Nat.le_of_lt_succ <| Finset.mem_range.1 hk)
theorem mem_degreeLT {n : ℕ} {f : R[X]} : f ∈ degreeLT R n ↔ degree f < n := by
rw [degreeLT, Submodule.mem_iInf]
conv_lhs => intro i; rw [Submodule.mem_iInf]
rw [degree, Finset.max_eq_sup_coe]
rw [Finset.sup_lt_iff ?_]
rotate_left
· apply WithBot.bot_lt_coe
conv_rhs =>
simp only [mem_support_iff]
intro b
rw [Nat.cast_withBot, WithBot.coe_lt_coe, lt_iff_not_le, Ne, not_imp_not]
rfl
@[mono]
theorem degreeLT_mono {m n : ℕ} (H : m ≤ n) : degreeLT R m ≤ degreeLT R n := fun _ hf =>
mem_degreeLT.2 (lt_of_lt_of_le (mem_degreeLT.1 hf) <| WithBot.coe_le_coe.2 H)
theorem degreeLT_eq_span_X_pow [DecidableEq R] {n : ℕ} :
degreeLT R n = Submodule.span R ↑((Finset.range n).image fun n => X ^ n : Finset R[X]) := by
apply le_antisymm
· intro p hp
replace hp := mem_degreeLT.1 hp
rw [← Polynomial.sum_monomial_eq p, Polynomial.sum]
refine Submodule.sum_mem _ fun k hk => ?_
have := WithBot.coe_lt_coe.1 ((Finset.sup_lt_iff <| WithBot.bot_lt_coe n).1 hp k hk)
rw [← C_mul_X_pow_eq_monomial, C_mul']
refine
Submodule.smul_mem _ _
(Submodule.subset_span <|
Finset.mem_coe.2 <| Finset.mem_image.2 ⟨_, Finset.mem_range.2 this, rfl⟩)
rw [Submodule.span_le, Finset.coe_image, Set.image_subset_iff]
intro k hk
apply mem_degreeLT.2
exact lt_of_le_of_lt (degree_X_pow_le _) (WithBot.coe_lt_coe.2 <| Finset.mem_range.1 hk)
/-- The first `n` coefficients on `degreeLT n` form a linear equivalence with `Fin n → R`. -/
def degreeLTEquiv (R) [Semiring R] (n : ℕ) : degreeLT R n ≃ₗ[R] Fin n → R where
toFun p n := (↑p : R[X]).coeff n
invFun f :=
⟨∑ i : Fin n, monomial i (f i),
(degreeLT R n).sum_mem fun i _ =>
mem_degreeLT.mpr
(lt_of_le_of_lt (degree_monomial_le i (f i)) (WithBot.coe_lt_coe.mpr i.is_lt))⟩
map_add' p q := by
ext
dsimp
rw [coeff_add]
map_smul' x p := by
ext
dsimp
rw [coeff_smul]
rfl
left_inv := by
rintro ⟨p, hp⟩
ext1
simp only [Submodule.coe_mk]
by_cases hp0 : p = 0
· subst hp0
simp only [coeff_zero, LinearMap.map_zero, Finset.sum_const_zero]
rw [mem_degreeLT, degree_eq_natDegree hp0, Nat.cast_lt] at hp
conv_rhs => rw [p.as_sum_range' n hp, ← Fin.sum_univ_eq_sum_range]
right_inv f := by
ext i
simp only [finset_sum_coeff, Submodule.coe_mk]
rw [Finset.sum_eq_single i, coeff_monomial, if_pos rfl]
· rintro j - hji
rw [coeff_monomial, if_neg]
rwa [← Fin.ext_iff]
· intro h
exact (h (Finset.mem_univ _)).elim
-- Porting note: removed @[simp] as simp can prove this
theorem degreeLTEquiv_eq_zero_iff_eq_zero {n : ℕ} {p : R[X]} (hp : p ∈ degreeLT R n) :
degreeLTEquiv _ _ ⟨p, hp⟩ = 0 ↔ p = 0 := by
rw [LinearEquiv.map_eq_zero_iff, Submodule.mk_eq_zero]
theorem eval_eq_sum_degreeLTEquiv {n : ℕ} {p : R[X]} (hp : p ∈ degreeLT R n) (x : R) :
p.eval x = ∑ i, degreeLTEquiv _ _ ⟨p, hp⟩ i * x ^ (i : ℕ) := by
simp_rw [eval_eq_sum]
exact (sum_fin _ (by simp_rw [zero_mul, forall_const]) (mem_degreeLT.mp hp)).symm
theorem degreeLT_succ_eq_degreeLE {n : ℕ} : degreeLT R (n + 1) = degreeLE R n := by
ext x
by_cases x_zero : x = 0
· simp_rw [x_zero, Submodule.zero_mem]
· rw [mem_degreeLT, mem_degreeLE, ← natDegree_lt_iff_degree_lt (by rwa [ne_eq]),
← natDegree_le_iff_degree_le, Nat.lt_succ]
/-- For every polynomial `p` in the span of a set `s : Set R[X]`, there exists a polynomial of
`p' ∈ s` with higher degree. See also `Polynomial.exists_degree_le_of_mem_span_of_finite`. -/
theorem exists_degree_le_of_mem_span {s : Set R[X]} {p : R[X]}
(hs : s.Nonempty) (hp : p ∈ Submodule.span R s) :
∃ p' ∈ s, degree p ≤ degree p' := by
by_contra! h
by_cases hp_zero : p = 0
· rw [hp_zero, degree_zero] at h
rcases hs with ⟨x, hx⟩
exact not_lt_bot (h x hx)
· have : p ∈ degreeLT R (natDegree p) := by
refine (Submodule.span_le.mpr fun p' p'_mem => ?_) hp
rw [SetLike.mem_coe, mem_degreeLT, Nat.cast_withBot]
exact lt_of_lt_of_le (h p' p'_mem) degree_le_natDegree
rwa [mem_degreeLT, Nat.cast_withBot, degree_eq_natDegree hp_zero,
Nat.cast_withBot, lt_self_iff_false] at this
/-- A stronger version of `Polynomial.exists_degree_le_of_mem_span` under the assumption that the
set `s : R[X]` is finite. There exists a polynomial `p' ∈ s` whose degree dominates the degree of
every element of `p ∈ span R s`-/
theorem exists_degree_le_of_mem_span_of_finite {s : Set R[X]} (s_fin : s.Finite) (hs : s.Nonempty) :
∃ p' ∈ s, ∀ (p : R[X]), p ∈ Submodule.span R s → degree p ≤ degree p' := by
rcases Set.Finite.exists_maximal_wrt degree s s_fin hs with ⟨a, has, hmax⟩
refine ⟨a, has, fun p hp => ?_⟩
rcases exists_degree_le_of_mem_span hs hp with ⟨p', hp'⟩
by_cases h : degree a ≤ degree p'
· rw [← hmax p' hp'.left h] at hp'; exact hp'.right
· exact le_trans hp'.right (not_le.mp h).le
/-- The span of every finite set of polynomials is contained in a `degreeLE n` for some `n`. -/
theorem span_le_degreeLE_of_finite {s : Set R[X]} (s_fin : s.Finite) :
∃ n : ℕ, Submodule.span R s ≤ degreeLE R n := by
by_cases s_emp : s.Nonempty
· rcases exists_degree_le_of_mem_span_of_finite s_fin s_emp with ⟨p', _, hp'max⟩
exact ⟨natDegree p', fun p hp => mem_degreeLE.mpr ((hp'max _ hp).trans degree_le_natDegree)⟩
· rw [Set.not_nonempty_iff_eq_empty] at s_emp
rw [s_emp, Submodule.span_empty]
exact ⟨0, bot_le⟩
/-- The span of every finite set of polynomials is contained in a `degreeLT n` for some `n`. -/
theorem span_of_finite_le_degreeLT {s : Set R[X]} (s_fin : s.Finite) :
∃ n : ℕ, Submodule.span R s ≤ degreeLT R n := by
rcases span_le_degreeLE_of_finite s_fin with ⟨n, _⟩
exact ⟨n + 1, by rwa [degreeLT_succ_eq_degreeLE]⟩
/-- If `R` is a nontrivial ring, the polynomials `R[X]` are not finite as an `R`-module. When `R` is
a field, this is equivalent to `R[X]` being an infinite-dimensional vector space over `R`. -/
theorem not_finite [Nontrivial R] : ¬ Module.Finite R R[X] := by
rw [Module.finite_def, Submodule.fg_def]
push_neg
intro s hs contra
rcases span_le_degreeLE_of_finite hs with ⟨n,hn⟩
have : ((X : R[X]) ^ (n + 1)) ∈ Polynomial.degreeLE R ↑n := by
rw [contra] at hn
exact hn Submodule.mem_top
rw [mem_degreeLE, degree_X_pow, Nat.cast_le, add_le_iff_nonpos_right, nonpos_iff_eq_zero] at this
exact one_ne_zero this
/-- The finset of nonzero coefficients of a polynomial. -/
def coeffs (p : R[X]) : Finset R :=
letI := Classical.decEq R
Finset.image (fun n => p.coeff n) p.support
@[deprecated (since := "2024-05-17")] noncomputable alias frange := coeffs
theorem coeffs_zero : coeffs (0 : R[X]) = ∅ :=
rfl
@[deprecated (since := "2024-05-17")] alias frange_zero := coeffs_zero
theorem mem_coeffs_iff {p : R[X]} {c : R} : c ∈ p.coeffs ↔ ∃ n ∈ p.support, c = p.coeff n := by
simp [coeffs, eq_comm, (Finset.mem_image)]
@[deprecated (since := "2024-05-17")] alias mem_frange_iff := mem_coeffs_iff
theorem coeffs_one : coeffs (1 : R[X]) ⊆ {1} := by
classical
simp_rw [coeffs, Finset.image_subset_iff]
simp_all [coeff_one]
@[deprecated (since := "2024-05-17")] alias frange_one := coeffs_one
theorem coeff_mem_coeffs (p : R[X]) (n : ℕ) (h : p.coeff n ≠ 0) : p.coeff n ∈ p.coeffs := by
classical
simp only [coeffs, exists_prop, mem_support_iff, Finset.mem_image, Ne]
exact ⟨n, h, rfl⟩
@[deprecated (since := "2024-05-17")] alias coeff_mem_frange := coeff_mem_coeffs
theorem geom_sum_X_comp_X_add_one_eq_sum (n : ℕ) :
(∑ i ∈ range n, (X : R[X]) ^ i).comp (X + 1) =
(Finset.range n).sum fun i : ℕ => (n.choose (i + 1) : R[X]) * X ^ i := by
ext i
trans (n.choose (i + 1) : R); swap
· simp only [finset_sum_coeff, ← C_eq_natCast, coeff_C_mul_X_pow]
rw [Finset.sum_eq_single i, if_pos rfl]
· simp (config := { contextual := true }) only [@eq_comm _ i, if_false, eq_self_iff_true,
imp_true_iff]
· simp (config := { contextual := true }) only [Nat.lt_add_one_iff, Nat.choose_eq_zero_of_lt,
Nat.cast_zero, Finset.mem_range, not_lt, eq_self_iff_true, if_true, imp_true_iff]
induction' n with n ih generalizing i
· dsimp; simp only [zero_comp, coeff_zero, Nat.cast_zero]
· simp only [geom_sum_succ', ih, add_comp, X_pow_comp, coeff_add, Nat.choose_succ_succ,
Nat.cast_add, coeff_X_add_one_pow]
theorem Monic.geom_sum {P : R[X]} (hP : P.Monic) (hdeg : 0 < P.natDegree) {n : ℕ} (hn : n ≠ 0) :
(∑ i ∈ range n, P ^ i).Monic := by
nontriviality R
obtain ⟨n, rfl⟩ := Nat.exists_eq_succ_of_ne_zero hn
rw [geom_sum_succ']
refine (hP.pow _).add_of_left ?_
refine lt_of_le_of_lt (degree_sum_le _ _) ?_
rw [Finset.sup_lt_iff]
· simp only [Finset.mem_range, degree_eq_natDegree (hP.pow _).ne_zero]
simp only [Nat.cast_lt, hP.natDegree_pow]
intro k
exact nsmul_lt_nsmul_left hdeg
· rw [bot_lt_iff_ne_bot, Ne, degree_eq_bot]
exact (hP.pow _).ne_zero
theorem Monic.geom_sum' {P : R[X]} (hP : P.Monic) (hdeg : 0 < P.degree) {n : ℕ} (hn : n ≠ 0) :
(∑ i ∈ range n, P ^ i).Monic :=
hP.geom_sum (natDegree_pos_iff_degree_pos.2 hdeg) hn
theorem monic_geom_sum_X {n : ℕ} (hn : n ≠ 0) : (∑ i ∈ range n, (X : R[X]) ^ i).Monic := by
nontriviality R
apply monic_X.geom_sum _ hn
simp only [natDegree_X, zero_lt_one]
end Semiring
section Ring
variable [Ring R]
/-- Given a polynomial, return the polynomial whose coefficients are in
the ring closure of the original coefficients. -/
def restriction (p : R[X]) : Polynomial (Subring.closure (↑p.coeffs : Set R)) :=
∑ i ∈ p.support,
monomial i
(⟨p.coeff i,
letI := Classical.decEq R
if H : p.coeff i = 0 then H.symm ▸ (Subring.closure _).zero_mem
else Subring.subset_closure (p.coeff_mem_coeffs _ H)⟩ :
Subring.closure (↑p.coeffs : Set R))
@[simp]
theorem coeff_restriction {p : R[X]} {n : ℕ} : ↑(coeff (restriction p) n) = coeff p n := by
classical
simp only [restriction, coeff_monomial, finset_sum_coeff, mem_support_iff, Finset.sum_ite_eq',
Ne, ite_not]
split_ifs with h
· rw [h]
rfl
· rfl
-- Porting note: removed @[simp] as simp can prove this
theorem coeff_restriction' {p : R[X]} {n : ℕ} : (coeff (restriction p) n).1 = coeff p n :=
coeff_restriction
@[simp]
theorem support_restriction (p : R[X]) : support (restriction p) = support p := by
ext i
simp only [mem_support_iff, not_iff_not, Ne]
conv_rhs => rw [← coeff_restriction]
exact ⟨fun H => by rw [H, ZeroMemClass.coe_zero], fun H => Subtype.coe_injective H⟩
@[simp]
theorem map_restriction {R : Type u} [CommRing R] (p : R[X]) :
p.restriction.map (algebraMap _ _) = p :=
ext fun n => by rw [coeff_map, Algebra.algebraMap_ofSubring_apply, coeff_restriction]
@[simp]
theorem degree_restriction {p : R[X]} : (restriction p).degree = p.degree := by simp [degree]
@[simp]
theorem natDegree_restriction {p : R[X]} : (restriction p).natDegree = p.natDegree := by
simp [natDegree]
@[simp]
theorem monic_restriction {p : R[X]} : Monic (restriction p) ↔ Monic p := by
simp only [Monic, leadingCoeff, natDegree_restriction]
rw [← @coeff_restriction _ _ p]
exact ⟨fun H => by rw [H, OneMemClass.coe_one], fun H => Subtype.coe_injective H⟩
@[simp]
theorem restriction_zero : restriction (0 : R[X]) = 0 := by
simp only [restriction, Finset.sum_empty, support_zero]
@[simp]
theorem restriction_one : restriction (1 : R[X]) = 1 :=
ext fun i => Subtype.eq <| by rw [coeff_restriction', coeff_one, coeff_one]; split_ifs <;> rfl
variable [Semiring S] {f : R →+* S} {x : S}
theorem eval₂_restriction {p : R[X]} :
eval₂ f x p =
eval₂ (f.comp (Subring.subtype (Subring.closure (p.coeffs : Set R)))) x p.restriction := by
simp only [eval₂_eq_sum, sum, support_restriction, ← @coeff_restriction _ _ p, RingHom.comp_apply,
Subring.coeSubtype]
section ToSubring
variable (p : R[X]) (T : Subring R)
/-- Given a polynomial `p` and a subring `T` that contains the coefficients of `p`,
return the corresponding polynomial whose coefficients are in `T`. -/
def toSubring (hp : (↑p.coeffs : Set R) ⊆ T) : T[X] :=
∑ i ∈ p.support,
monomial i
(⟨p.coeff i,
letI := Classical.decEq R
if H : p.coeff i = 0 then H.symm ▸ T.zero_mem else hp (p.coeff_mem_coeffs _ H)⟩ : T)
variable (hp : (↑p.coeffs : Set R) ⊆ T)
@[simp]
theorem coeff_toSubring {n : ℕ} : ↑(coeff (toSubring p T hp) n) = coeff p n := by
classical
simp only [toSubring, coeff_monomial, finset_sum_coeff, mem_support_iff, Finset.sum_ite_eq',
Ne, ite_not]
split_ifs with h
· rw [h]
rfl
· rfl
-- Porting note: removed @[simp] as simp can prove this
theorem coeff_toSubring' {n : ℕ} : (coeff (toSubring p T hp) n).1 = coeff p n :=
coeff_toSubring _ _ hp
@[simp]
theorem support_toSubring : support (toSubring p T hp) = support p := by
ext i
simp only [mem_support_iff, not_iff_not, Ne]
conv_rhs => rw [← coeff_toSubring p T hp]
exact ⟨fun H => by rw [H, ZeroMemClass.coe_zero], fun H => Subtype.coe_injective H⟩
@[simp]
theorem degree_toSubring : (toSubring p T hp).degree = p.degree := by simp [degree]
@[simp]
theorem natDegree_toSubring : (toSubring p T hp).natDegree = p.natDegree := by simp [natDegree]
@[simp]
theorem monic_toSubring : Monic (toSubring p T hp) ↔ Monic p := by
simp_rw [Monic, leadingCoeff, natDegree_toSubring, ← coeff_toSubring p T hp]
exact ⟨fun H => by rw [H, OneMemClass.coe_one], fun H => Subtype.coe_injective H⟩
@[simp]
theorem toSubring_zero : toSubring (0 : R[X]) T (by simp [coeffs]) = 0 := by
ext i
simp
@[simp]
theorem toSubring_one :
toSubring (1 : R[X]) T
(Set.Subset.trans coeffs_one <| Finset.singleton_subset_set_iff.2 T.one_mem) =
1 :=
ext fun i => Subtype.eq <| by
rw [coeff_toSubring', coeff_one, coeff_one, apply_ite Subtype.val, ZeroMemClass.coe_zero,
OneMemClass.coe_one]
@[simp]
theorem map_toSubring : (p.toSubring T hp).map (Subring.subtype T) = p := by
ext n
simp [coeff_map]
end ToSubring
variable (T : Subring R)
/-- Given a polynomial whose coefficients are in some subring, return
the corresponding polynomial whose coefficients are in the ambient ring. -/
def ofSubring (p : T[X]) : R[X] :=
∑ i ∈ p.support, monomial i (p.coeff i : R)
theorem coeff_ofSubring (p : T[X]) (n : ℕ) : coeff (ofSubring T p) n = (coeff p n : T) := by
simp only [ofSubring, coeff_monomial, finset_sum_coeff, mem_support_iff, Finset.sum_ite_eq',
ite_eq_right_iff, Ne, ite_not, Classical.not_not, ite_eq_left_iff]
intro h
rw [h, ZeroMemClass.coe_zero]
@[simp]
theorem coeffs_ofSubring {p : T[X]} : (↑(p.ofSubring T).coeffs : Set R) ⊆ T := by
classical
intro i hi
simp only [coeffs, Set.mem_image, mem_support_iff, Ne, Finset.mem_coe,
(Finset.coe_image)] at hi
rcases hi with ⟨n, _, h'n⟩
rw [← h'n, coeff_ofSubring]
exact Subtype.mem (coeff p n : T)
@[deprecated (since := "2024-05-17")] alias frange_ofSubring := coeffs_ofSubring
end Ring
section CommRing
variable [CommRing R]
section ModByMonic
variable {q : R[X]}
theorem mem_ker_modByMonic (hq : q.Monic) {p : R[X]} :
p ∈ LinearMap.ker (modByMonicHom q) ↔ q ∣ p :=
LinearMap.mem_ker.trans (modByMonic_eq_zero_iff_dvd hq)
@[simp]
theorem ker_modByMonicHom (hq : q.Monic) :
LinearMap.ker (Polynomial.modByMonicHom q) = (Ideal.span {q}).restrictScalars R :=
Submodule.ext fun _ => (mem_ker_modByMonic hq).trans Ideal.mem_span_singleton.symm
end ModByMonic
end CommRing
end Polynomial
namespace Ideal
open Polynomial
section Semiring
variable [Semiring R]
/-- Transport an ideal of `R[X]` to an `R`-submodule of `R[X]`. -/
def ofPolynomial (I : Ideal R[X]) : Submodule R R[X] where
carrier := I.carrier
zero_mem' := I.zero_mem
add_mem' := I.add_mem
smul_mem' c x H := by
rw [← C_mul']
exact I.mul_mem_left _ H
variable {I : Ideal R[X]}
theorem mem_ofPolynomial (x) : x ∈ I.ofPolynomial ↔ x ∈ I :=
Iff.rfl
variable (I)
/-- Given an ideal `I` of `R[X]`, make the `R`-submodule of `I`
consisting of polynomials of degree ≤ `n`. -/
def degreeLE (n : WithBot ℕ) : Submodule R R[X] :=
Polynomial.degreeLE R n ⊓ I.ofPolynomial
/-- Given an ideal `I` of `R[X]`, make the ideal in `R` of
leading coefficients of polynomials in `I` with degree ≤ `n`. -/
def leadingCoeffNth (n : ℕ) : Ideal R :=
(I.degreeLE n).map <| lcoeff R n
/-- Given an ideal `I` in `R[X]`, make the ideal in `R` of the
leading coefficients in `I`. -/
def leadingCoeff : Ideal R :=
⨆ n : ℕ, I.leadingCoeffNth n
end Semiring
section CommSemiring
variable [CommSemiring R] [Semiring S]
/-- If every coefficient of a polynomial is in an ideal `I`, then so is the polynomial itself -/
theorem polynomial_mem_ideal_of_coeff_mem_ideal (I : Ideal R[X]) (p : R[X])
(hp : ∀ n : ℕ, p.coeff n ∈ I.comap (C : R →+* R[X])) : p ∈ I :=
sum_C_mul_X_pow_eq p ▸ Submodule.sum_mem I fun n _ => I.mul_mem_right _ (hp n)
/-- The push-forward of an ideal `I` of `R` to `R[X]` via inclusion
is exactly the set of polynomials whose coefficients are in `I` -/
theorem mem_map_C_iff {I : Ideal R} {f : R[X]} :
f ∈ (Ideal.map (C : R →+* R[X]) I : Ideal R[X]) ↔ ∀ n : ℕ, f.coeff n ∈ I := by
constructor
· intro hf
apply @Submodule.span_induction _ _ _ _ _ f _ _ hf
· intro f hf n
cases' (Set.mem_image _ _ _).mp hf with x hx
rw [← hx.right, coeff_C]
by_cases h : n = 0
· simpa [h] using hx.left
· simp [h]
· simp
· exact fun f g hf hg n => by simp [I.add_mem (hf n) (hg n)]
· refine fun f g hg n => ?_
rw [smul_eq_mul, coeff_mul]
exact I.sum_mem fun c _ => I.mul_mem_left (f.coeff c.fst) (hg c.snd)
· intro hf
rw [← sum_monomial_eq f]
refine (I.map C : Ideal R[X]).sum_mem fun n _ => ?_
simp only [← C_mul_X_pow_eq_monomial, ne_eq]
rw [mul_comm]
exact (I.map C : Ideal R[X]).mul_mem_left _ (mem_map_of_mem _ (hf n))
theorem _root_.Polynomial.ker_mapRingHom (f : R →+* S) :
LinearMap.ker (Polynomial.mapRingHom f).toSemilinearMap = f.ker.map (C : R →+* R[X]) := by
ext
simp only [LinearMap.mem_ker, RingHom.toSemilinearMap_apply, coe_mapRingHom]
rw [mem_map_C_iff, Polynomial.ext_iff]
simp_rw [RingHom.mem_ker f]
simp
variable (I : Ideal R[X])
theorem mem_leadingCoeffNth (n : ℕ) (x) :
x ∈ I.leadingCoeffNth n ↔ ∃ p ∈ I, degree p ≤ n ∧ p.leadingCoeff = x := by
simp only [leadingCoeffNth, degreeLE, Submodule.mem_map, lcoeff_apply, Submodule.mem_inf,
mem_degreeLE]
constructor
· rintro ⟨p, ⟨hpdeg, hpI⟩, rfl⟩
rcases lt_or_eq_of_le hpdeg with hpdeg | hpdeg
· refine ⟨0, I.zero_mem, bot_le, ?_⟩
rw [leadingCoeff_zero, eq_comm]
exact coeff_eq_zero_of_degree_lt hpdeg
· refine ⟨p, hpI, le_of_eq hpdeg, ?_⟩
rw [Polynomial.leadingCoeff, natDegree, hpdeg, Nat.cast_withBot, WithBot.unbot'_coe]
· rintro ⟨p, hpI, hpdeg, rfl⟩
have : natDegree p + (n - natDegree p) = n :=
add_tsub_cancel_of_le (natDegree_le_of_degree_le hpdeg)
refine ⟨p * X ^ (n - natDegree p), ⟨?_, I.mul_mem_right _ hpI⟩, ?_⟩
· apply le_trans (degree_mul_le _ _) _
apply le_trans (add_le_add degree_le_natDegree (degree_X_pow_le _)) _
rw [← Nat.cast_add, this]
· rw [Polynomial.leadingCoeff, ← coeff_mul_X_pow p (n - natDegree p), this]
theorem mem_leadingCoeffNth_zero (x) : x ∈ I.leadingCoeffNth 0 ↔ C x ∈ I :=
(mem_leadingCoeffNth _ _ _).trans
⟨fun ⟨p, hpI, hpdeg, hpx⟩ => by
rwa [← hpx, Polynomial.leadingCoeff,
Nat.eq_zero_of_le_zero (natDegree_le_of_degree_le hpdeg), ← eq_C_of_degree_le_zero hpdeg],
fun hx => ⟨C x, hx, degree_C_le, leadingCoeff_C x⟩⟩
theorem leadingCoeffNth_mono {m n : ℕ} (H : m ≤ n) : I.leadingCoeffNth m ≤ I.leadingCoeffNth n := by
intro r hr
simp only [SetLike.mem_coe, mem_leadingCoeffNth] at hr ⊢
rcases hr with ⟨p, hpI, hpdeg, rfl⟩
refine ⟨p * X ^ (n - m), I.mul_mem_right _ hpI, ?_, leadingCoeff_mul_X_pow⟩
refine le_trans (degree_mul_le _ _) ?_
refine le_trans (add_le_add hpdeg (degree_X_pow_le _)) ?_
rw [← Nat.cast_add, add_tsub_cancel_of_le H]
theorem mem_leadingCoeff (x) : x ∈ I.leadingCoeff ↔ ∃ p ∈ I, Polynomial.leadingCoeff p = x := by
rw [leadingCoeff, Submodule.mem_iSup_of_directed]
· simp only [mem_leadingCoeffNth]
constructor
· rintro ⟨i, p, hpI, _, rfl⟩
exact ⟨p, hpI, rfl⟩
rintro ⟨p, hpI, rfl⟩
exact ⟨natDegree p, p, hpI, degree_le_natDegree, rfl⟩
intro i j
exact
⟨i + j, I.leadingCoeffNth_mono (Nat.le_add_right _ _),
I.leadingCoeffNth_mono (Nat.le_add_left _ _)⟩
/-- If `I` is an ideal, and `pᵢ` is a finite family of polynomials each satisfying
`∀ k, (pᵢ)ₖ ∈ Iⁿⁱ⁻ᵏ` for some `nᵢ`, then `p = ∏ pᵢ` also satisfies `∀ k, pₖ ∈ Iⁿ⁻ᵏ` with `n = ∑ nᵢ`.
-/
theorem _root_.Polynomial.coeff_prod_mem_ideal_pow_tsub {ι : Type*} (s : Finset ι) (f : ι → R[X])
(I : Ideal R) (n : ι → ℕ) (h : ∀ i ∈ s, ∀ (k), (f i).coeff k ∈ I ^ (n i - k)) (k : ℕ) :
(s.prod f).coeff k ∈ I ^ (s.sum n - k) := by
classical
induction' s using Finset.induction with a s ha hs generalizing k
· rw [sum_empty, prod_empty, coeff_one, zero_tsub, pow_zero, Ideal.one_eq_top]
exact Submodule.mem_top
· rw [sum_insert ha, prod_insert ha, coeff_mul]
apply sum_mem
rintro ⟨i, j⟩ e
obtain rfl : i + j = k := mem_antidiagonal.mp e
apply Ideal.pow_le_pow_right add_tsub_add_le_tsub_add_tsub
rw [pow_add]
exact
Ideal.mul_mem_mul (h _ (Finset.mem_insert.mpr <| Or.inl rfl) _)
(hs (fun i hi k => h _ (Finset.mem_insert.mpr <| Or.inr hi) _) j)
end CommSemiring
section Ring
variable [Ring R]
/-- `R[X]` is never a field for any ring `R`. -/
theorem polynomial_not_isField : ¬IsField R[X] := by
nontriviality R
intro hR
obtain ⟨p, hp⟩ := hR.mul_inv_cancel X_ne_zero
have hp0 : p ≠ 0 := right_ne_zero_of_mul_eq_one hp
have := degree_lt_degree_mul_X hp0
rw [← X_mul, congr_arg degree hp, degree_one, Nat.WithBot.lt_zero_iff, degree_eq_bot] at this
exact hp0 this
/-- The only constant in a maximal ideal over a field is `0`. -/
theorem eq_zero_of_constant_mem_of_maximal (hR : IsField R) (I : Ideal R[X]) [hI : I.IsMaximal]
(x : R) (hx : C x ∈ I) : x = 0 := by
refine Classical.by_contradiction fun hx0 => hI.ne_top ((eq_top_iff_one I).2 ?_)
obtain ⟨y, hy⟩ := hR.mul_inv_cancel hx0
convert I.mul_mem_left (C y) hx
rw [← C.map_mul, hR.mul_comm y x, hy, RingHom.map_one]
end Ring
section CommRing
variable [CommRing R]
/-- If `P` is a prime ideal of `R`, then `P.R[x]` is a prime ideal of `R[x]`. -/
theorem isPrime_map_C_iff_isPrime (P : Ideal R) :
IsPrime (map (C : R →+* R[X]) P : Ideal R[X]) ↔ IsPrime P := by
-- Note: the following proof avoids quotient rings
-- It can be golfed substantially by using something like
-- `(Quotient.isDomain_iff_prime (map C P : Ideal R[X]))`
constructor
· intro H
have := comap_isPrime C (map C P)
convert this using 1
ext x
simp only [mem_comap, mem_map_C_iff]
constructor
· rintro h (- | n)
· rwa [coeff_C_zero]
· simp only [coeff_C_ne_zero (Nat.succ_ne_zero _), Submodule.zero_mem]
· intro h
simpa only [coeff_C_zero] using h 0
· intro h
constructor
· rw [Ne, eq_top_iff_one, mem_map_C_iff, not_forall]
use 0
rw [coeff_one_zero, ← eq_top_iff_one]
exact h.1
· intro f g
simp only [mem_map_C_iff]
contrapose!
rintro ⟨hf, hg⟩
classical
let m := Nat.find hf
let n := Nat.find hg
refine ⟨m + n, ?_⟩
rw [coeff_mul, ← Finset.insert_erase ((Finset.mem_antidiagonal (a := (m,n))).mpr rfl),
Finset.sum_insert (Finset.not_mem_erase _ _), (P.add_mem_iff_left _).not]
· apply mt h.2
rw [not_or]
exact ⟨Nat.find_spec hf, Nat.find_spec hg⟩
apply P.sum_mem
rintro ⟨i, j⟩ hij
rw [Finset.mem_erase, Finset.mem_antidiagonal] at hij
simp only [Ne, Prod.mk.inj_iff, not_and_or] at hij
obtain hi | hj : i < m ∨ j < n := by
rw [or_iff_not_imp_left, not_lt, le_iff_lt_or_eq]
rintro (hmi | rfl)
· rw [← not_le]
intro hnj
exact (add_lt_add_of_lt_of_le hmi hnj).ne hij.2.symm
· simp only [eq_self_iff_true, not_true, false_or_iff, add_right_inj,
not_and_self_iff] at hij
· rw [mul_comm]
apply P.mul_mem_left
exact Classical.not_not.1 (Nat.find_min hf hi)
· apply P.mul_mem_left
exact Classical.not_not.1 (Nat.find_min hg hj)
/-- If `P` is a prime ideal of `R`, then `P.R[x]` is a prime ideal of `R[x]`. -/
theorem isPrime_map_C_of_isPrime {P : Ideal R} (H : IsPrime P) :
IsPrime (map (C : R →+* R[X]) P : Ideal R[X]) :=
(isPrime_map_C_iff_isPrime P).mpr H
theorem is_fg_degreeLE [IsNoetherianRing R] (I : Ideal R[X]) (n : ℕ) :
Submodule.FG (I.degreeLE n) :=
letI := Classical.decEq R
isNoetherian_submodule_left.1
(isNoetherian_of_fg_of_noetherian _ ⟨_, degreeLE_eq_span_X_pow.symm⟩) _
end CommRing
end Ideal
variable {σ : Type v} {M : Type w}
variable [CommRing R] [CommRing S] [AddCommGroup M] [Module R M]
section Prime
variable (σ) {r : R}
namespace Polynomial
theorem prime_C_iff : Prime (C r) ↔ Prime r :=
⟨comap_prime C (evalRingHom (0 : R)) fun r => eval_C, fun hr => by
have := hr.1
rw [← Ideal.span_singleton_prime] at hr ⊢
· rw [← Set.image_singleton, ← Ideal.map_span]
apply Ideal.isPrime_map_C_of_isPrime hr
· intro h; apply (this (C_eq_zero.mp h))
· assumption⟩
end Polynomial
namespace MvPolynomial
private theorem prime_C_iff_of_fintype {R : Type u} (σ : Type v) {r : R} [CommRing R] [Fintype σ] :
Prime (C r : MvPolynomial σ R) ↔ Prime r := by
rw [(renameEquiv R (Fintype.equivFin σ)).toMulEquiv.prime_iff]
convert_to Prime (C r) ↔ _
· congr!
apply rename_C
· symm
induction' Fintype.card σ with d hd
· exact (isEmptyAlgEquiv R (Fin 0)).toMulEquiv.symm.prime_iff
· rw [hd, ← Polynomial.prime_C_iff]
convert (finSuccEquiv R d).toMulEquiv.symm.prime_iff (p := Polynomial.C (C r))
rw [← finSuccEquiv_comp_C_eq_C]
simp_rw [RingHom.coe_comp, RingHom.coe_coe, Function.comp_apply, MulEquiv.symm_mk,
AlgEquiv.toEquiv_eq_coe, AlgEquiv.symm_toEquiv_eq_symm, MulEquiv.coe_mk, EquivLike.coe_coe]
theorem prime_C_iff : Prime (C r : MvPolynomial σ R) ↔ Prime r :=
⟨comap_prime C constantCoeff (constantCoeff_C _), fun hr =>
⟨fun h => hr.1 <| by
rw [← C_inj, h]
simp,
fun h =>
hr.2.1 <| by
rw [← constantCoeff_C _ r]
exact h.map _,
fun a b hd => by
obtain ⟨s, a', b', rfl, rfl⟩ := exists_finset_rename₂ a b
rw [← algebraMap_eq] at hd
have : algebraMap R _ r ∣ a' * b' := by
convert killCompl Subtype.coe_injective |>.toRingHom.map_dvd hd <;> simp
rw [← rename_C ((↑) : s → σ)]
let f := (rename (R := R) ((↑) : s → σ)).toRingHom
exact (((prime_C_iff_of_fintype s).2 hr).2.2 a' b' this).imp f.map_dvd f.map_dvd⟩⟩
variable {σ}
theorem prime_rename_iff (s : Set σ) {p : MvPolynomial s R} :
Prime (rename ((↑) : s → σ) p) ↔ Prime (p : MvPolynomial s R) := by
classical
symm
let eqv :=
(sumAlgEquiv R (↥sᶜ) s).symm.trans
(renameEquiv R <| (Equiv.sumComm (↥sᶜ) s).trans <| Equiv.Set.sumCompl s)
have : (rename (↑)).toRingHom = eqv.toAlgHom.toRingHom.comp C := by
apply ringHom_ext
· intro
simp only [eqv, AlgHom.toRingHom_eq_coe, RingHom.coe_coe, rename_C,
AlgEquiv.toAlgHom_eq_coe, AlgEquiv.toAlgHom_toRingHom, RingHom.coe_comp,
AlgEquiv.coe_trans, Function.comp_apply, MvPolynomial.sumAlgEquiv_symm_apply,
iterToSum_C_C, renameEquiv_apply, Equiv.coe_trans, Equiv.sumComm_apply]
· intro
simp only [eqv, AlgHom.toRingHom_eq_coe, RingHom.coe_coe, rename_X,
AlgEquiv.toAlgHom_eq_coe, AlgEquiv.toAlgHom_toRingHom, RingHom.coe_comp,
AlgEquiv.coe_trans, Function.comp_apply, MvPolynomial.sumAlgEquiv_symm_apply,
iterToSum_C_X, renameEquiv_apply, Equiv.coe_trans, Equiv.sumComm_apply, Sum.swap_inr,
Equiv.Set.sumCompl_apply_inl]
apply_fun (· p) at this
simp_rw [AlgHom.toRingHom_eq_coe, RingHom.coe_coe] at this
rw [← prime_C_iff, eqv.toMulEquiv.prime_iff, this]
simp only [MulEquiv.coe_mk, AlgEquiv.toEquiv_eq_coe, EquivLike.coe_coe, AlgEquiv.trans_apply,
MvPolynomial.sumAlgEquiv_symm_apply, renameEquiv_apply, Equiv.coe_trans, Equiv.sumComm_apply,
AlgEquiv.toAlgHom_eq_coe, AlgEquiv.toAlgHom_toRingHom, RingHom.coe_comp, RingHom.coe_coe,
AlgEquiv.coe_trans, Function.comp_apply]
end MvPolynomial
end Prime
namespace Polynomial
instance (priority := 100) wfDvdMonoid {R : Type*} [CommRing R] [IsDomain R] [WfDvdMonoid R] :
WfDvdMonoid R[X] where
wellFounded_dvdNotUnit := by
classical
refine
RelHomClass.wellFounded
(⟨fun p : R[X] =>
((if p = 0 then ⊤ else ↑p.degree : WithTop (WithBot ℕ)), p.leadingCoeff), ?_⟩ :
DvdNotUnit →r Prod.Lex (· < ·) DvdNotUnit)
(wellFounded_lt.prod_lex ‹WfDvdMonoid R›.wellFounded_dvdNotUnit)
rintro a b ⟨ane0, ⟨c, ⟨not_unit_c, rfl⟩⟩⟩
dsimp
rw [Polynomial.degree_mul, if_neg ane0]
split_ifs with hac
· rw [hac, Polynomial.leadingCoeff_zero]
apply Prod.Lex.left
exact WithTop.coe_lt_top _
have cne0 : c ≠ 0 := right_ne_zero_of_mul hac
simp only [cne0, ane0, Polynomial.leadingCoeff_mul]
by_cases hdeg : c.degree = (0 : ℕ)
· simp only [hdeg, Nat.cast_zero, add_zero]
refine Prod.Lex.right _ ⟨?_, ⟨c.leadingCoeff, fun unit_c => not_unit_c ?_, rfl⟩⟩
· rwa [Ne, Polynomial.leadingCoeff_eq_zero]
rw [Polynomial.isUnit_iff, Polynomial.eq_C_of_degree_eq_zero hdeg]
use c.leadingCoeff, unit_c
rw [Polynomial.leadingCoeff, Polynomial.natDegree_eq_of_degree_eq_some hdeg]
· apply Prod.Lex.left
rw [Polynomial.degree_eq_natDegree cne0] at *
simp only [Nat.cast_inj] at hdeg
rw [WithTop.coe_lt_coe, Polynomial.degree_eq_natDegree ane0, ← Nat.cast_add, Nat.cast_lt]
exact lt_add_of_pos_right _ (Nat.pos_of_ne_zero hdeg)
end Polynomial
/-- Hilbert basis theorem: a polynomial ring over a noetherian ring is a noetherian ring. -/
protected theorem Polynomial.isNoetherianRing [inst : IsNoetherianRing R] : IsNoetherianRing R[X] :=
isNoetherianRing_iff.2
⟨fun I : Ideal R[X] =>
let M :=
WellFounded.min (isNoetherian_iff_wellFounded.1 (by infer_instance))
(Set.range I.leadingCoeffNth) ⟨_, ⟨0, rfl⟩⟩
have hm : M ∈ Set.range I.leadingCoeffNth := WellFounded.min_mem _ _ _
let ⟨N, HN⟩ := hm
let ⟨s, hs⟩ := I.is_fg_degreeLE N
have hm2 : ∀ k, I.leadingCoeffNth k ≤ M := fun k =>
Or.casesOn (le_or_lt k N) (fun h => HN ▸ I.leadingCoeffNth_mono h) fun h x hx =>
Classical.by_contradiction fun hxm =>
haveI : IsNoetherian R R := inst
have : ¬M < I.leadingCoeffNth k := by
refine WellFounded.not_lt_min (wellFounded_submodule_gt R R) _ _ ?_; exact ⟨k, rfl⟩
this ⟨HN ▸ I.leadingCoeffNth_mono (le_of_lt h), fun H => hxm (H hx)⟩
have hs2 : ∀ {x}, x ∈ I.degreeLE N → x ∈ Ideal.span (↑s : Set R[X]) :=
hs ▸ fun hx =>
Submodule.span_induction hx (fun _ hx => Ideal.subset_span hx) (Ideal.zero_mem _)
(fun _ _ => Ideal.add_mem _) fun c f hf => f.C_mul' c ▸ Ideal.mul_mem_left _ _ hf
⟨s, le_antisymm (Ideal.span_le.2 fun x hx =>
have : x ∈ I.degreeLE N := hs ▸ Submodule.subset_span hx
this.2) <| by
have : Submodule.span R[X] ↑s = Ideal.span ↑s := rfl
rw [this]
intro p hp
generalize hn : p.natDegree = k
induction' k using Nat.strong_induction_on with k ih generalizing p
rcases le_or_lt k N with h | h
· subst k
refine hs2 ⟨Polynomial.mem_degreeLE.2
(le_trans Polynomial.degree_le_natDegree <| WithBot.coe_le_coe.2 h), hp⟩
· have hp0 : p ≠ 0 := by
rintro rfl
cases hn
exact Nat.not_lt_zero _ h
have : (0 : R) ≠ 1 := by
intro h
apply hp0
ext i
refine (mul_one _).symm.trans ?_
rw [← h, mul_zero]
rfl
haveI : Nontrivial R := ⟨⟨0, 1, this⟩⟩
have : p.leadingCoeff ∈ I.leadingCoeffNth N := by
rw [HN]
exact hm2 k ((I.mem_leadingCoeffNth _ _).2
⟨_, hp, hn ▸ Polynomial.degree_le_natDegree, rfl⟩)
rw [I.mem_leadingCoeffNth] at this
rcases this with ⟨q, hq, hdq, hlqp⟩
have hq0 : q ≠ 0 := by
intro H
rw [← Polynomial.leadingCoeff_eq_zero] at H
rw [hlqp, Polynomial.leadingCoeff_eq_zero] at H
exact hp0 H
have h1 : p.degree = (q * Polynomial.X ^ (k - q.natDegree)).degree := by
rw [Polynomial.degree_mul', Polynomial.degree_X_pow]
· rw [Polynomial.degree_eq_natDegree hp0, Polynomial.degree_eq_natDegree hq0]
rw [← Nat.cast_add, add_tsub_cancel_of_le, hn]
· refine le_trans (Polynomial.natDegree_le_of_degree_le hdq) (le_of_lt h)
rw [Polynomial.leadingCoeff_X_pow, mul_one]
exact mt Polynomial.leadingCoeff_eq_zero.1 hq0
have h2 : p.leadingCoeff = (q * Polynomial.X ^ (k - q.natDegree)).leadingCoeff := by
rw [← hlqp, Polynomial.leadingCoeff_mul_X_pow]
have := Polynomial.degree_sub_lt h1 hp0 h2
rw [Polynomial.degree_eq_natDegree hp0] at this
rw [← sub_add_cancel p (q * Polynomial.X ^ (k - q.natDegree))]
convert (Ideal.span ↑s).add_mem _ ((Ideal.span (s : Set R[X])).mul_mem_right _ _)
· by_cases hpq : p - q * Polynomial.X ^ (k - q.natDegree) = 0
· rw [hpq]
exact Ideal.zero_mem _
refine ih _ ?_ (I.sub_mem hp (I.mul_mem_right _ hq)) rfl
rwa [Polynomial.degree_eq_natDegree hpq, Nat.cast_lt, hn] at this
exact hs2 ⟨Polynomial.mem_degreeLE.2 hdq, hq⟩⟩⟩
attribute [instance] Polynomial.isNoetherianRing
namespace Polynomial
theorem exists_irreducible_of_degree_pos {R : Type u} [CommRing R] [IsDomain R] [WfDvdMonoid R]
{f : R[X]} (hf : 0 < f.degree) : ∃ g, Irreducible g ∧ g ∣ f :=
WfDvdMonoid.exists_irreducible_factor (fun huf => ne_of_gt hf <| degree_eq_zero_of_isUnit huf)
fun hf0 => not_lt_of_lt hf <| hf0.symm ▸ (@degree_zero R _).symm ▸ WithBot.bot_lt_coe _
theorem exists_irreducible_of_natDegree_pos {R : Type u} [CommRing R] [IsDomain R] [WfDvdMonoid R]
{f : R[X]} (hf : 0 < f.natDegree) : ∃ g, Irreducible g ∧ g ∣ f :=
exists_irreducible_of_degree_pos <| by
contrapose! hf
exact natDegree_le_of_degree_le hf
theorem exists_irreducible_of_natDegree_ne_zero {R : Type u} [CommRing R] [IsDomain R]
[WfDvdMonoid R] {f : R[X]} (hf : f.natDegree ≠ 0) : ∃ g, Irreducible g ∧ g ∣ f :=
exists_irreducible_of_natDegree_pos <| Nat.pos_of_ne_zero hf
theorem linearIndependent_powers_iff_aeval (f : M →ₗ[R] M) (v : M) :
(LinearIndependent R fun n : ℕ => (f ^ n) v) ↔ ∀ p : R[X], aeval f p v = 0 → p = 0 := by
rw [linearIndependent_iff]
simp only [Finsupp.total_apply, aeval_endomorphism, forall_iff_forall_finsupp, Sum, support,
coeff, ofFinsupp_eq_zero]
exact Iff.rfl
attribute [-instance] Ring.toNonAssocRing
theorem disjoint_ker_aeval_of_coprime (f : M →ₗ[R] M) {p q : R[X]} (hpq : IsCoprime p q) :
Disjoint (LinearMap.ker (aeval f p)) (LinearMap.ker (aeval f q)) := by
rw [disjoint_iff_inf_le]
intro v hv
rcases hpq with ⟨p', q', hpq'⟩
simpa [LinearMap.mem_ker.1 (Submodule.mem_inf.1 hv).1,
LinearMap.mem_ker.1 (Submodule.mem_inf.1 hv).2] using
congr_arg (fun p : R[X] => aeval f p v) hpq'.symm
theorem sup_aeval_range_eq_top_of_coprime (f : M →ₗ[R] M) {p q : R[X]} (hpq : IsCoprime p q) :
LinearMap.range (aeval f p) ⊔ LinearMap.range (aeval f q) = ⊤ := by
rw [eq_top_iff]
intro v _
rw [Submodule.mem_sup]
rcases hpq with ⟨p', q', hpq'⟩
use aeval f (p * p') v
use LinearMap.mem_range.2 ⟨aeval f p' v, by simp only [LinearMap.mul_apply, aeval_mul]⟩
use aeval f (q * q') v
use LinearMap.mem_range.2 ⟨aeval f q' v, by simp only [LinearMap.mul_apply, aeval_mul]⟩
simpa only [mul_comm p p', mul_comm q q', aeval_one, aeval_add] using
congr_arg (fun p : R[X] => aeval f p v) hpq'
theorem sup_ker_aeval_le_ker_aeval_mul {f : M →ₗ[R] M} {p q : R[X]} :
LinearMap.ker (aeval f p) ⊔ LinearMap.ker (aeval f q) ≤ LinearMap.ker (aeval f (p * q)) := by
intro v hv
rcases Submodule.mem_sup.1 hv with ⟨x, hx, y, hy, hxy⟩
have h_eval_x : aeval f (p * q) x = 0 := by
rw [mul_comm, aeval_mul, LinearMap.mul_apply, LinearMap.mem_ker.1 hx, LinearMap.map_zero]
have h_eval_y : aeval f (p * q) y = 0 := by
rw [aeval_mul, LinearMap.mul_apply, LinearMap.mem_ker.1 hy, LinearMap.map_zero]
rw [LinearMap.mem_ker, ← hxy, LinearMap.map_add, h_eval_x, h_eval_y, add_zero]
theorem sup_ker_aeval_eq_ker_aeval_mul_of_coprime (f : M →ₗ[R] M) {p q : R[X]}
(hpq : IsCoprime p q) :
LinearMap.ker (aeval f p) ⊔ LinearMap.ker (aeval f q) = LinearMap.ker (aeval f (p * q)) := by
apply le_antisymm sup_ker_aeval_le_ker_aeval_mul
intro v hv
rw [Submodule.mem_sup]
rcases hpq with ⟨p', q', hpq'⟩
have h_eval₂_qpp' :=
calc
aeval f (q * (p * p')) v = aeval f (p' * (p * q)) v := by
rw [mul_comm, mul_assoc, mul_comm, mul_assoc, mul_comm q p]
_ = 0 := by rw [aeval_mul, LinearMap.mul_apply, LinearMap.mem_ker.1 hv, LinearMap.map_zero]
have h_eval₂_pqq' :=
calc
aeval f (p * (q * q')) v = aeval f (q' * (p * q)) v := by rw [← mul_assoc, mul_comm]
_ = 0 := by rw [aeval_mul, LinearMap.mul_apply, LinearMap.mem_ker.1 hv, LinearMap.map_zero]
rw [aeval_mul] at h_eval₂_qpp' h_eval₂_pqq'
refine
⟨aeval f (q * q') v, LinearMap.mem_ker.1 h_eval₂_pqq', aeval f (p * p') v,
LinearMap.mem_ker.1 h_eval₂_qpp', ?_⟩
rw [add_comm, mul_comm p p', mul_comm q q']
simpa only [map_add, map_mul, aeval_one] using congr_arg (fun p : R[X] => aeval f p v) hpq'
end Polynomial
namespace MvPolynomial
lemma aeval_natDegree_le {R : Type*} [CommSemiring R] {m n : ℕ}
(F : MvPolynomial σ R) (hF : F.totalDegree ≤ m)
(f : σ → Polynomial R) (hf : ∀ i, (f i).natDegree ≤ n) :
(MvPolynomial.aeval f F).natDegree ≤ m * n := by
rw [MvPolynomial.aeval_def, MvPolynomial.eval₂]
apply (Polynomial.natDegree_sum_le _ _).trans
apply Finset.sup_le
intro d hd
simp_rw [Function.comp_apply, ← C_eq_algebraMap]
apply (Polynomial.natDegree_C_mul_le _ _).trans
apply (Polynomial.natDegree_prod_le _ _).trans
have : ∑ i ∈ d.support, (d i) * n ≤ m * n := by
rw [← Finset.sum_mul]
apply mul_le_mul' (.trans _ hF) le_rfl
rw [MvPolynomial.totalDegree]
exact Finset.le_sup_of_le hd le_rfl
apply (Finset.sum_le_sum _).trans this
rintro i -
apply Polynomial.natDegree_pow_le.trans
exact mul_le_mul' le_rfl (hf i)
theorem isNoetherianRing_fin_0 [IsNoetherianRing R] :
IsNoetherianRing (MvPolynomial (Fin 0) R) := by
apply isNoetherianRing_of_ringEquiv R
symm; apply MvPolynomial.isEmptyRingEquiv R (Fin 0)
theorem isNoetherianRing_fin [IsNoetherianRing R] :
∀ {n : ℕ}, IsNoetherianRing (MvPolynomial (Fin n) R)
| 0 => isNoetherianRing_fin_0
| n + 1 =>
@isNoetherianRing_of_ringEquiv (Polynomial (MvPolynomial (Fin n) R)) _ _ _
(MvPolynomial.finSuccEquiv _ n).toRingEquiv.symm
(@Polynomial.isNoetherianRing (MvPolynomial (Fin n) R) _ isNoetherianRing_fin)
/-- The multivariate polynomial ring in finitely many variables over a noetherian ring
is itself a noetherian ring. -/
instance isNoetherianRing [Finite σ] [IsNoetherianRing R] :
IsNoetherianRing (MvPolynomial σ R) := by
cases nonempty_fintype σ
exact
@isNoetherianRing_of_ringEquiv (MvPolynomial (Fin (Fintype.card σ)) R) _ _ _
(renameEquiv R (Fintype.equivFin σ).symm).toRingEquiv isNoetherianRing_fin
/-- Auxiliary lemma:
Multivariate polynomials over an integral domain
with variables indexed by `Fin n` form an integral domain.
This fact is proven inductively,
and then used to prove the general case without any finiteness hypotheses.
See `MvPolynomial.noZeroDivisors` for the general case. -/
theorem noZeroDivisors_fin (R : Type u) [CommSemiring R] [NoZeroDivisors R] :
∀ n : ℕ, NoZeroDivisors (MvPolynomial (Fin n) R)
| 0 => (MvPolynomial.isEmptyAlgEquiv R _).injective.noZeroDivisors _ (map_zero _) (map_mul _)
| n + 1 =>
haveI := noZeroDivisors_fin R n
(MvPolynomial.finSuccEquiv R n).injective.noZeroDivisors _ (map_zero _) (map_mul _)
/-- Auxiliary definition:
Multivariate polynomials in finitely many variables over an integral domain form an integral domain.
This fact is proven by transport of structure from the `MvPolynomial.noZeroDivisors_fin`,
and then used to prove the general case without finiteness hypotheses.
See `MvPolynomial.noZeroDivisors` for the general case. -/
theorem noZeroDivisors_of_finite (R : Type u) (σ : Type v) [CommSemiring R] [Finite σ]
[NoZeroDivisors R] : NoZeroDivisors (MvPolynomial σ R) := by
cases nonempty_fintype σ
haveI := noZeroDivisors_fin R (Fintype.card σ)
exact (renameEquiv R (Fintype.equivFin σ)).injective.noZeroDivisors _ (map_zero _) (map_mul _)
instance {R : Type u} [CommSemiring R] [NoZeroDivisors R] {σ : Type v} :
NoZeroDivisors (MvPolynomial σ R) where
eq_zero_or_eq_zero_of_mul_eq_zero {p q} h := by
obtain ⟨s, p, q, rfl, rfl⟩ := exists_finset_rename₂ p q
let _nzd := MvPolynomial.noZeroDivisors_of_finite R s
have : p * q = 0 := by
apply rename_injective _ Subtype.val_injective
simpa using h
rw [mul_eq_zero] at this
apply this.imp <;> rintro rfl <;> simp
/-- The multivariate polynomial ring over an integral domain is an integral domain. -/
instance isDomain {R : Type u} {σ : Type v} [CommRing R] [IsDomain R] :
IsDomain (MvPolynomial σ R) := by
apply @NoZeroDivisors.to_isDomain (MvPolynomial σ R) _ ?_ _
apply AddMonoidAlgebra.nontrivial
-- instance {R : Type u} {σ : Type v} [CommRing R] [IsDomain R] :
-- IsDomain (MvPolynomial σ R)[X] := inferInstance
theorem map_mvPolynomial_eq_eval₂ {S : Type*} [CommRing S] [Finite σ] (ϕ : MvPolynomial σ R →+* S)
(p : MvPolynomial σ R) :
ϕ p = MvPolynomial.eval₂ (ϕ.comp MvPolynomial.C) (fun s => ϕ (MvPolynomial.X s)) p := by
cases nonempty_fintype σ
refine Trans.trans (congr_arg ϕ (MvPolynomial.as_sum p)) ?_
rw [MvPolynomial.eval₂_eq', map_sum ϕ]
congr
ext
simp only [monomial_eq, ϕ.map_pow, map_prod ϕ, ϕ.comp_apply, ϕ.map_mul, Finsupp.prod_pow]
/-- If every coefficient of a polynomial is in an ideal `I`, then so is the polynomial itself,
multivariate version. -/
theorem mem_ideal_of_coeff_mem_ideal (I : Ideal (MvPolynomial σ R)) (p : MvPolynomial σ R)
(hcoe : ∀ m : σ →₀ ℕ, p.coeff m ∈ I.comap (C : R →+* MvPolynomial σ R)) : p ∈ I := by
rw [as_sum p]
suffices ∀ m ∈ p.support, monomial m (MvPolynomial.coeff m p) ∈ I by
exact Submodule.sum_mem I this
intro m _
rw [← mul_one (coeff m p), ← C_mul_monomial]
suffices C (coeff m p) ∈ I by exact I.mul_mem_right (monomial m 1) this
simpa [Ideal.mem_comap] using hcoe m
/-- The push-forward of an ideal `I` of `R` to `MvPolynomial σ R` via inclusion
is exactly the set of polynomials whose coefficients are in `I` -/
theorem mem_map_C_iff {I : Ideal R} {f : MvPolynomial σ R} :
f ∈ (Ideal.map (C : R →+* MvPolynomial σ R) I : Ideal (MvPolynomial σ R)) ↔
∀ m : σ →₀ ℕ, f.coeff m ∈ I := by
classical
constructor
· intro hf
apply @Submodule.span_induction _ _ _ _ Semiring.toModule f _ _ hf
· intro f hf n
cases' (Set.mem_image _ _ _).mp hf with x hx
rw [← hx.right, coeff_C]
by_cases h : n = 0
· simpa [h] using hx.left
· simp [Ne.symm h]
· simp
· exact fun f g hf hg n => by simp [I.add_mem (hf n) (hg n)]
· refine fun f g hg n => ?_
rw [smul_eq_mul, coeff_mul]
exact I.sum_mem fun c _ => I.mul_mem_left (f.coeff c.fst) (hg c.snd)
· intro hf
rw [as_sum f]
suffices ∀ m ∈ f.support, monomial m (coeff m f) ∈ (Ideal.map C I : Ideal (MvPolynomial σ R)) by
exact Submodule.sum_mem _ this
intro m _
rw [← mul_one (coeff m f), ← C_mul_monomial]
suffices C (coeff m f) ∈ (Ideal.map C I : Ideal (MvPolynomial σ R)) by
exact Ideal.mul_mem_right _ _ this
apply Ideal.mem_map_of_mem _
exact hf m
attribute [-instance] Ring.toNonAssocRing in
theorem ker_map (f : R →+* S) :
RingHom.ker (map f : MvPolynomial σ R →+* MvPolynomial σ S) =
Ideal.map (C : R →+* MvPolynomial σ R) (RingHom.ker f) := by
ext
rw [MvPolynomial.mem_map_C_iff, RingHom.mem_ker, MvPolynomial.ext_iff]
simp_rw [coeff_map, coeff_zero, RingHom.mem_ker]
end MvPolynomial
section UniqueFactorizationDomain
variable {D : Type u} [CommRing D] [IsDomain D] [UniqueFactorizationMonoid D] (σ)
open UniqueFactorizationMonoid
namespace Polynomial
instance (priority := 100) uniqueFactorizationMonoid : UniqueFactorizationMonoid D[X] := by
letI := Classical.arbitrary (NormalizedGCDMonoid D)
exact ufm_of_decomposition_of_wfDvdMonoid
/-- If `D` is a unique factorization domain, `f` is a non-zero polynomial in `D[X]`, then `f` has
only finitely many monic factors.
(Note that its factors up to unit may be more than monic factors.)
See also `UniqueFactorizationMonoid.fintypeSubtypeDvd`. -/
noncomputable def fintypeSubtypeMonicDvd (f : D[X]) (hf : f ≠ 0) :
Fintype { g : D[X] // g.Monic ∧ g ∣ f } := by
set G := { g : D[X] // g.Monic ∧ g ∣ f }
let y : Associates D[X] := Associates.mk f
have hy : y ≠ 0 := Associates.mk_ne_zero.mpr hf
let H := { x : Associates D[X] // x ∣ y }
let hfin : Fintype H := UniqueFactorizationMonoid.fintypeSubtypeDvd y hy
let i : G → H := fun x ↦ ⟨Associates.mk x.1, Associates.mk_dvd_mk.2 x.2.2⟩
refine Fintype.ofInjective i fun x y heq ↦ ?_
rw [Subtype.mk.injEq] at heq ⊢
exact eq_of_monic_of_associated x.2.1 y.2.1 (Associates.mk_eq_mk_iff_associated.mp heq)
end Polynomial
namespace MvPolynomial
variable (d : ℕ)
private theorem uniqueFactorizationMonoid_of_fintype [Fintype σ] :
UniqueFactorizationMonoid (MvPolynomial σ D) :=
(renameEquiv D (Fintype.equivFin σ)).toMulEquiv.symm.uniqueFactorizationMonoid <| by
induction' Fintype.card σ with d hd
· apply (isEmptyAlgEquiv D (Fin 0)).toMulEquiv.symm.uniqueFactorizationMonoid
infer_instance
· apply (finSuccEquiv D d).toMulEquiv.symm.uniqueFactorizationMonoid
exact Polynomial.uniqueFactorizationMonoid
instance (priority := 100) uniqueFactorizationMonoid :
UniqueFactorizationMonoid (MvPolynomial σ D) := by
rw [iff_exists_prime_factors]
intro a ha; obtain ⟨s, a', rfl⟩ := exists_finset_rename a
obtain ⟨w, h, u, hw⟩ :=
iff_exists_prime_factors.1 (uniqueFactorizationMonoid_of_fintype s) a' fun h =>
ha <| by simp [h]
exact
⟨w.map (rename (↑)), fun b hb =>
let ⟨b', hb', he⟩ := Multiset.mem_map.1 hb
he ▸ (prime_rename_iff ↑s).2 (h b' hb'),
Units.map (@rename s σ D _ (↑)).toRingHom.toMonoidHom u, by
erw [Multiset.prod_hom, ← map_mul, hw]⟩
end MvPolynomial
end UniqueFactorizationDomain
/-- A polynomial over a field which is not a unit must have a monic irreducible factor.
See also `WfDvdMonoid.exists_irreducible_factor`. -/
theorem Polynomial.exists_monic_irreducible_factor {F : Type*} [Field F] (f : F[X])
(hu : ¬IsUnit f) : ∃ g : F[X], g.Monic ∧ Irreducible g ∧ g ∣ f := by
by_cases hf : f = 0
· exact ⟨X, monic_X, irreducible_X, hf ▸ dvd_zero X⟩
obtain ⟨g, hi, hf⟩ := WfDvdMonoid.exists_irreducible_factor hu hf
have ha : Associated g (g * C g.leadingCoeff⁻¹) := associated_mul_unit_right _ _ <|
isUnit_C.2 (leadingCoeff_ne_zero.2 hi.ne_zero).isUnit.inv
exact ⟨_, monic_mul_leadingCoeff_inv hi.ne_zero, ha.irreducible hi, ha.dvd_iff_dvd_left.1 hf⟩
|
RingTheory\Polynomial\Bernstein.lean | /-
Copyright (c) 2020 Scott Morrison. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Scott Morrison
-/
import Mathlib.Algebra.MvPolynomial.PDeriv
import Mathlib.Algebra.Polynomial.AlgebraMap
import Mathlib.Algebra.Polynomial.Derivative
import Mathlib.Data.Nat.Choose.Sum
import Mathlib.LinearAlgebra.LinearIndependent
import Mathlib.RingTheory.Polynomial.Pochhammer
/-!
# Bernstein polynomials
The definition of the Bernstein polynomials
```
bernsteinPolynomial (R : Type*) [CommRing R] (n ν : ℕ) : R[X] :=
(choose n ν) * X^ν * (1 - X)^(n - ν)
```
and the fact that for `ν : Fin (n+1)` these are linearly independent over `ℚ`.
We prove the basic identities
* `(Finset.range (n + 1)).sum (fun ν ↦ bernsteinPolynomial R n ν) = 1`
* `(Finset.range (n + 1)).sum (fun ν ↦ ν • bernsteinPolynomial R n ν) = n • X`
* `(Finset.range (n + 1)).sum (fun ν ↦ (ν * (ν-1)) • bernsteinPolynomial R n ν) = (n * (n-1)) • X^2`
## Notes
See also `Mathlib.Analysis.SpecialFunctions.Bernstein`, which defines the Bernstein approximations
of a continuous function `f : C([0,1], ℝ)`, and shows that these converge uniformly to `f`.
-/
noncomputable section
open Nat (choose)
open Polynomial (X)
open scoped Polynomial
variable (R : Type*) [CommRing R]
/-- `bernsteinPolynomial R n ν` is `(choose n ν) * X^ν * (1 - X)^(n - ν)`.
Although the coefficients are integers, it is convenient to work over an arbitrary commutative ring.
-/
def bernsteinPolynomial (n ν : ℕ) : R[X] :=
(choose n ν : R[X]) * X ^ ν * (1 - X) ^ (n - ν)
example : bernsteinPolynomial ℤ 3 2 = 3 * X ^ 2 - 3 * X ^ 3 := by
norm_num [bernsteinPolynomial, choose]
ring
namespace bernsteinPolynomial
theorem eq_zero_of_lt {n ν : ℕ} (h : n < ν) : bernsteinPolynomial R n ν = 0 := by
simp [bernsteinPolynomial, Nat.choose_eq_zero_of_lt h]
section
variable {R} {S : Type*} [CommRing S]
@[simp]
theorem map (f : R →+* S) (n ν : ℕ) :
(bernsteinPolynomial R n ν).map f = bernsteinPolynomial S n ν := by simp [bernsteinPolynomial]
end
theorem flip (n ν : ℕ) (h : ν ≤ n) :
(bernsteinPolynomial R n ν).comp (1 - X) = bernsteinPolynomial R n (n - ν) := by
simp [bernsteinPolynomial, h, tsub_tsub_assoc, mul_right_comm]
theorem flip' (n ν : ℕ) (h : ν ≤ n) :
bernsteinPolynomial R n ν = (bernsteinPolynomial R n (n - ν)).comp (1 - X) := by
simp [← flip _ _ _ h, Polynomial.comp_assoc]
theorem eval_at_0 (n ν : ℕ) : (bernsteinPolynomial R n ν).eval 0 = if ν = 0 then 1 else 0 := by
rw [bernsteinPolynomial]
split_ifs with h
· subst h; simp
· simp [zero_pow h]
theorem eval_at_1 (n ν : ℕ) : (bernsteinPolynomial R n ν).eval 1 = if ν = n then 1 else 0 := by
rw [bernsteinPolynomial]
split_ifs with h
· subst h; simp
· obtain hνn | hnν := Ne.lt_or_lt h
· simp [zero_pow $ Nat.sub_ne_zero_of_lt hνn]
· simp [Nat.choose_eq_zero_of_lt hnν]
theorem derivative_succ_aux (n ν : ℕ) :
Polynomial.derivative (bernsteinPolynomial R (n + 1) (ν + 1)) =
(n + 1) * (bernsteinPolynomial R n ν - bernsteinPolynomial R n (ν + 1)) := by
rw [bernsteinPolynomial]
suffices ((n + 1).choose (ν + 1) : R[X]) * ((↑(ν + 1 : ℕ) : R[X]) * X ^ ν) * (1 - X) ^ (n - ν) -
((n + 1).choose (ν + 1) : R[X]) * X ^ (ν + 1) * ((↑(n - ν) : R[X]) * (1 - X) ^ (n - ν - 1)) =
(↑(n + 1) : R[X]) * ((n.choose ν : R[X]) * X ^ ν * (1 - X) ^ (n - ν) -
(n.choose (ν + 1) : R[X]) * X ^ (ν + 1) * (1 - X) ^ (n - (ν + 1))) by
simpa [Polynomial.derivative_pow, ← sub_eq_add_neg, Nat.succ_sub_succ_eq_sub,
Polynomial.derivative_mul, Polynomial.derivative_natCast, zero_mul,
Nat.cast_add, algebraMap.coe_one, Polynomial.derivative_X, mul_one, zero_add,
Polynomial.derivative_sub, Polynomial.derivative_one, zero_sub, mul_neg, Nat.sub_zero,
bernsteinPolynomial, map_add, map_natCast, Nat.cast_one]
conv_rhs => rw [mul_sub]
-- We'll prove the two terms match up separately.
refine congr (congr_arg Sub.sub ?_) ?_
· simp only [← mul_assoc]
apply congr (congr_arg (· * ·) (congr (congr_arg (· * ·) _) rfl)) rfl
-- Now it's just about binomial coefficients
exact mod_cast congr_arg (fun m : ℕ => (m : R[X])) (Nat.succ_mul_choose_eq n ν).symm
· rw [← tsub_add_eq_tsub_tsub, ← mul_assoc, ← mul_assoc]; congr 1
rw [mul_comm, ← mul_assoc, ← mul_assoc]; congr 1
norm_cast
congr 1
convert (Nat.choose_mul_succ_eq n (ν + 1)).symm using 1
· -- Porting note: was
-- convert mul_comm _ _ using 2
-- simp
rw [mul_comm, Nat.succ_sub_succ_eq_sub]
· apply mul_comm
theorem derivative_succ (n ν : ℕ) : Polynomial.derivative (bernsteinPolynomial R n (ν + 1)) =
n * (bernsteinPolynomial R (n - 1) ν - bernsteinPolynomial R (n - 1) (ν + 1)) := by
cases n
· simp [bernsteinPolynomial]
· rw [Nat.cast_succ]; apply derivative_succ_aux
theorem derivative_zero (n : ℕ) :
Polynomial.derivative (bernsteinPolynomial R n 0) = -n * bernsteinPolynomial R (n - 1) 0 := by
simp [bernsteinPolynomial, Polynomial.derivative_pow]
theorem iterate_derivative_at_0_eq_zero_of_lt (n : ℕ) {ν k : ℕ} :
k < ν → (Polynomial.derivative^[k] (bernsteinPolynomial R n ν)).eval 0 = 0 := by
cases' ν with ν
· rintro ⟨⟩
· rw [Nat.lt_succ_iff]
induction' k with k ih generalizing n ν
· simp [eval_at_0]
· simp only [derivative_succ, Int.natCast_eq_zero, mul_eq_zero, Function.comp_apply,
Function.iterate_succ, Polynomial.iterate_derivative_sub,
Polynomial.iterate_derivative_natCast_mul, Polynomial.eval_mul, Polynomial.eval_natCast,
Polynomial.eval_sub]
intro h
apply mul_eq_zero_of_right
rw [ih _ _ (Nat.le_of_succ_le h), sub_zero]
convert ih _ _ (Nat.pred_le_pred h)
exact (Nat.succ_pred_eq_of_pos (k.succ_pos.trans_le h)).symm
@[simp]
theorem iterate_derivative_succ_at_0_eq_zero (n ν : ℕ) :
(Polynomial.derivative^[ν] (bernsteinPolynomial R n (ν + 1))).eval 0 = 0 :=
iterate_derivative_at_0_eq_zero_of_lt R n (lt_add_one ν)
open Polynomial
@[simp]
theorem iterate_derivative_at_0 (n ν : ℕ) :
(Polynomial.derivative^[ν] (bernsteinPolynomial R n ν)).eval 0 =
(ascPochhammer R ν).eval ((n - (ν - 1) : ℕ) : R) := by
by_cases h : ν ≤ n
· induction' ν with ν ih generalizing n
· simp [eval_at_0]
· have h' : ν ≤ n - 1 := le_tsub_of_add_le_right h
simp only [derivative_succ, ih (n - 1) h', iterate_derivative_succ_at_0_eq_zero,
Nat.succ_sub_succ_eq_sub, tsub_zero, sub_zero, iterate_derivative_sub,
iterate_derivative_natCast_mul, eval_one, eval_mul, eval_add, eval_sub, eval_X, eval_comp,
eval_natCast, Function.comp_apply, Function.iterate_succ, ascPochhammer_succ_left]
obtain rfl | h'' := ν.eq_zero_or_pos
· simp
· have : n - 1 - (ν - 1) = n - ν := by
rw [gt_iff_lt, ← Nat.succ_le_iff] at h''
rw [← tsub_add_eq_tsub_tsub, add_comm, tsub_add_cancel_of_le h'']
rw [this, ascPochhammer_eval_succ]
rw_mod_cast [tsub_add_cancel_of_le (h'.trans n.pred_le)]
· simp only [not_le] at h
rw [tsub_eq_zero_iff_le.mpr (Nat.le_sub_one_of_lt h), eq_zero_of_lt R h]
simp [pos_iff_ne_zero.mp (pos_of_gt h)]
theorem iterate_derivative_at_0_ne_zero [CharZero R] (n ν : ℕ) (h : ν ≤ n) :
(Polynomial.derivative^[ν] (bernsteinPolynomial R n ν)).eval 0 ≠ 0 := by
simp only [Int.natCast_eq_zero, bernsteinPolynomial.iterate_derivative_at_0, Ne, Nat.cast_eq_zero]
simp only [← ascPochhammer_eval_cast]
norm_cast
apply ne_of_gt
obtain rfl | h' := Nat.eq_zero_or_pos ν
· simp
· rw [← Nat.succ_pred_eq_of_pos h'] at h
exact ascPochhammer_pos _ _ (tsub_pos_of_lt (Nat.lt_of_succ_le h))
/-!
Rather than redoing the work of evaluating the derivatives at 1,
we use the symmetry of the Bernstein polynomials.
-/
theorem iterate_derivative_at_1_eq_zero_of_lt (n : ℕ) {ν k : ℕ} :
k < n - ν → (Polynomial.derivative^[k] (bernsteinPolynomial R n ν)).eval 1 = 0 := by
intro w
rw [flip' _ _ _ (tsub_pos_iff_lt.mp (pos_of_gt w)).le]
simp [Polynomial.eval_comp, iterate_derivative_at_0_eq_zero_of_lt R n w]
@[simp]
theorem iterate_derivative_at_1 (n ν : ℕ) (h : ν ≤ n) :
(Polynomial.derivative^[n - ν] (bernsteinPolynomial R n ν)).eval 1 =
(-1) ^ (n - ν) * (ascPochhammer R (n - ν)).eval (ν + 1 : R) := by
rw [flip' _ _ _ h]
simp [Polynomial.eval_comp, h]
obtain rfl | h' := h.eq_or_lt
· simp
· norm_cast
congr
omega
theorem iterate_derivative_at_1_ne_zero [CharZero R] (n ν : ℕ) (h : ν ≤ n) :
(Polynomial.derivative^[n - ν] (bernsteinPolynomial R n ν)).eval 1 ≠ 0 := by
rw [bernsteinPolynomial.iterate_derivative_at_1 _ _ _ h, Ne, neg_one_pow_mul_eq_zero_iff, ←
Nat.cast_succ, ← ascPochhammer_eval_cast, ← Nat.cast_zero, Nat.cast_inj]
exact (ascPochhammer_pos _ _ (Nat.succ_pos ν)).ne'
open Submodule
theorem linearIndependent_aux (n k : ℕ) (h : k ≤ n + 1) :
LinearIndependent ℚ fun ν : Fin k => bernsteinPolynomial ℚ n ν := by
induction' k with k ih
· apply linearIndependent_empty_type
· apply linearIndependent_fin_succ'.mpr
fconstructor
· exact ih (le_of_lt h)
· -- The actual work!
-- We show that the (n-k)-th derivative at 1 doesn't vanish,
-- but vanishes for everything in the span.
clear ih
simp only [Nat.succ_eq_add_one, add_le_add_iff_right] at h
simp only [Fin.val_last, Fin.init_def]
dsimp
apply not_mem_span_of_apply_not_mem_span_image (@Polynomial.derivative ℚ _ ^ (n - k))
-- Note: #8386 had to change `span_image` into `span_image _`
simp only [not_exists, not_and, Submodule.mem_map, Submodule.span_image _]
intro p m
apply_fun Polynomial.eval (1 : ℚ)
simp only [LinearMap.pow_apply]
-- The right hand side is nonzero,
-- so it will suffice to show the left hand side is always zero.
suffices (Polynomial.derivative^[n - k] p).eval 1 = 0 by
rw [this]
exact (iterate_derivative_at_1_ne_zero ℚ n k h).symm
refine span_induction m ?_ ?_ ?_ ?_
· simp only [Set.mem_range, forall_exists_index, forall_apply_eq_imp_iff]
rintro ⟨a, w⟩; simp only [Fin.val_mk]
rw [iterate_derivative_at_1_eq_zero_of_lt ℚ n ((tsub_lt_tsub_iff_left_of_le h).mpr w)]
· simp
· intro x y hx hy; simp [hx, hy]
· intro a x h; simp [h]
/-- The Bernstein polynomials are linearly independent.
We prove by induction that the collection of `bernsteinPolynomial n ν` for `ν = 0, ..., k`
are linearly independent.
The inductive step relies on the observation that the `(n-k)`-th derivative, evaluated at 1,
annihilates `bernsteinPolynomial n ν` for `ν < k`, but has a nonzero value at `ν = k`.
-/
theorem linearIndependent (n : ℕ) :
LinearIndependent ℚ fun ν : Fin (n + 1) => bernsteinPolynomial ℚ n ν :=
linearIndependent_aux n (n + 1) le_rfl
theorem sum (n : ℕ) : (∑ ν ∈ Finset.range (n + 1), bernsteinPolynomial R n ν) = 1 :=
calc
(∑ ν ∈ Finset.range (n + 1), bernsteinPolynomial R n ν) = (X + (1 - X)) ^ n := by
rw [add_pow]
simp only [bernsteinPolynomial, mul_comm, mul_assoc, mul_left_comm]
_ = 1 := by simp
open Polynomial
open MvPolynomial hiding X
theorem sum_smul (n : ℕ) :
(∑ ν ∈ Finset.range (n + 1), ν • bernsteinPolynomial R n ν) = n • X := by
-- We calculate the `x`-derivative of `(x+y)^n`, evaluated at `y=(1-x)`,
-- either directly or by using the binomial theorem.
-- We'll work in `MvPolynomial Bool R`.
let x : MvPolynomial Bool R := MvPolynomial.X true
let y : MvPolynomial Bool R := MvPolynomial.X false
have pderiv_true_x : pderiv true x = 1 := by rw [pderiv_X]; rfl
have pderiv_true_y : pderiv true y = 0 := by rw [pderiv_X]; rfl
let e : Bool → R[X] := fun i => cond i X (1 - X)
-- Start with `(x+y)^n = (x+y)^n`,
-- take the `x`-derivative, evaluate at `x=X, y=1-X`, and multiply by `X`:
trans MvPolynomial.aeval e (pderiv true ((x + y) ^ n)) * X
-- On the left hand side we'll use the binomial theorem, then simplify.
· -- We first prepare a tedious rewrite:
have w : ∀ k : ℕ, k • bernsteinPolynomial R n k =
(k : R[X]) * Polynomial.X ^ (k - 1) * (1 - Polynomial.X) ^ (n - k) * (n.choose k : R[X]) *
Polynomial.X := by
rintro (_ | k)
· simp
· rw [bernsteinPolynomial]
simp only [← natCast_mul, Nat.succ_eq_add_one, Nat.add_succ_sub_one, add_zero, pow_succ]
push_cast
ring
rw [add_pow, map_sum (pderiv true), map_sum (MvPolynomial.aeval e), Finset.sum_mul]
-- Step inside the sum:
refine Finset.sum_congr rfl fun k _ => (w k).trans ?_
simp only [x, y, e, pderiv_true_x, pderiv_true_y, Algebra.id.smul_eq_mul, nsmul_eq_mul,
Bool.cond_true, Bool.cond_false, add_zero, mul_one, mul_zero, smul_zero, MvPolynomial.aeval_X,
MvPolynomial.pderiv_mul, Derivation.leibniz_pow, Derivation.map_natCast, map_natCast, map_pow,
map_mul]
· rw [(pderiv true).leibniz_pow, (pderiv true).map_add, pderiv_true_x, pderiv_true_y]
simp only [x, y, e, Algebra.id.smul_eq_mul, nsmul_eq_mul, map_natCast, map_pow, map_add,
map_mul, Bool.cond_true, Bool.cond_false, MvPolynomial.aeval_X, add_sub_cancel,
one_pow, add_zero, mul_one]
theorem sum_mul_smul (n : ℕ) :
(∑ ν ∈ Finset.range (n + 1), (ν * (ν - 1)) • bernsteinPolynomial R n ν) =
(n * (n - 1)) • X ^ 2 := by
-- We calculate the second `x`-derivative of `(x+y)^n`, evaluated at `y=(1-x)`,
-- either directly or by using the binomial theorem.
-- We'll work in `MvPolynomial Bool R`.
let x : MvPolynomial Bool R := MvPolynomial.X true
let y : MvPolynomial Bool R := MvPolynomial.X false
have pderiv_true_x : pderiv true x = 1 := by rw [pderiv_X]; rfl
have pderiv_true_y : pderiv true y = 0 := by rw [pderiv_X]; rfl
let e : Bool → R[X] := fun i => cond i X (1 - X)
-- Start with `(x+y)^n = (x+y)^n`,
-- take the second `x`-derivative, evaluate at `x=X, y=1-X`, and multiply by `X`:
trans MvPolynomial.aeval e (pderiv true (pderiv true ((x + y) ^ n))) * X ^ 2
-- On the left hand side we'll use the binomial theorem, then simplify.
· -- We first prepare a tedious rewrite:
have w : ∀ k : ℕ, (k * (k - 1)) • bernsteinPolynomial R n k =
(n.choose k : R[X]) * ((1 - Polynomial.X) ^ (n - k) *
((k : R[X]) * ((↑(k - 1) : R[X]) * Polynomial.X ^ (k - 1 - 1)))) * Polynomial.X ^ 2 := by
rintro (_ | _ | k)
· simp
· simp
· rw [bernsteinPolynomial]
simp only [← natCast_mul, Nat.succ_eq_add_one, Nat.add_succ_sub_one, add_zero, pow_succ]
push_cast
ring
rw [add_pow, map_sum (pderiv true), map_sum (pderiv true), map_sum (MvPolynomial.aeval e),
Finset.sum_mul]
-- Step inside the sum:
refine Finset.sum_congr rfl fun k _ => (w k).trans ?_
simp only [x, y, e, pderiv_true_x, pderiv_true_y, Algebra.id.smul_eq_mul, nsmul_eq_mul,
Bool.cond_true, Bool.cond_false, add_zero, zero_add, mul_zero, smul_zero, mul_one,
MvPolynomial.aeval_X, MvPolynomial.pderiv_X_self, MvPolynomial.pderiv_X_of_ne,
Derivation.leibniz_pow, Derivation.leibniz, Derivation.map_natCast, map_natCast, map_pow,
map_mul, map_add]
-- On the right hand side, we'll just simplify.
· simp only [x, y, e, pderiv_one, pderiv_mul, (pderiv _).leibniz_pow, (pderiv _).map_natCast,
(pderiv true).map_add, pderiv_true_x, pderiv_true_y, Algebra.id.smul_eq_mul, add_zero,
mul_one, Derivation.map_smul_of_tower, map_nsmul, map_pow, map_add, Bool.cond_true,
Bool.cond_false, MvPolynomial.aeval_X, add_sub_cancel, one_pow, smul_smul,
smul_one_mul]
/-- A certain linear combination of the previous three identities,
which we'll want later.
-/
theorem variance (n : ℕ) :
(∑ ν ∈ Finset.range (n + 1), (n • Polynomial.X - (ν : R[X])) ^ 2 * bernsteinPolynomial R n ν) =
n • Polynomial.X * ((1 : R[X]) - Polynomial.X) := by
have p : ((((Finset.range (n + 1)).sum fun ν => (ν * (ν - 1)) • bernsteinPolynomial R n ν) +
(1 - (2 * n) • Polynomial.X) * (Finset.range (n + 1)).sum fun ν =>
ν • bernsteinPolynomial R n ν) + n ^ 2 • X ^ 2 *
(Finset.range (n + 1)).sum fun ν => bernsteinPolynomial R n ν) = _ :=
rfl
conv at p =>
lhs
rw [Finset.mul_sum, Finset.mul_sum, ← Finset.sum_add_distrib, ← Finset.sum_add_distrib]
simp only [← natCast_mul]
simp only [← mul_assoc]
simp only [← add_mul]
conv at p =>
rhs
rw [sum, sum_smul, sum_mul_smul, ← natCast_mul]
calc
_ = _ := Finset.sum_congr rfl fun k m => ?_
_ = _ := p
_ = _ := ?_
· congr 1; simp only [← natCast_mul, push_cast]
cases k <;> · simp; ring
· simp only [← natCast_mul, push_cast]
cases n
· simp
· simp; ring
end bernsteinPolynomial
|
RingTheory\Polynomial\Chebyshev.lean | /-
Copyright (c) 2020 Johan Commelin. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johan Commelin, Julian Kuelshammer, Heather Macbeth, Mitchell Lee
-/
import Mathlib.Algebra.Polynomial.Derivative
import Mathlib.Tactic.LinearCombination
/-!
# Chebyshev polynomials
The Chebyshev polynomials are families of polynomials indexed by `ℤ`,
with integral coefficients.
## Main definitions
* `Polynomial.Chebyshev.T`: the Chebyshev polynomials of the first kind.
* `Polynomial.Chebyshev.U`: the Chebyshev polynomials of the second kind.
## Main statements
* The formal derivative of the Chebyshev polynomials of the first kind is a scalar multiple of the
Chebyshev polynomials of the second kind.
* `Polynomial.Chebyshev.mul_T`, twice the product of the `m`-th and `k`-th Chebyshev polynomials of
the first kind is the sum of the `m + k`-th and `m - k`-th Chebyshev polynomials of the first
kind.
* `Polynomial.Chebyshev.T_mul`, the `(m * n)`-th Chebyshev polynomial of the first kind is the
composition of the `m`-th and `n`-th Chebyshev polynomials of the first kind.
## Implementation details
Since Chebyshev polynomials have interesting behaviour over the complex numbers and modulo `p`,
we define them to have coefficients in an arbitrary commutative ring, even though
technically `ℤ` would suffice.
The benefit of allowing arbitrary coefficient rings, is that the statements afterwards are clean,
and do not have `map (Int.castRingHom R)` interfering all the time.
## References
[Lionel Ponton, _Roots of the Chebyshev polynomials: A purely algebraic approach_]
[ponton2020chebyshev]
## TODO
* Redefine and/or relate the definition of Chebyshev polynomials to `LinearRecurrence`.
* Add explicit formula involving square roots for Chebyshev polynomials
* Compute zeroes and extrema of Chebyshev polynomials.
* Prove that the roots of the Chebyshev polynomials (except 0) are irrational.
* Prove minimax properties of Chebyshev polynomials.
-/
namespace Polynomial.Chebyshev
open Polynomial
variable (R S : Type*) [CommRing R] [CommRing S]
/-- `T n` is the `n`-th Chebyshev polynomial of the first kind. -/
-- Well-founded definitions are now irreducible by default;
-- as this was implemented before this change,
-- we just set it back to semireducible to avoid needing to change any proofs.
@[semireducible] noncomputable def T : ℤ → R[X]
| 0 => 1
| 1 => X
| (n : ℕ) + 2 => 2 * X * T (n + 1) - T n
| -((n : ℕ) + 1) => 2 * X * T (-n) - T (-n + 1)
termination_by n => Int.natAbs n + Int.natAbs (n - 1)
/-- Induction principle used for proving facts about Chebyshev polynomials. -/
@[elab_as_elim]
protected theorem induct (motive : ℤ → Prop)
(zero : motive 0)
(one : motive 1)
(add_two : ∀ (n : ℕ), motive (↑n + 1) → motive ↑n → motive (↑n + 2))
(neg_add_one : ∀ (n : ℕ), motive (-↑n) → motive (-↑n + 1) → motive (-↑n - 1)) :
∀ (a : ℤ), motive a :=
T.induct Unit motive zero one add_two fun n hn hnm => by
simpa only [Int.negSucc_eq, neg_add] using neg_add_one n hn hnm
@[simp]
theorem T_add_two : ∀ n, T R (n + 2) = 2 * X * T R (n + 1) - T R n
| (k : ℕ) => T.eq_3 R k
| -(k + 1 : ℕ) => by linear_combination (norm := (simp [Int.negSucc_eq]; ring_nf)) T.eq_4 R k
theorem T_add_one (n : ℤ) : T R (n + 1) = 2 * X * T R n - T R (n - 1) := by
linear_combination (norm := ring_nf) T_add_two R (n - 1)
theorem T_sub_two (n : ℤ) : T R (n - 2) = 2 * X * T R (n - 1) - T R n := by
linear_combination (norm := ring_nf) T_add_two R (n - 2)
theorem T_sub_one (n : ℤ) : T R (n - 1) = 2 * X * T R n - T R (n + 1) := by
linear_combination (norm := ring_nf) T_add_two R (n - 1)
theorem T_eq (n : ℤ) : T R n = 2 * X * T R (n - 1) - T R (n - 2) := by
linear_combination (norm := ring_nf) T_add_two R (n - 2)
@[simp]
theorem T_zero : T R 0 = 1 := rfl
@[simp]
theorem T_one : T R 1 = X := rfl
theorem T_neg_one : T R (-1) = X := (by ring : 2 * X * 1 - X = X)
theorem T_two : T R 2 = 2 * X ^ 2 - 1 := by
simpa [pow_two, mul_assoc] using T_add_two R 0
@[simp]
theorem T_neg (n : ℤ) : T R (-n) = T R n := by
induction n using Polynomial.Chebyshev.induct with
| zero => rfl
| one => show 2 * X * 1 - X = X; ring
| add_two n ih1 ih2 =>
have h₁ := T_add_two R n
have h₂ := T_sub_two R (-n)
linear_combination (norm := ring_nf) (2 * (X : R[X])) * ih1 - ih2 - h₁ + h₂
| neg_add_one n ih1 ih2 =>
have h₁ := T_add_one R n
have h₂ := T_sub_one R (-n)
linear_combination (norm := ring_nf) (2 * (X : R[X])) * ih1 - ih2 + h₁ - h₂
theorem T_natAbs (n : ℤ) : T R n.natAbs = T R n := by
obtain h | h := Int.natAbs_eq n <;> nth_rw 2 [h]; simp
theorem T_neg_two : T R (-2) = 2 * X ^ 2 - 1 := by simp [T_two]
/-- `U n` is the `n`-th Chebyshev polynomial of the second kind. -/
-- Well-founded definitions are now irreducible by default;
-- as this was implemented before this change,
-- we just set it back to semireducible to avoid needing to change any proofs.
@[semireducible] noncomputable def U : ℤ → R[X]
| 0 => 1
| 1 => 2 * X
| (n : ℕ) + 2 => 2 * X * U (n + 1) - U n
| -((n : ℕ) + 1) => 2 * X * U (-n) - U (-n + 1)
termination_by n => Int.natAbs n + Int.natAbs (n - 1)
@[simp]
theorem U_add_two : ∀ n, U R (n + 2) = 2 * X * U R (n + 1) - U R n
| (k : ℕ) => U.eq_3 R k
| -(k + 1 : ℕ) => by linear_combination (norm := (simp [Int.negSucc_eq]; ring_nf)) U.eq_4 R k
theorem U_add_one (n : ℤ) : U R (n + 1) = 2 * X * U R n - U R (n - 1) := by
linear_combination (norm := ring_nf) U_add_two R (n - 1)
theorem U_sub_two (n : ℤ) : U R (n - 2) = 2 * X * U R (n - 1) - U R n := by
linear_combination (norm := ring_nf) U_add_two R (n - 2)
theorem U_sub_one (n : ℤ) : U R (n - 1) = 2 * X * U R n - U R (n + 1) := by
linear_combination (norm := ring_nf) U_add_two R (n - 1)
theorem U_eq (n : ℤ) : U R n = 2 * X * U R (n - 1) - U R (n - 2) := by
linear_combination (norm := ring_nf) U_add_two R (n - 2)
@[simp]
theorem U_zero : U R 0 = 1 := rfl
@[simp]
theorem U_one : U R 1 = 2 * X := rfl
@[simp]
theorem U_neg_one : U R (-1) = 0 := by simpa using U_sub_one R 0
theorem U_two : U R 2 = 4 * X ^ 2 - 1 := by
have := U_add_two R 0
simp only [zero_add, U_one, U_zero] at this
linear_combination this
@[simp]
theorem U_neg_two : U R (-2) = -1 := by
simpa [zero_sub, Int.reduceNeg, U_neg_one, mul_zero, U_zero] using U_sub_two R 0
theorem U_neg_sub_one (n : ℤ) : U R (-n - 1) = -U R (n - 1) := by
induction n using Polynomial.Chebyshev.induct with
| zero => simp
| one => simp
| add_two n ih1 ih2 =>
have h₁ := U_add_one R n
have h₂ := U_sub_two R (-n - 1)
linear_combination (norm := ring_nf) 2 * (X : R[X]) * ih1 - ih2 + h₁ + h₂
| neg_add_one n ih1 ih2 =>
have h₁ := U_eq R n
have h₂ := U_sub_two R (-n)
linear_combination (norm := ring_nf) 2 * (X : R[X]) * ih1 - ih2 + h₁ + h₂
theorem U_neg (n : ℤ) : U R (-n) = -U R (n - 2) := by simpa [sub_sub] using U_neg_sub_one R (n - 1)
@[simp]
theorem U_neg_sub_two (n : ℤ) : U R (-n - 2) = -U R n := by
simpa [sub_eq_add_neg, add_comm] using U_neg R (n + 2)
theorem U_eq_X_mul_U_add_T (n : ℤ) : U R (n + 1) = X * U R n + T R (n + 1) := by
induction n using Polynomial.Chebyshev.induct with
| zero => simp [two_mul]
| one => simp [U_two, T_two]; ring
| add_two n ih1 ih2 =>
have h₁ := U_add_two R (n + 1)
have h₂ := U_add_two R n
have h₃ := T_add_two R (n + 1)
linear_combination (norm := ring_nf) -h₃ - (X : R[X]) * h₂ + h₁ + 2 * (X : R[X]) * ih1 - ih2
| neg_add_one n ih1 ih2 =>
have h₁ := U_add_two R (-n - 1)
have h₂ := U_add_two R (-n)
have h₃ := T_add_two R (-n)
linear_combination (norm := ring_nf) -h₃ + h₂ - (X : R[X]) * h₁ - ih2 + 2 * (X : R[X]) * ih1
theorem T_eq_U_sub_X_mul_U (n : ℤ) : T R n = U R n - X * U R (n - 1) := by
linear_combination (norm := ring_nf) - U_eq_X_mul_U_add_T R (n - 1)
theorem T_eq_X_mul_T_sub_pol_U (n : ℤ) : T R (n + 2) = X * T R (n + 1) - (1 - X ^ 2) * U R n := by
have h₁ := U_eq_X_mul_U_add_T R n
have h₂ := U_eq_X_mul_U_add_T R (n + 1)
have h₃ := U_add_two R n
linear_combination (norm := ring_nf) h₃ - h₂ + (X : R[X]) * h₁
theorem one_sub_X_sq_mul_U_eq_pol_in_T (n : ℤ) :
(1 - X ^ 2) * U R n = X * T R (n + 1) - T R (n + 2) := by
linear_combination T_eq_X_mul_T_sub_pol_U R n
variable {R S}
@[simp]
theorem map_T (f : R →+* S) (n : ℤ) : map f (T R n) = T S n := by
induction n using Polynomial.Chebyshev.induct with
| zero => simp
| one => simp
| add_two n ih1 ih2 =>
simp_rw [T_add_two, Polynomial.map_sub, Polynomial.map_mul, Polynomial.map_ofNat, map_X,
ih1, ih2]
| neg_add_one n ih1 ih2 =>
simp_rw [T_sub_one, Polynomial.map_sub, Polynomial.map_mul, Polynomial.map_ofNat, map_X, ih1,
ih2]
@[simp]
theorem map_U (f : R →+* S) (n : ℤ) : map f (U R n) = U S n := by
induction n using Polynomial.Chebyshev.induct with
| zero => simp
| one => simp
| add_two n ih1 ih2 =>
simp_rw [U_add_two, Polynomial.map_sub, Polynomial.map_mul, Polynomial.map_ofNat, map_X, ih1,
ih2]
| neg_add_one n ih1 ih2 =>
simp_rw [U_sub_one, Polynomial.map_sub, Polynomial.map_mul, Polynomial.map_ofNat, map_X, ih1,
ih2]
theorem T_derivative_eq_U (n : ℤ) : derivative (T R n) = n * U R (n - 1) := by
induction n using Polynomial.Chebyshev.induct with
| zero => simp
| one =>
simp [T_two, U_one, derivative_sub, derivative_one, derivative_mul, derivative_X_pow, add_mul]
| add_two n ih1 ih2 =>
have h₁ := congr_arg derivative (T_add_two R n)
have h₂ := U_sub_one R n
have h₃ := T_eq_U_sub_X_mul_U R (n + 1)
simp only [derivative_sub, derivative_mul, derivative_ofNat, derivative_X] at h₁
linear_combination (norm := (push_cast; ring_nf))
h₁ - ih2 + 2 * (X : R[X]) * ih1 + 2 * h₃ - n * h₂
| neg_add_one n ih1 ih2 =>
have h₁ := congr_arg derivative (T_sub_one R (-n))
have h₂ := U_sub_two R (-n)
have h₃ := T_eq_U_sub_X_mul_U R (-n)
simp only [derivative_sub, derivative_mul, derivative_ofNat, derivative_X] at h₁
linear_combination (norm := (push_cast; ring_nf))
-ih2 + 2 * (X : R[X]) * ih1 + h₁ + 2 * h₃ + (n + 1) * h₂
theorem one_sub_X_sq_mul_derivative_T_eq_poly_in_T (n : ℤ) :
(1 - X ^ 2) * derivative (T R (n + 1)) = (n + 1 : R[X]) * (T R n - X * T R (n + 1)) := by
have H₁ := one_sub_X_sq_mul_U_eq_pol_in_T R n
have H₂ := T_derivative_eq_U (R := R) (n + 1)
have h₁ := T_add_two R n
linear_combination (norm := (push_cast; ring_nf))
(-n - 1) * h₁ + (-(X : R[X]) ^ 2 + 1) * H₂ + (n + 1) * H₁
theorem add_one_mul_T_eq_poly_in_U (n : ℤ) :
((n : R[X]) + 1) * T R (n + 1) = X * U R n - (1 - X ^ 2) * derivative (U R n) := by
have h₁ := congr_arg derivative <| T_eq_X_mul_T_sub_pol_U R n
simp only [derivative_sub, derivative_mul, derivative_X, derivative_one, derivative_X_pow,
T_derivative_eq_U, C_eq_natCast] at h₁
have h₂ := T_eq_U_sub_X_mul_U R (n + 1)
linear_combination (norm := (push_cast; ring_nf))
h₁ + (n + 2) * h₂
variable (R)
/-- Twice the product of two Chebyshev `T` polynomials is the sum of two other Chebyshev `T`
polynomials. -/
theorem mul_T (m k : ℤ) : 2 * T R m * T R k = T R (m + k) + T R (m - k) := by
induction k using Polynomial.Chebyshev.induct with
| zero => simp [two_mul]
| one => rw [T_add_one, T_one]; ring
| add_two k ih1 ih2 =>
have h₁ := T_add_two R (m + k)
have h₂ := T_sub_two R (m - k)
have h₃ := T_add_two R k
linear_combination (norm := ring_nf) 2 * T R m * h₃ - h₂ - h₁ - ih2 + 2 * (X : R[X]) * ih1
| neg_add_one k ih1 ih2 =>
have h₁ := T_add_two R (m + (-k - 1))
have h₂ := T_sub_two R (m - (-k - 1))
have h₃ := T_add_two R (-k - 1)
linear_combination (norm := ring_nf) 2 * T R m * h₃ - h₂ - h₁ - ih2 + 2 * (X : R[X]) * ih1
/-- The `(m * n)`-th Chebyshev `T` polynomial is the composition of the `m`-th and `n`-th. -/
theorem T_mul (m n : ℤ) : T R (m * n) = (T R m).comp (T R n) := by
induction m using Polynomial.Chebyshev.induct with
| zero => simp
| one => simp
| add_two m ih1 ih2 =>
have h₁ := mul_T R ((m + 1) * n) n
have h₂ := congr_arg (comp · (T R n)) <| T_add_two R m
simp only [sub_comp, mul_comp, ofNat_comp, X_comp] at h₂
linear_combination (norm := ring_nf) -ih2 - h₂ - h₁ + 2 * T R n * ih1
| neg_add_one m ih1 ih2 =>
have h₁ := mul_T R ((-m) * n) n
have h₂ := congr_arg (comp · (T R n)) <| T_add_two R (-m - 1)
simp only [sub_comp, mul_comp, ofNat_comp, X_comp] at h₂
linear_combination (norm := ring_nf) -ih2 - h₂ - h₁ + 2 * T R n * ih1
end Polynomial.Chebyshev
|
RingTheory\Polynomial\Content.lean | /-
Copyright (c) 2020 Aaron Anderson. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Aaron Anderson
-/
import Mathlib.Algebra.GCDMonoid.Finset
import Mathlib.Algebra.Polynomial.CancelLeads
import Mathlib.Algebra.Polynomial.EraseLead
import Mathlib.Algebra.Polynomial.FieldDivision
/-!
# GCD structures on polynomials
Definitions and basic results about polynomials over GCD domains, particularly their contents
and primitive polynomials.
## Main Definitions
Let `p : R[X]`.
- `p.content` is the `gcd` of the coefficients of `p`.
- `p.IsPrimitive` indicates that `p.content = 1`.
## Main Results
- `Polynomial.content_mul`:
If `p q : R[X]`, then `(p * q).content = p.content * q.content`.
- `Polynomial.NormalizedGcdMonoid`:
The polynomial ring of a GCD domain is itself a GCD domain.
-/
namespace Polynomial
open Polynomial
section Primitive
variable {R : Type*} [CommSemiring R]
/-- A polynomial is primitive when the only constant polynomials dividing it are units -/
def IsPrimitive (p : R[X]) : Prop :=
∀ r : R, C r ∣ p → IsUnit r
theorem isPrimitive_iff_isUnit_of_C_dvd {p : R[X]} : p.IsPrimitive ↔ ∀ r : R, C r ∣ p → IsUnit r :=
Iff.rfl
@[simp]
theorem isPrimitive_one : IsPrimitive (1 : R[X]) := fun _ h =>
isUnit_C.mp (isUnit_of_dvd_one h)
theorem Monic.isPrimitive {p : R[X]} (hp : p.Monic) : p.IsPrimitive := by
rintro r ⟨q, h⟩
exact isUnit_of_mul_eq_one r (q.coeff p.natDegree) (by rwa [← coeff_C_mul, ← h])
theorem IsPrimitive.ne_zero [Nontrivial R] {p : R[X]} (hp : p.IsPrimitive) : p ≠ 0 := by
rintro rfl
exact (hp 0 (dvd_zero (C 0))).ne_zero rfl
theorem isPrimitive_of_dvd {p q : R[X]} (hp : IsPrimitive p) (hq : q ∣ p) : IsPrimitive q :=
fun a ha => isPrimitive_iff_isUnit_of_C_dvd.mp hp a (dvd_trans ha hq)
end Primitive
variable {R : Type*} [CommRing R] [IsDomain R]
section NormalizedGCDMonoid
variable [NormalizedGCDMonoid R]
/-- `p.content` is the `gcd` of the coefficients of `p`. -/
def content (p : R[X]) : R :=
p.support.gcd p.coeff
theorem content_dvd_coeff {p : R[X]} (n : ℕ) : p.content ∣ p.coeff n := by
by_cases h : n ∈ p.support
· apply Finset.gcd_dvd h
rw [mem_support_iff, Classical.not_not] at h
rw [h]
apply dvd_zero
@[simp]
theorem content_C {r : R} : (C r).content = normalize r := by
rw [content]
by_cases h0 : r = 0
· simp [h0]
have h : (C r).support = {0} := support_monomial _ h0
simp [h]
@[simp]
theorem content_zero : content (0 : R[X]) = 0 := by rw [← C_0, content_C, normalize_zero]
@[simp]
theorem content_one : content (1 : R[X]) = 1 := by rw [← C_1, content_C, normalize_one]
theorem content_X_mul {p : R[X]} : content (X * p) = content p := by
rw [content, content, Finset.gcd_def, Finset.gcd_def]
refine congr rfl ?_
have h : (X * p).support = p.support.map ⟨Nat.succ, Nat.succ_injective⟩ := by
ext a
simp only [exists_prop, Finset.mem_map, Function.Embedding.coeFn_mk, Ne, mem_support_iff]
cases' a with a
· simp [coeff_X_mul_zero, Nat.succ_ne_zero]
rw [mul_comm, coeff_mul_X]
constructor
· intro h
use a
· rintro ⟨b, ⟨h1, h2⟩⟩
rw [← Nat.succ_injective h2]
apply h1
rw [h]
simp only [Finset.map_val, Function.comp_apply, Function.Embedding.coeFn_mk, Multiset.map_map]
refine congr (congr rfl ?_) rfl
ext a
rw [mul_comm]
simp [coeff_mul_X]
@[simp]
theorem content_X_pow {k : ℕ} : content ((X : R[X]) ^ k) = 1 := by
induction' k with k hi
· simp
rw [pow_succ', content_X_mul, hi]
@[simp]
theorem content_X : content (X : R[X]) = 1 := by rw [← mul_one X, content_X_mul, content_one]
theorem content_C_mul (r : R) (p : R[X]) : (C r * p).content = normalize r * p.content := by
by_cases h0 : r = 0; · simp [h0]
rw [content]; rw [content]; rw [← Finset.gcd_mul_left]
refine congr (congr rfl ?_) ?_ <;> ext <;> simp [h0, mem_support_iff]
@[simp]
theorem content_monomial {r : R} {k : ℕ} : content (monomial k r) = normalize r := by
rw [← C_mul_X_pow_eq_monomial, content_C_mul, content_X_pow, mul_one]
theorem content_eq_zero_iff {p : R[X]} : content p = 0 ↔ p = 0 := by
rw [content, Finset.gcd_eq_zero_iff]
constructor <;> intro h
· ext n
by_cases h0 : n ∈ p.support
· rw [h n h0, coeff_zero]
· rw [mem_support_iff] at h0
push_neg at h0
simp [h0]
· intro x
simp [h]
-- Porting note: this reduced with simp so created `normUnit_content` and put simp on it
theorem normalize_content {p : R[X]} : normalize p.content = p.content :=
Finset.normalize_gcd
@[simp]
theorem normUnit_content {p : R[X]} : normUnit (content p) = 1 := by
by_cases hp0 : p.content = 0
· simp [hp0]
· ext
apply mul_left_cancel₀ hp0
erw [← normalize_apply, normalize_content, mul_one]
theorem content_eq_gcd_range_of_lt (p : R[X]) (n : ℕ) (h : p.natDegree < n) :
p.content = (Finset.range n).gcd p.coeff := by
apply dvd_antisymm_of_normalize_eq normalize_content Finset.normalize_gcd
· rw [Finset.dvd_gcd_iff]
intro i _
apply content_dvd_coeff _
· apply Finset.gcd_mono
intro i
simp only [Nat.lt_succ_iff, mem_support_iff, Ne, Finset.mem_range]
contrapose!
intro h1
apply coeff_eq_zero_of_natDegree_lt (lt_of_lt_of_le h h1)
theorem content_eq_gcd_range_succ (p : R[X]) :
p.content = (Finset.range p.natDegree.succ).gcd p.coeff :=
content_eq_gcd_range_of_lt _ _ (Nat.lt_succ_self _)
theorem content_eq_gcd_leadingCoeff_content_eraseLead (p : R[X]) :
p.content = GCDMonoid.gcd p.leadingCoeff (eraseLead p).content := by
by_cases h : p = 0
· simp [h]
rw [← leadingCoeff_eq_zero, leadingCoeff, ← Ne, ← mem_support_iff] at h
rw [content, ← Finset.insert_erase h, Finset.gcd_insert, leadingCoeff, content,
eraseLead_support]
refine congr rfl (Finset.gcd_congr rfl fun i hi => ?_)
rw [Finset.mem_erase] at hi
rw [eraseLead_coeff, if_neg hi.1]
theorem dvd_content_iff_C_dvd {p : R[X]} {r : R} : r ∣ p.content ↔ C r ∣ p := by
rw [C_dvd_iff_dvd_coeff]
constructor
· intro h i
apply h.trans (content_dvd_coeff _)
· intro h
rw [content, Finset.dvd_gcd_iff]
intro i _
apply h i
theorem C_content_dvd (p : R[X]) : C p.content ∣ p :=
dvd_content_iff_C_dvd.1 dvd_rfl
theorem isPrimitive_iff_content_eq_one {p : R[X]} : p.IsPrimitive ↔ p.content = 1 := by
rw [← normalize_content, normalize_eq_one, IsPrimitive]
simp_rw [← dvd_content_iff_C_dvd]
exact ⟨fun h => h p.content (dvd_refl p.content), fun h r hdvd => isUnit_of_dvd_unit hdvd h⟩
theorem IsPrimitive.content_eq_one {p : R[X]} (hp : p.IsPrimitive) : p.content = 1 :=
isPrimitive_iff_content_eq_one.mp hp
section PrimPart
/-- The primitive part of a polynomial `p` is the primitive polynomial gained by dividing `p` by
`p.content`. If `p = 0`, then `p.primPart = 1`. -/
noncomputable def primPart (p : R[X]) : R[X] :=
letI := Classical.decEq R
if p = 0 then 1 else Classical.choose (C_content_dvd p)
theorem eq_C_content_mul_primPart (p : R[X]) : p = C p.content * p.primPart := by
by_cases h : p = 0; · simp [h]
rw [primPart, if_neg h, ← Classical.choose_spec (C_content_dvd p)]
@[simp]
theorem primPart_zero : primPart (0 : R[X]) = 1 :=
if_pos rfl
theorem isPrimitive_primPart (p : R[X]) : p.primPart.IsPrimitive := by
by_cases h : p = 0; · simp [h]
rw [← content_eq_zero_iff] at h
rw [isPrimitive_iff_content_eq_one]
apply mul_left_cancel₀ h
conv_rhs => rw [p.eq_C_content_mul_primPart, mul_one, content_C_mul, normalize_content]
theorem content_primPart (p : R[X]) : p.primPart.content = 1 :=
p.isPrimitive_primPart.content_eq_one
theorem primPart_ne_zero (p : R[X]) : p.primPart ≠ 0 :=
p.isPrimitive_primPart.ne_zero
theorem natDegree_primPart (p : R[X]) : p.primPart.natDegree = p.natDegree := by
by_cases h : C p.content = 0
· rw [C_eq_zero, content_eq_zero_iff] at h
simp [h]
conv_rhs =>
rw [p.eq_C_content_mul_primPart, natDegree_mul h p.primPart_ne_zero, natDegree_C, zero_add]
@[simp]
theorem IsPrimitive.primPart_eq {p : R[X]} (hp : p.IsPrimitive) : p.primPart = p := by
rw [← one_mul p.primPart, ← C_1, ← hp.content_eq_one, ← p.eq_C_content_mul_primPart]
theorem isUnit_primPart_C (r : R) : IsUnit (C r).primPart := by
by_cases h0 : r = 0
· simp [h0]
unfold IsUnit
refine
⟨⟨C ↑(normUnit r)⁻¹, C ↑(normUnit r), by rw [← RingHom.map_mul, Units.inv_mul, C_1], by
rw [← RingHom.map_mul, Units.mul_inv, C_1]⟩,
?_⟩
rw [← normalize_eq_zero, ← C_eq_zero] at h0
apply mul_left_cancel₀ h0
conv_rhs => rw [← content_C, ← (C r).eq_C_content_mul_primPart]
simp only [Units.val_mk, normalize_apply, RingHom.map_mul]
rw [mul_assoc, ← RingHom.map_mul, Units.mul_inv, C_1, mul_one]
theorem primPart_dvd (p : R[X]) : p.primPart ∣ p :=
Dvd.intro_left (C p.content) p.eq_C_content_mul_primPart.symm
theorem aeval_primPart_eq_zero {S : Type*} [Ring S] [IsDomain S] [Algebra R S]
[NoZeroSMulDivisors R S] {p : R[X]} {s : S} (hpzero : p ≠ 0) (hp : aeval s p = 0) :
aeval s p.primPart = 0 := by
rw [eq_C_content_mul_primPart p, map_mul, aeval_C] at hp
have hcont : p.content ≠ 0 := fun h => hpzero (content_eq_zero_iff.1 h)
replace hcont := Function.Injective.ne (NoZeroSMulDivisors.algebraMap_injective R S) hcont
rw [map_zero] at hcont
exact eq_zero_of_ne_zero_of_mul_left_eq_zero hcont hp
theorem eval₂_primPart_eq_zero {S : Type*} [CommRing S] [IsDomain S] {f : R →+* S}
(hinj : Function.Injective f) {p : R[X]} {s : S} (hpzero : p ≠ 0) (hp : eval₂ f s p = 0) :
eval₂ f s p.primPart = 0 := by
rw [eq_C_content_mul_primPart p, eval₂_mul, eval₂_C] at hp
have hcont : p.content ≠ 0 := fun h => hpzero (content_eq_zero_iff.1 h)
replace hcont := Function.Injective.ne hinj hcont
rw [map_zero] at hcont
exact eq_zero_of_ne_zero_of_mul_left_eq_zero hcont hp
end PrimPart
theorem gcd_content_eq_of_dvd_sub {a : R} {p q : R[X]} (h : C a ∣ p - q) :
GCDMonoid.gcd a p.content = GCDMonoid.gcd a q.content := by
rw [content_eq_gcd_range_of_lt p (max p.natDegree q.natDegree).succ
(lt_of_le_of_lt (le_max_left _ _) (Nat.lt_succ_self _))]
rw [content_eq_gcd_range_of_lt q (max p.natDegree q.natDegree).succ
(lt_of_le_of_lt (le_max_right _ _) (Nat.lt_succ_self _))]
apply Finset.gcd_eq_of_dvd_sub
intro x _
cases' h with w hw
use w.coeff x
rw [← coeff_sub, hw, coeff_C_mul]
theorem content_mul_aux {p q : R[X]} :
GCDMonoid.gcd (p * q).eraseLead.content p.leadingCoeff =
GCDMonoid.gcd (p.eraseLead * q).content p.leadingCoeff := by
rw [gcd_comm (content _) _, gcd_comm (content _) _]
apply gcd_content_eq_of_dvd_sub
rw [← self_sub_C_mul_X_pow, ← self_sub_C_mul_X_pow, sub_mul, sub_sub, add_comm, sub_add,
sub_sub_cancel, leadingCoeff_mul, RingHom.map_mul, mul_assoc, mul_assoc]
apply dvd_sub (Dvd.intro _ rfl) (Dvd.intro _ rfl)
@[simp]
theorem content_mul {p q : R[X]} : (p * q).content = p.content * q.content := by
classical
suffices h :
∀ (n : ℕ) (p q : R[X]), (p * q).degree < n → (p * q).content = p.content * q.content by
apply h
apply lt_of_le_of_lt degree_le_natDegree (WithBot.coe_lt_coe.2 (Nat.lt_succ_self _))
intro n
induction' n with n ih
· intro p q hpq
rw [Nat.cast_zero,
Nat.WithBot.lt_zero_iff, degree_eq_bot, mul_eq_zero] at hpq
rcases hpq with (rfl | rfl) <;> simp
intro p q hpq
by_cases p0 : p = 0
· simp [p0]
by_cases q0 : q = 0
· simp [q0]
rw [degree_eq_natDegree (mul_ne_zero p0 q0), Nat.cast_lt,
Nat.lt_succ_iff_lt_or_eq, ← Nat.cast_lt (α := WithBot ℕ),
← degree_eq_natDegree (mul_ne_zero p0 q0), natDegree_mul p0 q0] at hpq
rcases hpq with (hlt | heq)
· apply ih _ _ hlt
rw [← p.natDegree_primPart, ← q.natDegree_primPart, ← Nat.cast_inj (R := WithBot ℕ),
Nat.cast_add, ← degree_eq_natDegree p.primPart_ne_zero,
← degree_eq_natDegree q.primPart_ne_zero] at heq
rw [p.eq_C_content_mul_primPart, q.eq_C_content_mul_primPart]
suffices h : (q.primPart * p.primPart).content = 1 by
rw [mul_assoc, content_C_mul, content_C_mul, mul_comm p.primPart, mul_assoc, content_C_mul,
content_C_mul, h, mul_one, content_primPart, content_primPart, mul_one, mul_one]
rw [← normalize_content, normalize_eq_one, isUnit_iff_dvd_one,
content_eq_gcd_leadingCoeff_content_eraseLead, leadingCoeff_mul, gcd_comm]
apply (gcd_mul_dvd_mul_gcd _ _ _).trans
rw [content_mul_aux, ih, content_primPart, mul_one, gcd_comm, ←
content_eq_gcd_leadingCoeff_content_eraseLead, content_primPart, one_mul,
mul_comm q.primPart, content_mul_aux, ih, content_primPart, mul_one, gcd_comm, ←
content_eq_gcd_leadingCoeff_content_eraseLead, content_primPart]
· rw [← heq, degree_mul, WithBot.add_lt_add_iff_right]
· apply degree_erase_lt p.primPart_ne_zero
· rw [Ne, degree_eq_bot]
apply q.primPart_ne_zero
· rw [mul_comm, ← heq, degree_mul, WithBot.add_lt_add_iff_left]
· apply degree_erase_lt q.primPart_ne_zero
· rw [Ne, degree_eq_bot]
apply p.primPart_ne_zero
theorem IsPrimitive.mul {p q : R[X]} (hp : p.IsPrimitive) (hq : q.IsPrimitive) :
(p * q).IsPrimitive := by
rw [isPrimitive_iff_content_eq_one, content_mul, hp.content_eq_one, hq.content_eq_one, mul_one]
@[simp]
theorem primPart_mul {p q : R[X]} (h0 : p * q ≠ 0) :
(p * q).primPart = p.primPart * q.primPart := by
rw [Ne, ← content_eq_zero_iff, ← C_eq_zero] at h0
apply mul_left_cancel₀ h0
conv_lhs =>
rw [← (p * q).eq_C_content_mul_primPart, p.eq_C_content_mul_primPart,
q.eq_C_content_mul_primPart]
rw [content_mul, RingHom.map_mul]
ring
theorem IsPrimitive.dvd_primPart_iff_dvd {p q : R[X]} (hp : p.IsPrimitive) (hq : q ≠ 0) :
p ∣ q.primPart ↔ p ∣ q := by
refine ⟨fun h => h.trans (Dvd.intro_left _ q.eq_C_content_mul_primPart.symm), fun h => ?_⟩
rcases h with ⟨r, rfl⟩
apply Dvd.intro _
rw [primPart_mul hq, hp.primPart_eq]
theorem exists_primitive_lcm_of_isPrimitive {p q : R[X]} (hp : p.IsPrimitive) (hq : q.IsPrimitive) :
∃ r : R[X], r.IsPrimitive ∧ ∀ s : R[X], p ∣ s ∧ q ∣ s ↔ r ∣ s := by
classical
have h : ∃ (n : ℕ) (r : R[X]), r.natDegree = n ∧ r.IsPrimitive ∧ p ∣ r ∧ q ∣ r :=
⟨(p * q).natDegree, p * q, rfl, hp.mul hq, dvd_mul_right _ _, dvd_mul_left _ _⟩
rcases Nat.find_spec h with ⟨r, rdeg, rprim, pr, qr⟩
refine ⟨r, rprim, fun s => ⟨?_, fun rs => ⟨pr.trans rs, qr.trans rs⟩⟩⟩
suffices hs : ∀ (n : ℕ) (s : R[X]), s.natDegree = n → p ∣ s ∧ q ∣ s → r ∣ s from
hs s.natDegree s rfl
clear s
by_contra! con
rcases Nat.find_spec con with ⟨s, sdeg, ⟨ps, qs⟩, rs⟩
have s0 : s ≠ 0 := by
contrapose! rs
simp [rs]
have hs :=
Nat.find_min' h
⟨_, s.natDegree_primPart, s.isPrimitive_primPart, (hp.dvd_primPart_iff_dvd s0).2 ps,
(hq.dvd_primPart_iff_dvd s0).2 qs⟩
rw [← rdeg] at hs
by_cases sC : s.natDegree ≤ 0
· rw [eq_C_of_natDegree_le_zero (le_trans hs sC), isPrimitive_iff_content_eq_one, content_C,
normalize_eq_one] at rprim
rw [eq_C_of_natDegree_le_zero (le_trans hs sC), ← dvd_content_iff_C_dvd] at rs
apply rs rprim.dvd
have hcancel := natDegree_cancelLeads_lt_of_natDegree_le_natDegree hs (lt_of_not_ge sC)
rw [sdeg] at hcancel
apply Nat.find_min con hcancel
refine
⟨_, rfl, ⟨dvd_cancelLeads_of_dvd_of_dvd pr ps, dvd_cancelLeads_of_dvd_of_dvd qr qs⟩,
fun rcs => rs ?_⟩
rw [← rprim.dvd_primPart_iff_dvd s0]
rw [cancelLeads, tsub_eq_zero_iff_le.mpr hs, pow_zero, mul_one] at rcs
have h :=
dvd_add rcs (Dvd.intro_left (C (leadingCoeff s) * X ^ (natDegree s - natDegree r)) rfl)
have hC0 := rprim.ne_zero
rw [Ne, ← leadingCoeff_eq_zero, ← C_eq_zero] at hC0
rw [sub_add_cancel, ← rprim.dvd_primPart_iff_dvd (mul_ne_zero hC0 s0)] at h
rcases isUnit_primPart_C r.leadingCoeff with ⟨u, hu⟩
apply h.trans (Associated.symm ⟨u, _⟩).dvd
rw [primPart_mul (mul_ne_zero hC0 s0), hu, mul_comm]
theorem dvd_iff_content_dvd_content_and_primPart_dvd_primPart {p q : R[X]} (hq : q ≠ 0) :
p ∣ q ↔ p.content ∣ q.content ∧ p.primPart ∣ q.primPart := by
constructor <;> intro h
· rcases h with ⟨r, rfl⟩
rw [content_mul, p.isPrimitive_primPart.dvd_primPart_iff_dvd hq]
exact ⟨Dvd.intro _ rfl, p.primPart_dvd.trans (Dvd.intro _ rfl)⟩
· rw [p.eq_C_content_mul_primPart, q.eq_C_content_mul_primPart]
exact mul_dvd_mul (RingHom.map_dvd C h.1) h.2
noncomputable instance (priority := 100) normalizedGcdMonoid : NormalizedGCDMonoid R[X] :=
letI := Classical.decEq R
normalizedGCDMonoidOfExistsLCM fun p q => by
rcases exists_primitive_lcm_of_isPrimitive p.isPrimitive_primPart
q.isPrimitive_primPart with
⟨r, rprim, hr⟩
refine ⟨C (lcm p.content q.content) * r, fun s => ?_⟩
by_cases hs : s = 0
· simp [hs]
by_cases hpq : C (lcm p.content q.content) = 0
· rw [C_eq_zero, lcm_eq_zero_iff, content_eq_zero_iff, content_eq_zero_iff] at hpq
rcases hpq with (hpq | hpq) <;> simp [hpq, hs]
iterate 3 rw [dvd_iff_content_dvd_content_and_primPart_dvd_primPart hs]
rw [content_mul, rprim.content_eq_one, mul_one, content_C, normalize_lcm, lcm_dvd_iff,
primPart_mul (mul_ne_zero hpq rprim.ne_zero), rprim.primPart_eq,
(isUnit_primPart_C (lcm p.content q.content)).mul_left_dvd, ← hr s.primPart]
tauto
theorem degree_gcd_le_left {p : R[X]} (hp : p ≠ 0) (q) : (gcd p q).degree ≤ p.degree := by
have := natDegree_le_iff_degree_le.mp (natDegree_le_of_dvd (gcd_dvd_left p q) hp)
rwa [degree_eq_natDegree hp]
theorem degree_gcd_le_right (p) {q : R[X]} (hq : q ≠ 0) : (gcd p q).degree ≤ q.degree := by
rw [gcd_comm]
exact degree_gcd_le_left hq p
end NormalizedGCDMonoid
end Polynomial
|
RingTheory\Polynomial\Dickson.lean | /-
Copyright (c) 2021 Julian Kuelshammer. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Julian Kuelshammer
-/
import Mathlib.Algebra.CharP.Invertible
import Mathlib.Data.ZMod.Basic
import Mathlib.RingTheory.Localization.FractionRing
import Mathlib.RingTheory.Polynomial.Chebyshev
import Mathlib.Algebra.CharP.Basic
import Mathlib.Algebra.EuclideanDomain.Field
import Mathlib.Algebra.Polynomial.Roots
/-!
# Dickson polynomials
The (generalised) Dickson polynomials are a family of polynomials indexed by `ℕ × ℕ`,
with coefficients in a commutative ring `R` depending on an element `a∈R`. More precisely, the
they satisfy the recursion `dickson k a (n + 2) = X * (dickson k a n + 1) - a * (dickson k a n)`
with starting values `dickson k a 0 = 3 - k` and `dickson k a 1 = X`. In the literature,
`dickson k a n` is called the `n`-th Dickson polynomial of the `k`-th kind associated to the
parameter `a : R`. They are closely related to the Chebyshev polynomials in the case that `a=1`.
When `a=0` they are just the family of monomials `X ^ n`.
## Main definition
* `Polynomial.dickson`: the generalised Dickson polynomials.
## Main statements
* `Polynomial.dickson_one_one_mul`, the `(m * n)`-th Dickson polynomial of the first kind for
parameter `1 : R` is the composition of the `m`-th and `n`-th Dickson polynomials of the first
kind for `1 : R`.
* `Polynomial.dickson_one_one_charP`, for a prime number `p`, the `p`-th Dickson polynomial of the
first kind associated to parameter `1 : R` is congruent to `X ^ p` modulo `p`.
## References
* [R. Lidl, G. L. Mullen and G. Turnwald, _Dickson polynomials_][MR1237403]
## TODO
* Redefine `dickson` in terms of `LinearRecurrence`.
* Show that `dickson 2 1` is equal to the characteristic polynomial of the adjacency matrix of a
type A Dynkin diagram.
* Prove that the adjacency matrices of simply laced Dynkin diagrams are precisely the adjacency
matrices of simple connected graphs which annihilate `dickson 2 1`.
-/
noncomputable section
namespace Polynomial
open Polynomial
variable {R S : Type*} [CommRing R] [CommRing S] (k : ℕ) (a : R)
/-- `dickson` is the `n`-th (generalised) Dickson polynomial of the `k`-th kind associated to the
element `a ∈ R`. -/
noncomputable def dickson : ℕ → R[X]
| 0 => 3 - k
| 1 => X
| n + 2 => X * dickson (n + 1) - C a * dickson n
@[simp]
theorem dickson_zero : dickson k a 0 = 3 - k :=
rfl
@[simp]
theorem dickson_one : dickson k a 1 = X :=
rfl
theorem dickson_two : dickson k a 2 = X ^ 2 - C a * (3 - k : R[X]) := by
simp only [dickson, sq]
@[simp]
theorem dickson_add_two (n : ℕ) :
dickson k a (n + 2) = X * dickson k a (n + 1) - C a * dickson k a n := by rw [dickson]
theorem dickson_of_two_le {n : ℕ} (h : 2 ≤ n) :
dickson k a n = X * dickson k a (n - 1) - C a * dickson k a (n - 2) := by
obtain ⟨n, rfl⟩ := Nat.exists_eq_add_of_le h
rw [add_comm]
exact dickson_add_two k a n
variable {k a}
theorem map_dickson (f : R →+* S) : ∀ n : ℕ, map f (dickson k a n) = dickson k (f a) n
| 0 => by
simp_rw [dickson_zero, Polynomial.map_sub, Polynomial.map_natCast, Polynomial.map_ofNat]
| 1 => by simp only [dickson_one, map_X]
| n + 2 => by
simp only [dickson_add_two, Polynomial.map_sub, Polynomial.map_mul, map_X, map_C]
rw [map_dickson f n, map_dickson f (n + 1)]
@[simp]
theorem dickson_two_zero : ∀ n : ℕ, dickson 2 (0 : R) n = X ^ n
| 0 => by
simp only [dickson_zero, pow_zero]
norm_num
| 1 => by simp only [dickson_one, pow_one]
| n + 2 => by
simp only [dickson_add_two, C_0, zero_mul, sub_zero]
rw [dickson_two_zero (n + 1), pow_add X (n + 1) 1, mul_comm, pow_one]
section Dickson
/-!
### A Lambda structure on `ℤ[X]`
Mathlib doesn't currently know what a Lambda ring is.
But once it does, we can endow `ℤ[X]` with a Lambda structure
in terms of the `dickson 1 1` polynomials defined below.
There is exactly one other Lambda structure on `ℤ[X]` in terms of binomial polynomials.
-/
theorem dickson_one_one_eval_add_inv (x y : R) (h : x * y = 1) :
∀ n, (dickson 1 (1 : R) n).eval (x + y) = x ^ n + y ^ n
| 0 => by
simp only [eval_one, eval_add, pow_zero, dickson_zero]; norm_num
| 1 => by simp only [eval_X, dickson_one, pow_one]
| n + 2 => by
simp only [eval_sub, eval_mul, dickson_one_one_eval_add_inv x y h _, eval_X, dickson_add_two,
C_1, eval_one]
conv_lhs => simp only [pow_succ', add_mul, mul_add, h, ← mul_assoc, mul_comm y x, one_mul]
ring
variable (R)
-- Porting note: Added 2 new theorems for convenience
private theorem two_mul_C_half_eq_one [Invertible (2 : R)] : 2 * C (⅟ 2 : R) = 1 := by
rw [two_mul, ← C_add, invOf_two_add_invOf_two, C_1]
private theorem C_half_mul_two_eq_one [Invertible (2 : R)] : C (⅟ 2 : R) * 2 = 1 := by
rw [mul_comm, two_mul_C_half_eq_one]
theorem dickson_one_one_eq_chebyshev_T [Invertible (2 : R)] :
∀ n, dickson 1 (1 : R) n = 2 * (Chebyshev.T R n).comp (C (⅟ 2) * X)
| 0 => by
simp only [Chebyshev.T_zero, mul_one, one_comp, dickson_zero]
norm_num
| 1 => by
rw [dickson_one, Nat.cast_one, Chebyshev.T_one, X_comp, ← mul_assoc, two_mul_C_half_eq_one,
one_mul]
| n + 2 => by
rw [dickson_add_two, C_1, Nat.cast_add, Nat.cast_two, Chebyshev.T_add_two,
dickson_one_one_eq_chebyshev_T (n + 1), dickson_one_one_eq_chebyshev_T n, sub_comp, mul_comp,
mul_comp, X_comp, ofNat_comp]
simp_rw [← mul_assoc, Nat.cast_ofNat, two_mul_C_half_eq_one, Nat.cast_add, Nat.cast_one]
ring
theorem chebyshev_T_eq_dickson_one_one [Invertible (2 : R)] (n : ℕ) :
Chebyshev.T R n = C (⅟ 2) * (dickson 1 1 n).comp (2 * X) := by
rw [dickson_one_one_eq_chebyshev_T, mul_comp, ofNat_comp, comp_assoc, mul_comp, C_comp, X_comp]
simp_rw [← mul_assoc, Nat.cast_ofNat, C_half_mul_two_eq_one, one_mul, comp_X]
/-- The `(m * n)`-th Dickson polynomial of the first kind is the composition of the `m`-th and
`n`-th. -/
theorem dickson_one_one_mul (m n : ℕ) :
dickson 1 (1 : R) (m * n) = (dickson 1 1 m).comp (dickson 1 1 n) := by
have h : (1 : R) = Int.castRingHom R 1 := by simp only [eq_intCast, Int.cast_one]
rw [h]
simp only [← map_dickson (Int.castRingHom R), ← map_comp]
congr 1
apply map_injective (Int.castRingHom ℚ) Int.cast_injective
simp only [map_dickson, map_comp, eq_intCast, Int.cast_one, dickson_one_one_eq_chebyshev_T,
Nat.cast_mul, Chebyshev.T_mul, two_mul, ← add_comp]
simp only [← two_mul, ← comp_assoc]
apply eval₂_congr rfl rfl
rw [comp_assoc]
apply eval₂_congr rfl _ rfl
rw [mul_comp, C_comp, X_comp, ← mul_assoc, C_half_mul_two_eq_one, one_mul]
theorem dickson_one_one_comp_comm (m n : ℕ) :
(dickson 1 (1 : R) m).comp (dickson 1 1 n) = (dickson 1 1 n).comp (dickson 1 1 m) := by
rw [← dickson_one_one_mul, mul_comm, dickson_one_one_mul]
theorem dickson_one_one_zmod_p (p : ℕ) [Fact p.Prime] : dickson 1 (1 : ZMod p) p = X ^ p := by
-- Recall that `dickson_one_one_eval_add_inv` characterises `dickson 1 1 p`
-- as a polynomial that maps `x + x⁻¹` to `x ^ p + (x⁻¹) ^ p`.
-- Since `X ^ p` also satisfies this property in characteristic `p`,
-- we can use a variant on `Polynomial.funext` to conclude that these polynomials are equal.
-- For this argument, we need an arbitrary infinite field of characteristic `p`.
obtain ⟨K, _, _, H⟩ : ∃ (K : Type) (_ : Field K), ∃ _ : CharP K p, Infinite K := by
let K := FractionRing (Polynomial (ZMod p))
let f : ZMod p →+* K := (algebraMap _ (FractionRing _)).comp C
have : CharP K p := by
rw [← f.charP_iff_charP]
infer_instance
haveI : Infinite K :=
Infinite.of_injective (algebraMap (Polynomial (ZMod p)) (FractionRing (Polynomial (ZMod p))))
(IsFractionRing.injective _ _)
refine ⟨K, ?_, ?_, ?_⟩ <;> infer_instance
apply map_injective (ZMod.castHom (dvd_refl p) K) (RingHom.injective _)
rw [map_dickson, Polynomial.map_pow, map_X]
apply eq_of_infinite_eval_eq
-- The two polynomials agree on all `x` of the form `x = y + y⁻¹`.
apply @Set.Infinite.mono _ { x : K | ∃ y, x = y + y⁻¹ ∧ y ≠ 0 }
· rintro _ ⟨x, rfl, hx⟩
simp only [eval_X, eval_pow, Set.mem_setOf_eq, @add_pow_char K _ p,
dickson_one_one_eval_add_inv _ _ (mul_inv_cancel hx), inv_pow, ZMod.castHom_apply,
ZMod.cast_one']
-- Now we need to show that the set of such `x` is infinite.
-- If the set is finite, then we will show that `K` is also finite.
· intro h
rw [← Set.infinite_univ_iff] at H
apply H
-- To each `x` of the form `x = y + y⁻¹`
-- we `bind` the set of `y` that solve the equation `x = y + y⁻¹`.
-- For every `x`, that set is finite (since it is governed by a quadratic equation).
-- For the moment, we claim that all these sets together cover `K`.
suffices (Set.univ : Set K) =
⋃ x ∈ { x : K | ∃ y : K, x = y + y⁻¹ ∧ y ≠ 0 }, { y | x = y + y⁻¹ ∨ y = 0 } by
rw [this]
clear this
refine h.biUnion fun x _ => ?_
-- The following quadratic polynomial has as solutions the `y` for which `x = y + y⁻¹`.
let φ : K[X] := X ^ 2 - C x * X + 1
have hφ : φ ≠ 0 := by
intro H
have : φ.eval 0 = 0 := by rw [H, eval_zero]
simpa [φ, eval_X, eval_one, eval_pow, eval_sub, sub_zero, eval_add, eval_mul,
mul_zero, sq, zero_add, one_ne_zero]
classical
convert (φ.roots ∪ {0}).toFinset.finite_toSet using 1
ext1 y
simp only [φ, Multiset.mem_toFinset, Set.mem_setOf_eq, Finset.mem_coe, Multiset.mem_union,
mem_roots hφ, IsRoot, eval_add, eval_sub, eval_pow, eval_mul, eval_X, eval_C, eval_one,
Multiset.mem_singleton]
by_cases hy : y = 0
· simp only [hy, eq_self_iff_true, or_true_iff]
apply or_congr _ Iff.rfl
rw [← mul_left_inj' hy, eq_comm, ← sub_eq_zero, add_mul, inv_mul_cancel hy]
apply eq_iff_eq_cancel_right.mpr
ring
-- Finally, we prove the claim that our finite union of finite sets covers all of `K`.
apply (Set.eq_univ_of_forall _).symm
intro x
simp only [exists_prop, Set.mem_iUnion, Set.bind_def, Ne, Set.mem_setOf_eq]
by_cases hx : x = 0
· simp only [hx, and_true_iff, eq_self_iff_true, inv_zero, or_true_iff]
exact ⟨_, 1, rfl, one_ne_zero⟩
· simp only [hx, or_false_iff, exists_eq_right]
exact ⟨_, rfl, hx⟩
theorem dickson_one_one_charP (p : ℕ) [Fact p.Prime] [CharP R p] : dickson 1 (1 : R) p = X ^ p := by
have h : (1 : R) = ZMod.castHom (dvd_refl p) R 1 := by
simp only [ZMod.castHom_apply, ZMod.cast_one']
rw [h, ← map_dickson (ZMod.castHom (dvd_refl p) R), dickson_one_one_zmod_p, Polynomial.map_pow,
map_X]
end Dickson
end Polynomial
|
RingTheory\Polynomial\GaussLemma.lean | /-
Copyright (c) 2020 Aaron Anderson. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Aaron Anderson
-/
import Mathlib.FieldTheory.SplittingField.Construction
import Mathlib.RingTheory.Int.Basic
import Mathlib.RingTheory.Localization.Integral
import Mathlib.RingTheory.IntegralClosure.IntegrallyClosed
/-!
# Gauss's Lemma
Gauss's Lemma is one of a few results pertaining to irreducibility of primitive polynomials.
## Main Results
- `IsIntegrallyClosed.eq_map_mul_C_of_dvd`: if `R` is integrally closed, `K = Frac(R)` and
`g : K[X]` divides a monic polynomial with coefficients in `R`, then `g * (C g.leadingCoeff⁻¹)`
has coefficients in `R`
- `Polynomial.Monic.irreducible_iff_irreducible_map_fraction_map`:
A monic polynomial over an integrally closed domain is irreducible iff it is irreducible in a
fraction field
- `isIntegrallyClosed_iff'`:
Integrally closed domains are precisely the domains for in which Gauss's lemma holds
for monic polynomials
- `Polynomial.IsPrimitive.irreducible_iff_irreducible_map_fraction_map`:
A primitive polynomial over a GCD domain is irreducible iff it is irreducible in a fraction field
- `Polynomial.IsPrimitive.Int.irreducible_iff_irreducible_map_cast`:
A primitive polynomial over `ℤ` is irreducible iff it is irreducible over `ℚ`.
- `Polynomial.IsPrimitive.dvd_iff_fraction_map_dvd_fraction_map`:
Two primitive polynomials over a GCD domain divide each other iff they do in a fraction field.
- `Polynomial.IsPrimitive.Int.dvd_iff_map_cast_dvd_map_cast`:
Two primitive polynomials over `ℤ` divide each other if they do in `ℚ`.
-/
open scoped nonZeroDivisors Polynomial
variable {R : Type*} [CommRing R]
section IsIntegrallyClosed
open Polynomial
open integralClosure
open IsIntegrallyClosed
variable (K : Type*) [Field K] [Algebra R K]
theorem integralClosure.mem_lifts_of_monic_of_dvd_map {f : R[X]} (hf : f.Monic) {g : K[X]}
(hg : g.Monic) (hd : g ∣ f.map (algebraMap R K)) :
g ∈ lifts (algebraMap (integralClosure R K) K) := by
have := mem_lift_of_splits_of_roots_mem_range (integralClosure R g.SplittingField)
((splits_id_iff_splits _).2 <| SplittingField.splits g) (hg.map _) fun a ha =>
(SetLike.ext_iff.mp (integralClosure R g.SplittingField).range_algebraMap _).mpr <|
roots_mem_integralClosure hf ?_
· rw [lifts_iff_coeff_lifts, ← RingHom.coe_range, Subalgebra.range_algebraMap] at this
refine (lifts_iff_coeff_lifts _).2 fun n => ?_
rw [← RingHom.coe_range, Subalgebra.range_algebraMap]
obtain ⟨p, hp, he⟩ := SetLike.mem_coe.mp (this n); use p, hp
rw [IsScalarTower.algebraMap_eq R K, coeff_map, ← eval₂_map, eval₂_at_apply] at he
rw [eval₂_eq_eval_map]; apply (injective_iff_map_eq_zero _).1 _ _ he
apply RingHom.injective
rw [aroots_def, IsScalarTower.algebraMap_eq R K _, ← map_map]
refine Multiset.mem_of_le (roots.le_of_dvd ((hf.map _).map _).ne_zero ?_) ha
exact map_dvd (algebraMap K g.SplittingField) hd
variable [IsDomain R] [IsFractionRing R K]
/-- If `K = Frac(R)` and `g : K[X]` divides a monic polynomial with coefficients in `R`, then
`g * (C g.leadingCoeff⁻¹)` has coefficients in `R` -/
theorem IsIntegrallyClosed.eq_map_mul_C_of_dvd [IsIntegrallyClosed R] {f : R[X]} (hf : f.Monic)
{g : K[X]} (hg : g ∣ f.map (algebraMap R K)) :
∃ g' : R[X], g'.map (algebraMap R K) * (C <| leadingCoeff g) = g := by
have g_ne_0 : g ≠ 0 := ne_zero_of_dvd_ne_zero (Monic.ne_zero <| hf.map (algebraMap R K)) hg
suffices lem : ∃ g' : R[X], g'.map (algebraMap R K) = g * C g.leadingCoeff⁻¹ by
obtain ⟨g', hg'⟩ := lem
use g'
rw [hg', mul_assoc, ← C_mul, inv_mul_cancel (leadingCoeff_ne_zero.mpr g_ne_0), C_1, mul_one]
have g_mul_dvd : g * C g.leadingCoeff⁻¹ ∣ f.map (algebraMap R K) := by
rwa [Associated.dvd_iff_dvd_left (show Associated (g * C g.leadingCoeff⁻¹) g from _)]
rw [associated_mul_isUnit_left_iff]
exact isUnit_C.mpr (inv_ne_zero <| leadingCoeff_ne_zero.mpr g_ne_0).isUnit
let algeq :=
(Subalgebra.equivOfEq _ _ <| integralClosure_eq_bot R _).trans
(Algebra.botEquivOfInjective <| IsFractionRing.injective R <| K)
have :
(algebraMap R _).comp algeq.toAlgHom.toRingHom = (integralClosure R _).toSubring.subtype := by
ext x; (conv_rhs => rw [← algeq.symm_apply_apply x]); rfl
have H :=
(mem_lifts _).1
(integralClosure.mem_lifts_of_monic_of_dvd_map K hf (monic_mul_leadingCoeff_inv g_ne_0)
g_mul_dvd)
refine ⟨map algeq.toAlgHom.toRingHom ?_, ?_⟩
· use! Classical.choose H
· rw [map_map, this]
exact Classical.choose_spec H
end IsIntegrallyClosed
namespace Polynomial
section
variable {S : Type*} [CommRing S] [IsDomain S]
variable {φ : R →+* S} (hinj : Function.Injective φ) {f : R[X]} (hf : f.IsPrimitive)
theorem IsPrimitive.isUnit_iff_isUnit_map_of_injective : IsUnit f ↔ IsUnit (map φ f) := by
refine ⟨(mapRingHom φ).isUnit_map, fun h => ?_⟩
rcases isUnit_iff.1 h with ⟨_, ⟨u, rfl⟩, hu⟩
have hdeg := degree_C u.ne_zero
rw [hu, degree_map_eq_of_injective hinj] at hdeg
rw [eq_C_of_degree_eq_zero hdeg] at hf ⊢
exact isUnit_C.mpr (isPrimitive_iff_isUnit_of_C_dvd.mp hf (f.coeff 0) dvd_rfl)
theorem IsPrimitive.irreducible_of_irreducible_map_of_injective (h_irr : Irreducible (map φ f)) :
Irreducible f := by
refine
⟨fun h => h_irr.not_unit (IsUnit.map (mapRingHom φ) h), fun a b h =>
(h_irr.isUnit_or_isUnit <| by rw [h, Polynomial.map_mul]).imp ?_ ?_⟩
all_goals apply ((isPrimitive_of_dvd hf _).isUnit_iff_isUnit_map_of_injective hinj).mpr
exacts [Dvd.intro _ h.symm, Dvd.intro_left _ h.symm]
end
section FractionMap
variable {K : Type*} [Field K] [Algebra R K] [IsFractionRing R K]
theorem IsPrimitive.isUnit_iff_isUnit_map {p : R[X]} (hp : p.IsPrimitive) :
IsUnit p ↔ IsUnit (p.map (algebraMap R K)) :=
hp.isUnit_iff_isUnit_map_of_injective (IsFractionRing.injective _ _)
variable [IsDomain R]
section IsIntegrallyClosed
open IsIntegrallyClosed
/-- **Gauss's Lemma** for integrally closed domains states that a monic polynomial is irreducible
iff it is irreducible in the fraction field. -/
theorem Monic.irreducible_iff_irreducible_map_fraction_map [IsIntegrallyClosed R] {p : R[X]}
(h : p.Monic) : Irreducible p ↔ Irreducible (p.map <| algebraMap R K) := by
/- The ← direction follows from `IsPrimitive.irreducible_of_irreducible_map_of_injective`.
For the → direction, it is enought to show that if `(p.map <| algebraMap R K) = a * b` and
`a` is not a unit then `b` is a unit -/
refine
⟨fun hp =>
irreducible_iff.mpr
⟨hp.not_unit.imp h.isPrimitive.isUnit_iff_isUnit_map.mpr, fun a b H =>
or_iff_not_imp_left.mpr fun hₐ => ?_⟩,
fun hp =>
h.isPrimitive.irreducible_of_irreducible_map_of_injective (IsFractionRing.injective R K) hp⟩
obtain ⟨a', ha⟩ := eq_map_mul_C_of_dvd K h (dvd_of_mul_right_eq b H.symm)
obtain ⟨b', hb⟩ := eq_map_mul_C_of_dvd K h (dvd_of_mul_left_eq a H.symm)
have : a.leadingCoeff * b.leadingCoeff = 1 := by
rw [← leadingCoeff_mul, ← H, Monic.leadingCoeff (h.map <| algebraMap R K)]
rw [← ha, ← hb, mul_comm _ (C b.leadingCoeff), mul_assoc, ← mul_assoc (C a.leadingCoeff), ←
C_mul, this, C_1, one_mul, ← Polynomial.map_mul] at H
rw [← hb, ← Polynomial.coe_mapRingHom]
refine
IsUnit.mul (IsUnit.map _ (Or.resolve_left (hp.isUnit_or_isUnit ?_) (show ¬IsUnit a' from ?_)))
(isUnit_iff_exists_inv'.mpr
-- Porting note(https://github.com/leanprover-community/mathlib4/issues/5073): was `rwa`
(Exists.intro (C a.leadingCoeff) <| by rw [← C_mul, this, C_1]))
· exact Polynomial.map_injective _ (IsFractionRing.injective R K) H
· by_contra h_contra
refine hₐ ?_
rw [← ha, ← Polynomial.coe_mapRingHom]
exact
IsUnit.mul (IsUnit.map _ h_contra)
(isUnit_iff_exists_inv.mpr
-- Porting note(https://github.com/leanprover-community/mathlib4/issues/5073): was `rwa`
(Exists.intro (C b.leadingCoeff) <| by rw [← C_mul, this, C_1]))
/-- Integrally closed domains are precisely the domains for in which Gauss's lemma holds
for monic polynomials -/
theorem isIntegrallyClosed_iff' :
IsIntegrallyClosed R ↔
∀ p : R[X], p.Monic → (Irreducible p ↔ Irreducible (p.map <| algebraMap R K)) := by
constructor
· intro hR p hp; exact Monic.irreducible_iff_irreducible_map_fraction_map hp
· intro H
refine
(isIntegrallyClosed_iff K).mpr fun {x} hx =>
RingHom.mem_range.mp <| minpoly.mem_range_of_degree_eq_one R x ?_
rw [← Monic.degree_map (minpoly.monic hx) (algebraMap R K)]
apply
degree_eq_one_of_irreducible_of_root ((H _ <| minpoly.monic hx).mp (minpoly.irreducible hx))
rw [IsRoot, eval_map, ← aeval_def, minpoly.aeval R x]
theorem Monic.dvd_of_fraction_map_dvd_fraction_map [IsIntegrallyClosed R] {p q : R[X]}
(hp : p.Monic) (hq : q.Monic)
(h : q.map (algebraMap R K) ∣ p.map (algebraMap R K)) : q ∣ p := by
obtain ⟨r, hr⟩ := h
obtain ⟨d', hr'⟩ := IsIntegrallyClosed.eq_map_mul_C_of_dvd K hp (dvd_of_mul_left_eq _ hr.symm)
rw [Monic.leadingCoeff, C_1, mul_one] at hr'
· rw [← hr', ← Polynomial.map_mul] at hr
exact dvd_of_mul_right_eq _ (Polynomial.map_injective _ (IsFractionRing.injective R K) hr.symm)
· exact Monic.of_mul_monic_left (hq.map (algebraMap R K)) (by simpa [← hr] using hp.map _)
theorem Monic.dvd_iff_fraction_map_dvd_fraction_map [IsIntegrallyClosed R] {p q : R[X]}
(hp : p.Monic) (hq : q.Monic) : q.map (algebraMap R K) ∣ p.map (algebraMap R K) ↔ q ∣ p :=
⟨fun h => hp.dvd_of_fraction_map_dvd_fraction_map hq h, fun ⟨a, b⟩ =>
⟨a.map (algebraMap R K), b.symm ▸ Polynomial.map_mul (algebraMap R K)⟩⟩
end IsIntegrallyClosed
open IsLocalization
section NormalizedGCDMonoid
variable [NormalizedGCDMonoid R]
theorem isUnit_or_eq_zero_of_isUnit_integerNormalization_primPart {p : K[X]} (h0 : p ≠ 0)
(h : IsUnit (integerNormalization R⁰ p).primPart) : IsUnit p := by
rcases isUnit_iff.1 h with ⟨_, ⟨u, rfl⟩, hu⟩
obtain ⟨⟨c, c0⟩, hc⟩ := integerNormalization_map_to_map R⁰ p
rw [Subtype.coe_mk, Algebra.smul_def, algebraMap_apply] at hc
apply isUnit_of_mul_isUnit_right
rw [← hc, (integerNormalization R⁰ p).eq_C_content_mul_primPart, ← hu, ← RingHom.map_mul,
isUnit_iff]
refine
⟨algebraMap R K ((integerNormalization R⁰ p).content * ↑u), isUnit_iff_ne_zero.2 fun con => ?_,
by simp⟩
replace con := (injective_iff_map_eq_zero (algebraMap R K)).1 (IsFractionRing.injective _ _) _ con
rw [mul_eq_zero, content_eq_zero_iff, IsFractionRing.integerNormalization_eq_zero_iff] at con
rcases con with (con | con)
· apply h0 con
· apply Units.ne_zero _ con
/-- **Gauss's Lemma** for GCD domains states that a primitive polynomial is irreducible iff it is
irreducible in the fraction field. -/
theorem IsPrimitive.irreducible_iff_irreducible_map_fraction_map {p : R[X]} (hp : p.IsPrimitive) :
Irreducible p ↔ Irreducible (p.map (algebraMap R K)) := by
-- Porting note: was `(IsFractionRing.injective _ _)`
refine
⟨fun hi => ⟨fun h => hi.not_unit (hp.isUnit_iff_isUnit_map.2 h), fun a b hab => ?_⟩,
hp.irreducible_of_irreducible_map_of_injective (IsFractionRing.injective R K)⟩
obtain ⟨⟨c, c0⟩, hc⟩ := integerNormalization_map_to_map R⁰ a
obtain ⟨⟨d, d0⟩, hd⟩ := integerNormalization_map_to_map R⁰ b
rw [Algebra.smul_def, algebraMap_apply, Subtype.coe_mk] at hc hd
rw [mem_nonZeroDivisors_iff_ne_zero] at c0 d0
have hcd0 : c * d ≠ 0 := mul_ne_zero c0 d0
rw [Ne, ← C_eq_zero] at hcd0
have h1 : C c * C d * p = integerNormalization R⁰ a * integerNormalization R⁰ b := by
apply map_injective (algebraMap R K) (IsFractionRing.injective _ _) _
rw [Polynomial.map_mul, Polynomial.map_mul, Polynomial.map_mul, hc, hd, map_C, map_C, hab]
ring
obtain ⟨u, hu⟩ :
Associated (c * d)
(content (integerNormalization R⁰ a) * content (integerNormalization R⁰ b)) := by
rw [← dvd_dvd_iff_associated, ← normalize_eq_normalize_iff, normalize.map_mul,
normalize.map_mul, normalize_content, normalize_content, ←
mul_one (normalize c * normalize d), ← hp.content_eq_one, ← content_C, ← content_C, ←
content_mul, ← content_mul, ← content_mul, h1]
rw [← RingHom.map_mul, eq_comm, (integerNormalization R⁰ a).eq_C_content_mul_primPart,
(integerNormalization R⁰ b).eq_C_content_mul_primPart, mul_assoc, mul_comm _ (C _ * _), ←
mul_assoc, ← mul_assoc, ← RingHom.map_mul, ← hu, RingHom.map_mul, mul_assoc, mul_assoc, ←
mul_assoc (C (u : R))] at h1
have h0 : a ≠ 0 ∧ b ≠ 0 := by
classical
rw [Ne, Ne, ← not_or, ← mul_eq_zero, ← hab]
intro con
apply hp.ne_zero (map_injective (algebraMap R K) (IsFractionRing.injective _ _) _)
simp [con]
rcases hi.isUnit_or_isUnit (mul_left_cancel₀ hcd0 h1).symm with (h | h)
· right
apply
isUnit_or_eq_zero_of_isUnit_integerNormalization_primPart h0.2
(isUnit_of_mul_isUnit_right h)
· left
apply isUnit_or_eq_zero_of_isUnit_integerNormalization_primPart h0.1 h
theorem IsPrimitive.dvd_of_fraction_map_dvd_fraction_map {p q : R[X]} (hp : p.IsPrimitive)
(hq : q.IsPrimitive) (h_dvd : p.map (algebraMap R K) ∣ q.map (algebraMap R K)) : p ∣ q := by
rcases h_dvd with ⟨r, hr⟩
obtain ⟨⟨s, s0⟩, hs⟩ := integerNormalization_map_to_map R⁰ r
rw [Subtype.coe_mk, Algebra.smul_def, algebraMap_apply] at hs
have h : p ∣ q * C s := by
use integerNormalization R⁰ r
apply map_injective (algebraMap R K) (IsFractionRing.injective _ _)
rw [Polynomial.map_mul, Polynomial.map_mul, hs, hr, mul_assoc, mul_comm r]
simp
rw [← hp.dvd_primPart_iff_dvd, primPart_mul, hq.primPart_eq, Associated.dvd_iff_dvd_right] at h
· exact h
· symm
rcases isUnit_primPart_C s with ⟨u, hu⟩
use u
rw [hu]
iterate 2
apply mul_ne_zero hq.ne_zero
rw [Ne, C_eq_zero]
contrapose! s0
simp [s0, mem_nonZeroDivisors_iff_ne_zero]
variable (K)
theorem IsPrimitive.dvd_iff_fraction_map_dvd_fraction_map {p q : R[X]} (hp : p.IsPrimitive)
(hq : q.IsPrimitive) : p ∣ q ↔ p.map (algebraMap R K) ∣ q.map (algebraMap R K) :=
⟨fun ⟨a, b⟩ => ⟨a.map (algebraMap R K), b.symm ▸ Polynomial.map_mul (algebraMap R K)⟩, fun h =>
hp.dvd_of_fraction_map_dvd_fraction_map hq h⟩
end NormalizedGCDMonoid
end FractionMap
/-- **Gauss's Lemma** for `ℤ` states that a primitive integer polynomial is irreducible iff it is
irreducible over `ℚ`. -/
theorem IsPrimitive.Int.irreducible_iff_irreducible_map_cast {p : ℤ[X]} (hp : p.IsPrimitive) :
Irreducible p ↔ Irreducible (p.map (Int.castRingHom ℚ)) :=
hp.irreducible_iff_irreducible_map_fraction_map
theorem IsPrimitive.Int.dvd_iff_map_cast_dvd_map_cast (p q : ℤ[X]) (hp : p.IsPrimitive)
(hq : q.IsPrimitive) : p ∣ q ↔ p.map (Int.castRingHom ℚ) ∣ q.map (Int.castRingHom ℚ) :=
hp.dvd_iff_fraction_map_dvd_fraction_map ℚ hq
end Polynomial
|
RingTheory\Polynomial\IntegralNormalization.lean | /-
Copyright (c) 2018 Chris Hughes. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Chris Hughes, Johannes Hölzl, Scott Morrison, Jens Wagemaker
-/
import Mathlib.Algebra.Polynomial.AlgebraMap
import Mathlib.Algebra.Polynomial.Degree.Lemmas
import Mathlib.Algebra.Polynomial.Monic
/-!
# Theory of monic polynomials
We define `integralNormalization`, which relate arbitrary polynomials to monic ones.
-/
open Polynomial
namespace Polynomial
universe u v y
variable {R : Type u} {S : Type v} {a b : R} {m n : ℕ} {ι : Type y}
section IntegralNormalization
section Semiring
variable [Semiring R]
/-- If `f : R[X]` is a nonzero polynomial with root `z`, `integralNormalization f` is
a monic polynomial with root `leadingCoeff f * z`.
Moreover, `integralNormalization 0 = 0`.
-/
noncomputable def integralNormalization (f : R[X]) : R[X] :=
∑ i ∈ f.support,
monomial i (if f.degree = i then 1 else coeff f i * f.leadingCoeff ^ (f.natDegree - 1 - i))
@[simp]
theorem integralNormalization_zero : integralNormalization (0 : R[X]) = 0 := by
simp [integralNormalization]
theorem integralNormalization_coeff {f : R[X]} {i : ℕ} :
(integralNormalization f).coeff i =
if f.degree = i then 1 else coeff f i * f.leadingCoeff ^ (f.natDegree - 1 - i) := by
have : f.coeff i = 0 → f.degree ≠ i := fun hc hd => coeff_ne_zero_of_eq_degree hd hc
simp (config := { contextual := true }) [integralNormalization, coeff_monomial, this,
mem_support_iff]
theorem integralNormalization_support {f : R[X]} :
(integralNormalization f).support ⊆ f.support := by
intro
simp (config := { contextual := true }) [integralNormalization, coeff_monomial, mem_support_iff]
theorem integralNormalization_coeff_degree {f : R[X]} {i : ℕ} (hi : f.degree = i) :
(integralNormalization f).coeff i = 1 := by rw [integralNormalization_coeff, if_pos hi]
theorem integralNormalization_coeff_natDegree {f : R[X]} (hf : f ≠ 0) :
(integralNormalization f).coeff (natDegree f) = 1 :=
integralNormalization_coeff_degree (degree_eq_natDegree hf)
theorem integralNormalization_coeff_ne_degree {f : R[X]} {i : ℕ} (hi : f.degree ≠ i) :
coeff (integralNormalization f) i = coeff f i * f.leadingCoeff ^ (f.natDegree - 1 - i) := by
rw [integralNormalization_coeff, if_neg hi]
theorem integralNormalization_coeff_ne_natDegree {f : R[X]} {i : ℕ} (hi : i ≠ natDegree f) :
coeff (integralNormalization f) i = coeff f i * f.leadingCoeff ^ (f.natDegree - 1 - i) :=
integralNormalization_coeff_ne_degree (degree_ne_of_natDegree_ne hi.symm)
theorem monic_integralNormalization {f : R[X]} (hf : f ≠ 0) : Monic (integralNormalization f) :=
monic_of_degree_le f.natDegree
(Finset.sup_le fun i h =>
WithBot.coe_le_coe.2 <| le_natDegree_of_mem_supp i <| integralNormalization_support h)
(integralNormalization_coeff_natDegree hf)
end Semiring
section IsDomain
variable [Ring R] [IsDomain R]
@[simp]
theorem support_integralNormalization {f : R[X]} :
(integralNormalization f).support = f.support := by
by_cases hf : f = 0; · simp [hf]
ext i
refine ⟨fun h => integralNormalization_support h, ?_⟩
simp only [integralNormalization_coeff, mem_support_iff]
intro hfi
split_ifs with hi <;> simp [hf, hfi, hi]
end IsDomain
section IsDomain
variable [CommRing R] [IsDomain R]
variable [CommSemiring S]
theorem integralNormalization_eval₂_eq_zero {p : R[X]} (f : R →+* S) {z : S} (hz : eval₂ f z p = 0)
(inj : ∀ x : R, f x = 0 → x = 0) :
eval₂ f (z * f p.leadingCoeff) (integralNormalization p) = 0 :=
calc
eval₂ f (z * f p.leadingCoeff) (integralNormalization p) =
p.support.attach.sum fun i =>
f (coeff (integralNormalization p) i.1 * p.leadingCoeff ^ i.1) * z ^ i.1 := by
rw [eval₂_eq_sum, sum_def, support_integralNormalization]
simp only [mul_comm z, mul_pow, mul_assoc, RingHom.map_pow, RingHom.map_mul]
rw [← Finset.sum_attach]
_ =
p.support.attach.sum fun i =>
f (coeff p i.1 * p.leadingCoeff ^ (natDegree p - 1)) * z ^ i.1 := by
by_cases hp : p = 0; · simp [hp]
have one_le_deg : 1 ≤ natDegree p :=
Nat.succ_le_of_lt (natDegree_pos_of_eval₂_root hp f hz inj)
congr with i
congr 2
by_cases hi : i.1 = natDegree p
· rw [hi, integralNormalization_coeff_degree, one_mul, leadingCoeff, ← pow_succ',
tsub_add_cancel_of_le one_le_deg]
exact degree_eq_natDegree hp
· have : i.1 ≤ p.natDegree - 1 :=
Nat.le_sub_one_of_lt
(lt_of_le_of_ne (le_natDegree_of_ne_zero (mem_support_iff.mp i.2)) hi)
rw [integralNormalization_coeff_ne_natDegree hi, mul_assoc, ← pow_add,
tsub_add_cancel_of_le this]
_ = f p.leadingCoeff ^ (natDegree p - 1) * eval₂ f z p := by
simp_rw [eval₂_eq_sum, sum_def, fun i => mul_comm (coeff p i), RingHom.map_mul,
RingHom.map_pow, mul_assoc, ← Finset.mul_sum]
congr 1
exact p.support.sum_attach fun i ↦ f (p.coeff i) * z ^ i
_ = 0 := by rw [hz, mul_zero]
theorem integralNormalization_aeval_eq_zero [Algebra R S] {f : R[X]} {z : S} (hz : aeval z f = 0)
(inj : ∀ x : R, algebraMap R S x = 0 → x = 0) :
aeval (z * algebraMap R S f.leadingCoeff) (integralNormalization f) = 0 :=
integralNormalization_eval₂_eq_zero (algebraMap R S) hz inj
end IsDomain
end IntegralNormalization
end Polynomial
|
RingTheory\Polynomial\IrreducibleRing.lean | /-
Copyright (c) 2024 Jz Pan. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jz Pan
-/
import Mathlib.Algebra.Polynomial.RingDivision
import Mathlib.RingTheory.Polynomial.Nilpotent
/-!
# Polynomials over an irreducible ring
This file contains results about the polynomials over an irreducible ring (i.e. a ring with only
one minimal prime ideal, equivalently, whose spectrum is an irreducible topological space).
## Main results
- `Polynomial.Monic.irreducible_of_irreducible_map_of_isPrime_nilradical`: a monic polynomial over
an irreducible ring is irreducible if it is irreducible after mapping into an integral domain.
A generalization to `Polynomial.Monic.irreducible_of_irreducible_map`.
## Tags
polynomial, irreducible ring, nilradical, prime ideal
-/
open scoped Classical Polynomial
open Polynomial
noncomputable section
/-- A polynomial over an irreducible ring `R` is irreducible if it is monic and irreducible after
mapping into an integral domain `S` (https://math.stackexchange.com/a/4843432/235999).
A generalization to `Polynomial.Monic.irreducible_of_irreducible_map`. -/
theorem Polynomial.Monic.irreducible_of_irreducible_map_of_isPrime_nilradical
{R S : Type*} [CommRing R] [(nilradical R).IsPrime] [CommRing S] [IsDomain S]
(φ : R →+* S) (f : R[X]) (hm : f.Monic) (hi : Irreducible (f.map φ)) : Irreducible f := by
let R' := R ⧸ nilradical R
let ψ : R' →+* S := Ideal.Quotient.lift (nilradical R) φ
(haveI := RingHom.ker_isPrime φ; nilradical_le_prime (RingHom.ker φ))
let ι := algebraMap R R'
rw [show φ = ψ.comp ι from rfl, ← map_map] at hi
replace hi := hm.map ι |>.irreducible_of_irreducible_map _ _ hi
refine ⟨fun h ↦ hi.1 <| (mapRingHom ι).isUnit_map h, fun a b h ↦ ?_⟩
wlog hb : IsUnit (b.map ι) generalizing a b
· exact (this b a (mul_comm a b ▸ h)
(hi.2 _ _ (by rw [h, Polynomial.map_mul]) |>.resolve_right hb)).symm
have hn (i : ℕ) (hi : i ≠ 0) : IsNilpotent (b.coeff i) := by
obtain ⟨_, _, h⟩ := Polynomial.isUnit_iff.1 hb
simpa only [coeff_map, coeff_C, hi, ite_false, ← RingHom.mem_ker,
show RingHom.ker ι = nilradical R from Ideal.mk_ker] using congr(coeff $(h.symm) i)
refine .inr <| isUnit_of_coeff_isUnit_isNilpotent (isUnit_of_mul_isUnit_right
(x := a.coeff f.natDegree) <| (IsUnit.neg_iff _).1 ?_) hn
have hc : f.leadingCoeff = _ := congr(coeff $h f.natDegree)
rw [hm, coeff_mul, Finset.Nat.sum_antidiagonal_eq_sum_range_succ fun i j ↦ a.coeff i * b.coeff j,
Finset.sum_range_succ, ← sub_eq_iff_eq_add, Nat.sub_self] at hc
rw [← add_sub_cancel_left 1 (-(_ * _)), ← sub_eq_add_neg, hc]
exact IsNilpotent.isUnit_sub_one <| show _ ∈ nilradical R from sum_mem fun i hi ↦
Ideal.mul_mem_left _ _ <| hn _ <| Nat.sub_ne_zero_of_lt (List.mem_range.1 hi)
|
RingTheory\Polynomial\Nilpotent.lean | /-
Copyright (c) 2023 Oliver Nash. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Emilie Uthaiwat, Oliver Nash
-/
import Mathlib.Algebra.Polynomial.AlgebraMap
import Mathlib.Algebra.Polynomial.Div
import Mathlib.Algebra.Polynomial.Identities
import Mathlib.RingTheory.Ideal.QuotientOperations
import Mathlib.RingTheory.Nilpotent.Lemmas
import Mathlib.RingTheory.Polynomial.Tower
import Mathlib.RingTheory.Nilpotent.Basic
/-!
# Nilpotency in polynomial rings.
This file is a place for results related to nilpotency in (single-variable) polynomial rings.
## Main results:
* `Polynomial.isNilpotent_iff`
* `Polynomial.isUnit_iff_coeff_isUnit_isNilpotent`
-/
namespace Polynomial
variable {R : Type*} {r : R}
section Semiring
variable [Semiring R] {P : R[X]}
lemma isNilpotent_C_mul_pow_X_of_isNilpotent (n : ℕ) (hnil : IsNilpotent r) :
IsNilpotent ((C r) * X ^ n) := by
refine Commute.isNilpotent_mul_left (commute_X_pow _ _).symm ?_
obtain ⟨m, hm⟩ := hnil
refine ⟨m, ?_⟩
rw [← C_pow, hm, C_0]
lemma isNilpotent_pow_X_mul_C_of_isNilpotent (n : ℕ) (hnil : IsNilpotent r) :
IsNilpotent (X ^ n * (C r)) := by
rw [commute_X_pow]
exact isNilpotent_C_mul_pow_X_of_isNilpotent n hnil
@[simp] lemma isNilpotent_monomial_iff {n : ℕ} :
IsNilpotent (monomial (R := R) n r) ↔ IsNilpotent r :=
exists_congr fun k ↦ by simp
@[simp] lemma isNilpotent_C_iff :
IsNilpotent (C r) ↔ IsNilpotent r :=
exists_congr fun k ↦ by simpa only [← C_pow] using C_eq_zero
@[simp] lemma isNilpotent_X_mul_iff :
IsNilpotent (X * P) ↔ IsNilpotent P := by
refine ⟨fun h ↦ ?_, ?_⟩
· rwa [Commute.isNilpotent_mul_right_iff (commute_X P) (by simp)] at h
· rintro ⟨k, hk⟩
exact ⟨k, by simp [(commute_X P).mul_pow, hk]⟩
@[simp] lemma isNilpotent_mul_X_iff :
IsNilpotent (P * X) ↔ IsNilpotent P := by
rw [← commute_X P]
exact isNilpotent_X_mul_iff
end Semiring
section CommRing
variable [CommRing R] {P : R[X]}
protected lemma isNilpotent_iff :
IsNilpotent P ↔ ∀ i, IsNilpotent (coeff P i) := by
refine
⟨P.recOnHorner (by simp) (fun p r hp₀ _ hp hpr i ↦ ?_) (fun p _ hnp hpX i ↦ ?_), fun h ↦ ?_⟩
· rw [← sum_monomial_eq P]
exact isNilpotent_sum (fun i _ ↦ by simpa only [isNilpotent_monomial_iff] using h i)
· have hr : IsNilpotent (C r) := by
obtain ⟨k, hk⟩ := hpr
replace hp : eval 0 p = 0 := by rwa [coeff_zero_eq_aeval_zero] at hp₀
refine isNilpotent_C_iff.mpr ⟨k, ?_⟩
simpa [coeff_zero_eq_aeval_zero, hp] using congr_arg (fun q ↦ coeff q 0) hk
cases' i with i
· simpa [hp₀] using hr
simp only [coeff_add, coeff_C_succ, add_zero]
apply hp
simpa using Commute.isNilpotent_sub (Commute.all _ _) hpr hr
· cases' i with i
· simp
simpa using hnp (isNilpotent_mul_X_iff.mp hpX) i
@[simp] lemma isNilpotent_reflect_iff {P : R[X]} {N : ℕ} (hN : P.natDegree ≤ N) :
IsNilpotent (reflect N P) ↔ IsNilpotent P := by
simp only [Polynomial.isNilpotent_iff, coeff_reverse]
refine ⟨fun h i ↦ ?_, fun h i ↦ ?_⟩ <;> rcases le_or_lt i N with hi | hi
· simpa [tsub_tsub_cancel_of_le hi] using h (N - i)
· simp [coeff_eq_zero_of_natDegree_lt <| lt_of_le_of_lt hN hi]
· simpa [hi, revAt_le] using h (N - i)
· simpa [revAt_eq_self_of_lt hi] using h i
@[simp] lemma isNilpotent_reverse_iff :
IsNilpotent P.reverse ↔ IsNilpotent P :=
isNilpotent_reflect_iff (le_refl _)
/-- Let `P` be a polynomial over `R`. If its constant term is a unit and its other coefficients are
nilpotent, then `P` is a unit.
See also `Polynomial.isUnit_iff_coeff_isUnit_isNilpotent`. -/
theorem isUnit_of_coeff_isUnit_isNilpotent (hunit : IsUnit (P.coeff 0))
(hnil : ∀ i, i ≠ 0 → IsNilpotent (P.coeff i)) : IsUnit P := by
induction' h : P.natDegree using Nat.strong_induction_on with k hind generalizing P
by_cases hdeg : P.natDegree = 0
{ rw [eq_C_of_natDegree_eq_zero hdeg]
exact hunit.map C }
set P₁ := P.eraseLead with hP₁
suffices IsUnit P₁ by
rw [← eraseLead_add_monomial_natDegree_leadingCoeff P, ← C_mul_X_pow_eq_monomial, ← hP₁]
refine IsNilpotent.isUnit_add_left_of_commute ?_ this (Commute.all _ _)
exact isNilpotent_C_mul_pow_X_of_isNilpotent _ (hnil _ hdeg)
have hdeg₂ := lt_of_le_of_lt P.eraseLead_natDegree_le (Nat.sub_lt
(Nat.pos_of_ne_zero hdeg) zero_lt_one)
refine hind P₁.natDegree ?_ ?_ (fun i hi => ?_) rfl
· simp_rw [← h, hdeg₂]
· simp_rw [eraseLead_coeff_of_ne _ (Ne.symm hdeg), hunit]
· by_cases H : i ≤ P₁.natDegree
· simp_rw [eraseLead_coeff_of_ne _ (ne_of_lt (lt_of_le_of_lt H hdeg₂)), hnil i hi]
· simp_rw [coeff_eq_zero_of_natDegree_lt (lt_of_not_ge H), IsNilpotent.zero]
/-- Let `P` be a polynomial over `R`. If `P` is a unit, then all its coefficients are nilpotent,
except its constant term which is a unit.
See also `Polynomial.isUnit_iff_coeff_isUnit_isNilpotent`. -/
theorem coeff_isUnit_isNilpotent_of_isUnit (hunit : IsUnit P) :
IsUnit (P.coeff 0) ∧ (∀ i, i ≠ 0 → IsNilpotent (P.coeff i)) := by
obtain ⟨Q, hQ⟩ := IsUnit.exists_right_inv hunit
constructor
· refine isUnit_of_mul_eq_one _ (Q.coeff 0) ?_
have h := (mul_coeff_zero P Q).symm
rwa [hQ, coeff_one_zero] at h
· intros n hn
rw [nilpotent_iff_mem_prime]
intros I hI
let f := mapRingHom (Ideal.Quotient.mk I)
have hPQ : degree (f P) = 0 ∧ degree (f Q) = 0 := by
rw [← Nat.WithBot.add_eq_zero_iff, ← degree_mul, ← _root_.map_mul, hQ, map_one, degree_one]
have hcoeff : (f P).coeff n = 0 := by
refine coeff_eq_zero_of_degree_lt ?_
rw [hPQ.1]
exact WithBot.coe_pos.2 hn.bot_lt
rw [coe_mapRingHom, coeff_map, ← RingHom.mem_ker, Ideal.mk_ker] at hcoeff
exact hcoeff
/-- Let `P` be a polynomial over `R`. `P` is a unit if and only if all its coefficients are
nilpotent, except its constant term which is a unit.
See also `Polynomial.isUnit_iff'`. -/
theorem isUnit_iff_coeff_isUnit_isNilpotent :
IsUnit P ↔ IsUnit (P.coeff 0) ∧ (∀ i, i ≠ 0 → IsNilpotent (P.coeff i)) :=
⟨coeff_isUnit_isNilpotent_of_isUnit, fun H => isUnit_of_coeff_isUnit_isNilpotent H.1 H.2⟩
@[simp] lemma isUnit_C_add_X_mul_iff :
IsUnit (C r + X * P) ↔ IsUnit r ∧ IsNilpotent P := by
have : ∀ i, coeff (C r + X * P) (i + 1) = coeff P i := by simp
simp_rw [isUnit_iff_coeff_isUnit_isNilpotent, Nat.forall_ne_zero_iff, this]
simp only [coeff_add, coeff_C_zero, mul_coeff_zero, coeff_X_zero, zero_mul, add_zero,
and_congr_right_iff, ← Polynomial.isNilpotent_iff]
lemma isUnit_iff' :
IsUnit P ↔ IsUnit (eval 0 P) ∧ IsNilpotent (P /ₘ X) := by
suffices P = C (eval 0 P) + X * (P /ₘ X) by
conv_lhs => rw [this]; simp
conv_lhs => rw [← modByMonic_add_div P monic_X]
simp [modByMonic_X]
theorem not_isUnit_of_natDegree_pos_of_isReduced [IsReduced R] (p : R[X])
(hpl : 0 < p.natDegree) : ¬ IsUnit p := by
simp only [ne_eq, isNilpotent_iff_eq_zero, not_and, not_forall, exists_prop,
Polynomial.isUnit_iff_coeff_isUnit_isNilpotent]
intro _
refine ⟨p.natDegree, hpl.ne', ?_⟩
contrapose! hpl
simp only [coeff_natDegree, leadingCoeff_eq_zero] at hpl
simp [hpl]
theorem not_isUnit_of_degree_pos_of_isReduced [IsReduced R] (p : R[X])
(hpl : 0 < p.degree) : ¬ IsUnit p :=
not_isUnit_of_natDegree_pos_of_isReduced _ (natDegree_pos_iff_degree_pos.mpr hpl)
end CommRing
section CommAlgebra
variable {R S : Type*} [CommRing R] [CommRing S] [Algebra R S] (P : R[X]) {a b : S}
lemma isNilpotent_aeval_sub_of_isNilpotent_sub (h : IsNilpotent (a - b)) :
IsNilpotent (aeval a P - aeval b P) := by
simp only [← eval_map_algebraMap]
have ⟨c, hc⟩ := evalSubFactor (map (algebraMap R S) P) a b
exact hc ▸ (Commute.all _ _).isNilpotent_mul_right h
variable {P}
lemma isUnit_aeval_of_isUnit_aeval_of_isNilpotent_sub
(hb : IsUnit (aeval b P)) (hab : IsNilpotent (a - b)) :
IsUnit (aeval a P) := by
rw [← add_sub_cancel (aeval b P) (aeval a P)]
refine IsNilpotent.isUnit_add_left_of_commute ?_ hb (Commute.all _ _)
exact isNilpotent_aeval_sub_of_isNilpotent_sub P hab
end CommAlgebra
end Polynomial
|
RingTheory\Polynomial\Opposites.lean | /-
Copyright (c) 2022 Damiano Testa. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Damiano Testa
-/
import Mathlib.Algebra.Polynomial.Degree.Definitions
/-! # Interactions between `R[X]` and `Rᵐᵒᵖ[X]`
This file contains the basic API for "pushing through" the isomorphism
`opRingEquiv : R[X]ᵐᵒᵖ ≃+* Rᵐᵒᵖ[X]`. It allows going back and forth between a polynomial ring
over a semiring and the polynomial ring over the opposite semiring. -/
open Polynomial
open Polynomial MulOpposite
variable {R : Type*} [Semiring R]
noncomputable section
namespace Polynomial
/-- Ring isomorphism between `R[X]ᵐᵒᵖ` and `Rᵐᵒᵖ[X]` sending each coefficient of a polynomial
to the corresponding element of the opposite ring. -/
def opRingEquiv (R : Type*) [Semiring R] : R[X]ᵐᵒᵖ ≃+* Rᵐᵒᵖ[X] :=
((toFinsuppIso R).op.trans AddMonoidAlgebra.opRingEquiv).trans (toFinsuppIso _).symm
/-! Lemmas to get started, using `opRingEquiv R` on the various expressions of
`Finsupp.single`: `monomial`, `C a`, `X`, `C a * X ^ n`. -/
@[simp]
theorem opRingEquiv_op_monomial (n : ℕ) (r : R) :
opRingEquiv R (op (monomial n r : R[X])) = monomial n (op r) := by
simp only [opRingEquiv, RingEquiv.coe_trans, Function.comp_apply,
AddMonoidAlgebra.opRingEquiv_apply, RingEquiv.op_apply_apply, toFinsuppIso_apply, unop_op,
toFinsupp_monomial, Finsupp.mapRange_single, toFinsuppIso_symm_apply, ofFinsupp_single]
@[simp]
theorem opRingEquiv_op_C (a : R) : opRingEquiv R (op (C a)) = C (op a) :=
opRingEquiv_op_monomial 0 a
@[simp]
theorem opRingEquiv_op_X : opRingEquiv R (op (X : R[X])) = X :=
opRingEquiv_op_monomial 1 1
theorem opRingEquiv_op_C_mul_X_pow (r : R) (n : ℕ) :
opRingEquiv R (op (C r * X ^ n : R[X])) = C (op r) * X ^ n := by
simp only [X_pow_mul, op_mul, op_pow, map_mul, map_pow, opRingEquiv_op_X, opRingEquiv_op_C]
/-! Lemmas to get started, using `(opRingEquiv R).symm` on the various expressions of
`Finsupp.single`: `monomial`, `C a`, `X`, `C a * X ^ n`. -/
@[simp]
theorem opRingEquiv_symm_monomial (n : ℕ) (r : Rᵐᵒᵖ) :
(opRingEquiv R).symm (monomial n r) = op (monomial n (unop r)) :=
(opRingEquiv R).injective (by simp)
@[simp]
theorem opRingEquiv_symm_C (a : Rᵐᵒᵖ) : (opRingEquiv R).symm (C a) = op (C (unop a)) :=
opRingEquiv_symm_monomial 0 a
@[simp]
theorem opRingEquiv_symm_X : (opRingEquiv R).symm (X : Rᵐᵒᵖ[X]) = op X :=
opRingEquiv_symm_monomial 1 1
theorem opRingEquiv_symm_C_mul_X_pow (r : Rᵐᵒᵖ) (n : ℕ) :
(opRingEquiv R).symm (C r * X ^ n : Rᵐᵒᵖ[X]) = op (C (unop r) * X ^ n) := by
rw [C_mul_X_pow_eq_monomial, opRingEquiv_symm_monomial, C_mul_X_pow_eq_monomial]
/-! Lemmas about more global properties of polynomials and opposites. -/
@[simp]
theorem coeff_opRingEquiv (p : R[X]ᵐᵒᵖ) (n : ℕ) :
(opRingEquiv R p).coeff n = op ((unop p).coeff n) := by
induction' p with p
cases p
rfl
@[simp]
theorem support_opRingEquiv (p : R[X]ᵐᵒᵖ) : (opRingEquiv R p).support = (unop p).support := by
induction' p with p
cases p
exact Finsupp.support_mapRange_of_injective (map_zero _) _ op_injective
@[simp]
theorem natDegree_opRingEquiv (p : R[X]ᵐᵒᵖ) : (opRingEquiv R p).natDegree = (unop p).natDegree := by
by_cases p0 : p = 0
· simp only [p0, _root_.map_zero, natDegree_zero, unop_zero]
· simp only [p0, natDegree_eq_support_max', Ne, AddEquivClass.map_eq_zero_iff, not_false_iff,
support_opRingEquiv, unop_eq_zero_iff]
@[simp]
theorem leadingCoeff_opRingEquiv (p : R[X]ᵐᵒᵖ) :
(opRingEquiv R p).leadingCoeff = op (unop p).leadingCoeff := by
rw [leadingCoeff, coeff_opRingEquiv, natDegree_opRingEquiv, leadingCoeff]
end Polynomial
|
RingTheory\Polynomial\Pochhammer.lean | /-
Copyright (c) 2020 Scott Morrison. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Scott Morrison
-/
import Mathlib.Algebra.Polynomial.Degree.Definitions
import Mathlib.Algebra.Polynomial.Eval
import Mathlib.Algebra.Polynomial.Monic
import Mathlib.Algebra.Polynomial.RingDivision
import Mathlib.Tactic.Abel
/-!
# The Pochhammer polynomials
We define and prove some basic relations about
`ascPochhammer S n : S[X] := X * (X + 1) * ... * (X + n - 1)`
which is also known as the rising factorial and about
`descPochhammer R n : R[X] := X * (X - 1) * ... * (X - n + 1)`
which is also known as the falling factorial. Versions of this definition
that are focused on `Nat` can be found in `Data.Nat.Factorial` as `Nat.ascFactorial` and
`Nat.descFactorial`.
## Implementation
As with many other families of polynomials, even though the coefficients are always in `ℕ` or `ℤ` ,
we define the polynomial with coefficients in any `[Semiring S]` or `[Ring R]`.
## TODO
There is lots more in this direction:
* q-factorials, q-binomials, q-Pochhammer.
-/
universe u v
open Polynomial
open Polynomial
section Semiring
variable (S : Type u) [Semiring S]
/-- `ascPochhammer S n` is the polynomial `X * (X + 1) * ... * (X + n - 1)`,
with coefficients in the semiring `S`.
-/
noncomputable def ascPochhammer : ℕ → S[X]
| 0 => 1
| n + 1 => X * (ascPochhammer n).comp (X + 1)
@[simp]
theorem ascPochhammer_zero : ascPochhammer S 0 = 1 :=
rfl
@[simp]
theorem ascPochhammer_one : ascPochhammer S 1 = X := by simp [ascPochhammer]
theorem ascPochhammer_succ_left (n : ℕ) :
ascPochhammer S (n + 1) = X * (ascPochhammer S n).comp (X + 1) := by
rw [ascPochhammer]
theorem monic_ascPochhammer (n : ℕ) [Nontrivial S] [NoZeroDivisors S] :
Monic <| ascPochhammer S n := by
induction' n with n hn
· simp
· have : leadingCoeff (X + 1 : S[X]) = 1 := leadingCoeff_X_add_C 1
rw [ascPochhammer_succ_left, Monic.def, leadingCoeff_mul,
leadingCoeff_comp (ne_zero_of_eq_one <| natDegree_X_add_C 1 : natDegree (X + 1) ≠ 0), hn,
monic_X, one_mul, one_mul, this, one_pow]
section
variable {S} {T : Type v} [Semiring T]
@[simp]
theorem ascPochhammer_map (f : S →+* T) (n : ℕ) :
(ascPochhammer S n).map f = ascPochhammer T n := by
induction' n with n ih
· simp
· simp [ih, ascPochhammer_succ_left, map_comp]
theorem ascPochhammer_eval₂ (f : S →+* T) (n : ℕ) (t : T) :
(ascPochhammer T n).eval t = (ascPochhammer S n).eval₂ f t := by
rw [← ascPochhammer_map f]
exact eval_map f t
theorem ascPochhammer_eval_comp {R : Type*} [CommSemiring R] (n : ℕ) (p : R[X]) [Algebra R S]
(x : S) : ((ascPochhammer S n).comp (p.map (algebraMap R S))).eval x =
(ascPochhammer S n).eval (p.eval₂ (algebraMap R S) x) := by
rw [ascPochhammer_eval₂ (algebraMap R S), ← eval₂_comp', ← ascPochhammer_map (algebraMap R S),
← map_comp, eval_map]
end
@[simp, norm_cast]
theorem ascPochhammer_eval_cast (n k : ℕ) :
(((ascPochhammer ℕ n).eval k : ℕ) : S) = ((ascPochhammer S n).eval k : S) := by
rw [← ascPochhammer_map (algebraMap ℕ S), eval_map, ← eq_natCast (algebraMap ℕ S),
eval₂_at_natCast,Nat.cast_id]
theorem ascPochhammer_eval_zero {n : ℕ} : (ascPochhammer S n).eval 0 = if n = 0 then 1 else 0 := by
cases n
· simp
· simp [X_mul, Nat.succ_ne_zero, ascPochhammer_succ_left]
theorem ascPochhammer_zero_eval_zero : (ascPochhammer S 0).eval 0 = 1 := by simp
@[simp]
theorem ascPochhammer_ne_zero_eval_zero {n : ℕ} (h : n ≠ 0) : (ascPochhammer S n).eval 0 = 0 := by
simp [ascPochhammer_eval_zero, h]
theorem ascPochhammer_succ_right (n : ℕ) :
ascPochhammer S (n + 1) = ascPochhammer S n * (X + (n : S[X])) := by
suffices h : ascPochhammer ℕ (n + 1) = ascPochhammer ℕ n * (X + (n : ℕ[X])) by
apply_fun Polynomial.map (algebraMap ℕ S) at h
simpa only [ascPochhammer_map, Polynomial.map_mul, Polynomial.map_add, map_X,
Polynomial.map_natCast] using h
induction' n with n ih
· simp
· conv_lhs =>
rw [ascPochhammer_succ_left, ih, mul_comp, ← mul_assoc, ← ascPochhammer_succ_left, add_comp,
X_comp, natCast_comp, add_assoc, add_comm (1 : ℕ[X]), ← Nat.cast_succ]
theorem ascPochhammer_succ_eval {S : Type*} [Semiring S] (n : ℕ) (k : S) :
(ascPochhammer S (n + 1)).eval k = (ascPochhammer S n).eval k * (k + n) := by
rw [ascPochhammer_succ_right, mul_add, eval_add, eval_mul_X, ← Nat.cast_comm, ← C_eq_natCast,
eval_C_mul, Nat.cast_comm, ← mul_add]
theorem ascPochhammer_succ_comp_X_add_one (n : ℕ) :
(ascPochhammer S (n + 1)).comp (X + 1) =
ascPochhammer S (n + 1) + (n + 1) • (ascPochhammer S n).comp (X + 1) := by
suffices (ascPochhammer ℕ (n + 1)).comp (X + 1) =
ascPochhammer ℕ (n + 1) + (n + 1) * (ascPochhammer ℕ n).comp (X + 1)
by simpa [map_comp] using congr_arg (Polynomial.map (Nat.castRingHom S)) this
nth_rw 2 [ascPochhammer_succ_left]
rw [← add_mul, ascPochhammer_succ_right ℕ n, mul_comp, mul_comm, add_comp, X_comp, natCast_comp,
add_comm, ← add_assoc]
ring
theorem ascPochhammer_mul (n m : ℕ) :
ascPochhammer S n * (ascPochhammer S m).comp (X + (n : S[X])) = ascPochhammer S (n + m) := by
induction' m with m ih
· simp
· rw [ascPochhammer_succ_right, Polynomial.mul_X_add_natCast_comp, ← mul_assoc, ih,
← add_assoc, ascPochhammer_succ_right, Nat.cast_add, add_assoc]
theorem ascPochhammer_nat_eq_ascFactorial (n : ℕ) :
∀ k, (ascPochhammer ℕ k).eval n = n.ascFactorial k
| 0 => by rw [ascPochhammer_zero, eval_one, Nat.ascFactorial_zero]
| t + 1 => by
rw [ascPochhammer_succ_right, eval_mul, ascPochhammer_nat_eq_ascFactorial n t, eval_add, eval_X,
eval_natCast, Nat.cast_id, Nat.ascFactorial_succ, mul_comm]
theorem ascPochhammer_nat_eq_descFactorial (a b : ℕ) :
(ascPochhammer ℕ b).eval a = (a + b - 1).descFactorial b := by
rw [ascPochhammer_nat_eq_ascFactorial, Nat.add_descFactorial_eq_ascFactorial']
@[simp]
theorem ascPochhammer_natDegree (n : ℕ) [NoZeroDivisors S] [Nontrivial S] :
(ascPochhammer S n).natDegree = n := by
induction' n with n hn
· simp
· have : natDegree (X + (n : S[X])) = 1 := natDegree_X_add_C (n : S)
rw [ascPochhammer_succ_right,
natDegree_mul _ (ne_zero_of_natDegree_gt <| this.symm ▸ Nat.zero_lt_one), hn, this]
cases n
· simp
· refine ne_zero_of_natDegree_gt <| hn.symm ▸ Nat.add_one_pos _
end Semiring
section StrictOrderedSemiring
variable {S : Type*} [StrictOrderedSemiring S]
theorem ascPochhammer_pos (n : ℕ) (s : S) (h : 0 < s) : 0 < (ascPochhammer S n).eval s := by
induction' n with n ih
· simp only [Nat.zero_eq, ascPochhammer_zero, eval_one]
exact zero_lt_one
· rw [ascPochhammer_succ_right, mul_add, eval_add, ← Nat.cast_comm, eval_natCast_mul, eval_mul_X,
Nat.cast_comm, ← mul_add]
exact mul_pos ih (lt_of_lt_of_le h (le_add_of_nonneg_right (Nat.cast_nonneg n)))
end StrictOrderedSemiring
section Factorial
open Nat
variable (S : Type*) [Semiring S] (r n : ℕ)
@[simp]
theorem ascPochhammer_eval_one (S : Type*) [Semiring S] (n : ℕ) :
(ascPochhammer S n).eval (1 : S) = (n ! : S) := by
rw_mod_cast [ascPochhammer_nat_eq_ascFactorial, Nat.one_ascFactorial]
theorem factorial_mul_ascPochhammer (S : Type*) [Semiring S] (r n : ℕ) :
(r ! : S) * (ascPochhammer S n).eval (r + 1 : S) = (r + n)! := by
rw_mod_cast [ascPochhammer_nat_eq_ascFactorial, Nat.factorial_mul_ascFactorial]
theorem ascPochhammer_nat_eval_succ (r : ℕ) :
∀ n : ℕ, n * (ascPochhammer ℕ r).eval (n + 1) = (n + r) * (ascPochhammer ℕ r).eval n
| 0 => by
by_cases h : r = 0
· simp only [h, zero_mul, zero_add]
· simp only [ascPochhammer_eval_zero, zero_mul, if_neg h, mul_zero]
| k + 1 => by simp only [ascPochhammer_nat_eq_ascFactorial, Nat.succ_ascFactorial, add_right_comm]
theorem ascPochhammer_eval_succ (r n : ℕ) :
(n : S) * (ascPochhammer S r).eval (n + 1 : S) =
(n + r) * (ascPochhammer S r).eval (n : S) :=
mod_cast congr_arg Nat.cast (ascPochhammer_nat_eval_succ r n)
end Factorial
section Ring
variable (R : Type u) [Ring R]
/-- `descPochhammer R n` is the polynomial `X * (X - 1) * ... * (X - n + 1)`,
with coefficients in the ring `R`.
-/
noncomputable def descPochhammer : ℕ → R[X]
| 0 => 1
| n + 1 => X * (descPochhammer n).comp (X - 1)
@[simp]
theorem descPochhammer_zero : descPochhammer R 0 = 1 :=
rfl
@[simp]
theorem descPochhammer_one : descPochhammer R 1 = X := by simp [descPochhammer]
theorem descPochhammer_succ_left (n : ℕ) :
descPochhammer R (n + 1) = X * (descPochhammer R n).comp (X - 1) := by
rw [descPochhammer]
theorem monic_descPochhammer (n : ℕ) [Nontrivial R] [NoZeroDivisors R] :
Monic <| descPochhammer R n := by
induction' n with n hn
· simp
· have h : leadingCoeff (X - 1 : R[X]) = 1 := leadingCoeff_X_sub_C 1
have : natDegree (X - (1 : R[X])) ≠ 0 := ne_zero_of_eq_one <| natDegree_X_sub_C (1 : R)
rw [descPochhammer_succ_left, Monic.def, leadingCoeff_mul, leadingCoeff_comp this, hn, monic_X,
one_mul, one_mul, h, one_pow]
section
variable {R} {T : Type v} [Ring T]
@[simp]
theorem descPochhammer_map (f : R →+* T) (n : ℕ) :
(descPochhammer R n).map f = descPochhammer T n := by
induction' n with n ih
· simp
· simp [ih, descPochhammer_succ_left, map_comp]
end
@[simp, norm_cast]
theorem descPochhammer_eval_cast (n : ℕ) (k : ℤ) :
(((descPochhammer ℤ n).eval k : ℤ) : R) = ((descPochhammer R n).eval k : R) := by
rw [← descPochhammer_map (algebraMap ℤ R), eval_map, ← eq_intCast (algebraMap ℤ R)]
simp only [algebraMap_int_eq, eq_intCast, eval₂_at_intCast, Nat.cast_id, eq_natCast, Int.cast_id]
theorem descPochhammer_eval_zero {n : ℕ} :
(descPochhammer R n).eval 0 = if n = 0 then 1 else 0 := by
cases n
· simp
· simp [X_mul, Nat.succ_ne_zero, descPochhammer_succ_left]
theorem descPochhammer_zero_eval_zero : (descPochhammer R 0).eval 0 = 1 := by simp
@[simp]
theorem descPochhammer_ne_zero_eval_zero {n : ℕ} (h : n ≠ 0) : (descPochhammer R n).eval 0 = 0 := by
simp [descPochhammer_eval_zero, h]
theorem descPochhammer_succ_right (n : ℕ) :
descPochhammer R (n + 1) = descPochhammer R n * (X - (n : R[X])) := by
suffices h : descPochhammer ℤ (n + 1) = descPochhammer ℤ n * (X - (n : ℤ[X])) by
apply_fun Polynomial.map (algebraMap ℤ R) at h
simpa [descPochhammer_map, Polynomial.map_mul, Polynomial.map_add, map_X,
Polynomial.map_intCast] using h
induction' n with n ih
· simp [descPochhammer]
· conv_lhs =>
rw [descPochhammer_succ_left, ih, mul_comp, ← mul_assoc, ← descPochhammer_succ_left, sub_comp,
X_comp, natCast_comp]
rw [Nat.cast_add, Nat.cast_one, sub_add_eq_sub_sub_swap]
@[simp]
theorem descPochhammer_natDegree (n : ℕ) [NoZeroDivisors R] [Nontrivial R] :
(descPochhammer R n).natDegree = n := by
induction' n with n hn
· simp
· have : natDegree (X - (n : R[X])) = 1 := natDegree_X_sub_C (n : R)
rw [descPochhammer_succ_right,
natDegree_mul _ (ne_zero_of_natDegree_gt <| this.symm ▸ Nat.zero_lt_one), hn, this]
cases n
· simp
· refine ne_zero_of_natDegree_gt <| hn.symm ▸ Nat.add_one_pos _
theorem descPochhammer_succ_eval {S : Type*} [Ring S] (n : ℕ) (k : S) :
(descPochhammer S (n + 1)).eval k = (descPochhammer S n).eval k * (k - n) := by
rw [descPochhammer_succ_right, mul_sub, eval_sub, eval_mul_X, ← Nat.cast_comm, ← C_eq_natCast,
eval_C_mul, Nat.cast_comm, ← mul_sub]
theorem descPochhammer_succ_comp_X_sub_one (n : ℕ) :
(descPochhammer R (n + 1)).comp (X - 1) =
descPochhammer R (n + 1) - (n + (1 : R[X])) • (descPochhammer R n).comp (X - 1) := by
suffices (descPochhammer ℤ (n + 1)).comp (X - 1) =
descPochhammer ℤ (n + 1) - (n + 1) * (descPochhammer ℤ n).comp (X - 1)
by simpa [map_comp] using congr_arg (Polynomial.map (Int.castRingHom R)) this
nth_rw 2 [descPochhammer_succ_left]
rw [← sub_mul, descPochhammer_succ_right ℤ n, mul_comp, mul_comm, sub_comp, X_comp, natCast_comp]
ring
theorem descPochhammer_eq_ascPochhammer (n : ℕ) :
descPochhammer ℤ n = (ascPochhammer ℤ n).comp ((X : ℤ[X]) - n + 1) := by
induction n with
| zero => rw [descPochhammer_zero, ascPochhammer_zero, one_comp]
| succ n ih =>
rw [Nat.cast_succ, sub_add, add_sub_cancel_right, descPochhammer_succ_right,
ascPochhammer_succ_left, ih, X_mul, mul_X_comp, comp_assoc, add_comp, X_comp, one_comp]
theorem descPochhammer_eval_eq_ascPochhammer (r : R) (n : ℕ) :
(descPochhammer R n).eval r = (ascPochhammer R n).eval (r - n + 1) := by
induction n with
| zero => rw [descPochhammer_zero, eval_one, ascPochhammer_zero, eval_one]
| succ n ih =>
rw [Nat.cast_succ, sub_add, add_sub_cancel_right, descPochhammer_succ_eval, ih,
ascPochhammer_succ_left, X_mul, eval_mul_X, show (X + 1 : R[X]) =
(X + 1 : ℕ[X]).map (algebraMap ℕ R) by simp only [Polynomial.map_add, map_X,
Polynomial.map_one], ascPochhammer_eval_comp, eval₂_add, eval₂_X, eval₂_one]
theorem descPochhammer_mul (n m : ℕ) :
descPochhammer R n * (descPochhammer R m).comp (X - (n : R[X])) = descPochhammer R (n + m) := by
induction' m with m ih
· simp
· rw [descPochhammer_succ_right, Polynomial.mul_X_sub_intCast_comp, ← mul_assoc, ih,
← add_assoc, descPochhammer_succ_right, Nat.cast_add, sub_add_eq_sub_sub]
theorem ascPochhammer_eval_neg_eq_descPochhammer (r : R) : ∀ (k : ℕ),
(ascPochhammer R k).eval (-r) = (-1)^k * (descPochhammer R k).eval r
| 0 => by
rw [ascPochhammer_zero, descPochhammer_zero]
simp only [eval_one, pow_zero, mul_one]
| (k+1) => by
rw [ascPochhammer_succ_right, mul_add, eval_add, eval_mul_X, ← Nat.cast_comm, eval_natCast_mul,
Nat.cast_comm, ← mul_add, ascPochhammer_eval_neg_eq_descPochhammer r k, mul_assoc,
descPochhammer_succ_right, mul_sub, eval_sub, eval_mul_X, ← Nat.cast_comm, eval_natCast_mul,
pow_add, pow_one, mul_assoc ((-1)^k) (-1), mul_sub, neg_one_mul, neg_mul_eq_mul_neg,
Nat.cast_comm, sub_eq_add_neg, neg_one_mul, neg_neg, ← mul_add]
theorem descPochhammer_eval_eq_descFactorial (n k : ℕ) :
(descPochhammer R k).eval (n : R) = n.descFactorial k := by
induction k with
| zero => rw [descPochhammer_zero, eval_one, Nat.descFactorial_zero, Nat.cast_one]
| succ k ih =>
rw [descPochhammer_succ_right, Nat.descFactorial_succ, mul_sub, eval_sub, eval_mul_X,
← Nat.cast_comm k, eval_natCast_mul, ← Nat.cast_comm n, ← sub_mul, ih]
by_cases h : n < k
· rw [Nat.descFactorial_eq_zero_iff_lt.mpr h, Nat.cast_zero, mul_zero, mul_zero, Nat.cast_zero]
· rw [Nat.cast_mul, Nat.cast_sub <| not_lt.mp h]
theorem descPochhammer_int_eq_ascFactorial (a b : ℕ) :
(descPochhammer ℤ b).eval (a + b : ℤ) = (a + 1).ascFactorial b := by
rw [← Nat.cast_add, descPochhammer_eval_eq_descFactorial ℤ (a + b) b,
Nat.add_descFactorial_eq_ascFactorial]
end Ring
|
RingTheory\Polynomial\Quotient.lean | /-
Copyright (c) 2019 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, David Kurniadi Angdinata, Devon Tuma, Riccardo Brasca
-/
import Mathlib.Algebra.Polynomial.Div
import Mathlib.RingTheory.Polynomial.Basic
import Mathlib.RingTheory.Ideal.QuotientOperations
/-!
# Quotients of polynomial rings
-/
open Polynomial
namespace Polynomial
variable {R : Type*} [CommRing R]
noncomputable def quotientSpanXSubCAlgEquivAux2 (x : R) :
(R[X] ⧸ (RingHom.ker (aeval x).toRingHom : Ideal R[X])) ≃ₐ[R] R :=
let e := RingHom.quotientKerEquivOfRightInverse (fun x => by
exact eval_C : Function.RightInverse (fun a : R => (C a : R[X])) (@aeval R R _ _ _ x))
{ e with commutes' := fun r => e.apply_symm_apply r }
noncomputable def quotientSpanXSubCAlgEquivAux1 (x : R) :
(R[X] ⧸ Ideal.span {X - C x}) ≃ₐ[R] (R[X] ⧸ (RingHom.ker (aeval x).toRingHom : Ideal R[X])) :=
@Ideal.quotientEquivAlgOfEq R R[X] _ _ _ _ _ (ker_evalRingHom x).symm
-- Porting note: need to split this definition into two sub-definitions to prevent time out
/-- For a commutative ring $R$, evaluating a polynomial at an element $x \in R$ induces an
isomorphism of $R$-algebras $R[X] / \langle X - x \rangle \cong R$. -/
noncomputable def quotientSpanXSubCAlgEquiv (x : R) :
(R[X] ⧸ Ideal.span ({X - C x} : Set R[X])) ≃ₐ[R] R :=
(quotientSpanXSubCAlgEquivAux1 x).trans (quotientSpanXSubCAlgEquivAux2 x)
@[simp]
theorem quotientSpanXSubCAlgEquiv_mk (x : R) (p : R[X]) :
quotientSpanXSubCAlgEquiv x (Ideal.Quotient.mk _ p) = p.eval x :=
rfl
@[simp]
theorem quotientSpanXSubCAlgEquiv_symm_apply (x : R) (y : R) :
(quotientSpanXSubCAlgEquiv x).symm y = algebraMap R _ y :=
rfl
/-- For a commutative ring $R$, evaluating a polynomial at an element $y \in R$ induces an
isomorphism of $R$-algebras $R[X] / \langle x, X - y \rangle \cong R / \langle x \rangle$. -/
noncomputable def quotientSpanCXSubCAlgEquiv (x y : R) :
(R[X] ⧸ (Ideal.span {C x, X - C y} : Ideal R[X])) ≃ₐ[R] R ⧸ (Ideal.span {x} : Ideal R) :=
(Ideal.quotientEquivAlgOfEq R <| by rw [Ideal.span_insert, sup_comm]).trans <|
(DoubleQuot.quotQuotEquivQuotSupₐ R _ _).symm.trans <|
(Ideal.quotientEquivAlg _ _ (quotientSpanXSubCAlgEquiv y) rfl).trans <|
Ideal.quotientEquivAlgOfEq R <| by
simp only [Ideal.map_span, Set.image_singleton]; congr 2; exact eval_C
/-- For a commutative ring $R$, evaluating a polynomial at elements $y(X) \in R[X]$ and $x \in R$
induces an isomorphism of $R$-algebras $R[X, Y] / \langle X - x, Y - y(X) \rangle \cong R$. -/
noncomputable def quotientSpanCXSubCXSubCAlgEquiv {x : R} {y : R[X]} :
@AlgEquiv R (R[X][X] ⧸ (Ideal.span {C (X - C x), X - C y} : Ideal <| R[X][X])) R _ _ _
(Ideal.Quotient.algebra R) _ :=
((quotientSpanCXSubCAlgEquiv (X - C x) y).restrictScalars R).trans <| quotientSpanXSubCAlgEquiv x
lemma modByMonic_eq_zero_iff_quotient_eq_zero (p q : R[X]) (hq : q.Monic) :
p %ₘ q = 0 ↔ (p : R[X] ⧸ Ideal.span {q}) = 0 := by
rw [modByMonic_eq_zero_iff_dvd hq, Ideal.Quotient.eq_zero_iff_dvd]
end Polynomial
namespace Ideal
noncomputable section
open Polynomial
variable {R : Type*} [CommRing R]
theorem quotient_map_C_eq_zero {I : Ideal R} :
∀ a ∈ I, ((Quotient.mk (map (C : R →+* R[X]) I : Ideal R[X])).comp C) a = 0 := by
intro a ha
rw [RingHom.comp_apply, Quotient.eq_zero_iff_mem]
exact mem_map_of_mem _ ha
theorem eval₂_C_mk_eq_zero {I : Ideal R} :
∀ f ∈ (map (C : R →+* R[X]) I : Ideal R[X]), eval₂RingHom (C.comp (Quotient.mk I)) X f = 0 := by
intro a ha
rw [← sum_monomial_eq a]
dsimp
rw [eval₂_sum]
refine Finset.sum_eq_zero fun n _ => ?_
dsimp
rw [eval₂_monomial (C.comp (Quotient.mk I)) X]
refine mul_eq_zero_of_left (Polynomial.ext fun m => ?_) (X ^ n)
erw [coeff_C]
by_cases h : m = 0
· simpa [h] using Quotient.eq_zero_iff_mem.2 ((mem_map_C_iff.1 ha) n)
· simp [h]
/-- If `I` is an ideal of `R`, then the ring polynomials over the quotient ring `I.quotient` is
isomorphic to the quotient of `R[X]` by the ideal `map C I`,
where `map C I` contains exactly the polynomials whose coefficients all lie in `I`. -/
def polynomialQuotientEquivQuotientPolynomial (I : Ideal R) :
(R ⧸ I)[X] ≃+* R[X] ⧸ (map C I : Ideal R[X]) where
toFun :=
eval₂RingHom
(Quotient.lift I ((Quotient.mk (map C I : Ideal R[X])).comp C) quotient_map_C_eq_zero)
(Quotient.mk (map C I : Ideal R[X]) X)
invFun :=
Quotient.lift (map C I : Ideal R[X]) (eval₂RingHom (C.comp (Quotient.mk I)) X)
eval₂_C_mk_eq_zero
map_mul' f g := by simp only [coe_eval₂RingHom, eval₂_mul]
map_add' f g := by simp only [eval₂_add, coe_eval₂RingHom]
left_inv := by
intro f
refine Polynomial.induction_on' f ?_ ?_
· intro p q hp hq
simp only [coe_eval₂RingHom] at hp hq
simp only [coe_eval₂RingHom, hp, hq, RingHom.map_add]
· rintro n ⟨x⟩
simp only [← smul_X_eq_monomial, C_mul', Quotient.lift_mk, Submodule.Quotient.quot_mk_eq_mk,
Quotient.mk_eq_mk, eval₂_X_pow, eval₂_smul, coe_eval₂RingHom, RingHom.map_pow, eval₂_C,
RingHom.coe_comp, RingHom.map_mul, eval₂_X, Function.comp_apply]
right_inv := by
rintro ⟨f⟩
refine Polynomial.induction_on' f ?_ ?_
· -- Porting note: was `simp_intro p q hp hq`
intros p q hp hq
simp only [Submodule.Quotient.quot_mk_eq_mk, Quotient.mk_eq_mk, map_add, Quotient.lift_mk,
coe_eval₂RingHom] at hp hq ⊢
rw [hp, hq]
· intro n a
simp only [← smul_X_eq_monomial, ← C_mul' a (X ^ n), Quotient.lift_mk,
Submodule.Quotient.quot_mk_eq_mk, Quotient.mk_eq_mk, eval₂_X_pow, eval₂_smul,
coe_eval₂RingHom, RingHom.map_pow, eval₂_C, RingHom.coe_comp, RingHom.map_mul, eval₂_X,
Function.comp_apply]
@[simp]
theorem polynomialQuotientEquivQuotientPolynomial_symm_mk (I : Ideal R) (f : R[X]) :
I.polynomialQuotientEquivQuotientPolynomial.symm (Quotient.mk _ f) = f.map (Quotient.mk I) := by
rw [polynomialQuotientEquivQuotientPolynomial, RingEquiv.symm_mk, RingEquiv.coe_mk,
Equiv.coe_fn_mk, Quotient.lift_mk, coe_eval₂RingHom, eval₂_eq_eval_map, ← Polynomial.map_map,
← eval₂_eq_eval_map, Polynomial.eval₂_C_X]
@[simp]
theorem polynomialQuotientEquivQuotientPolynomial_map_mk (I : Ideal R) (f : R[X]) :
I.polynomialQuotientEquivQuotientPolynomial (f.map <| Quotient.mk I) =
Quotient.mk (map C I : Ideal R[X]) f := by
apply (polynomialQuotientEquivQuotientPolynomial I).symm.injective
rw [RingEquiv.symm_apply_apply, polynomialQuotientEquivQuotientPolynomial_symm_mk]
/-- If `P` is a prime ideal of `R`, then `R[x]/(P)` is an integral domain. -/
theorem isDomain_map_C_quotient {P : Ideal R} (_ : IsPrime P) :
IsDomain (R[X] ⧸ (map (C : R →+* R[X]) P : Ideal R[X])) :=
MulEquiv.isDomain (Polynomial (R ⧸ P)) (polynomialQuotientEquivQuotientPolynomial P).symm
/-- Given any ring `R` and an ideal `I` of `R[X]`, we get a map `R → R[x] → R[x]/I`.
If we let `R` be the image of `R` in `R[x]/I` then we also have a map `R[x] → R'[x]`.
In particular we can map `I` across this map, to get `I'` and a new map `R' → R'[x] → R'[x]/I`.
This theorem shows `I'` will not contain any non-zero constant polynomials. -/
theorem eq_zero_of_polynomial_mem_map_range (I : Ideal R[X]) (x : ((Quotient.mk I).comp C).range)
(hx : C x ∈ I.map (Polynomial.mapRingHom ((Quotient.mk I).comp C).rangeRestrict)) : x = 0 := by
let i := ((Quotient.mk I).comp C).rangeRestrict
have hi' : RingHom.ker (Polynomial.mapRingHom i) ≤ I := by
refine fun f hf => polynomial_mem_ideal_of_coeff_mem_ideal I f fun n => ?_
rw [mem_comap, ← Quotient.eq_zero_iff_mem, ← RingHom.comp_apply]
rw [RingHom.mem_ker, coe_mapRingHom] at hf
replace hf := congr_arg (fun f : Polynomial _ => f.coeff n) hf
simp only [coeff_map, coeff_zero] at hf
rwa [Subtype.ext_iff, RingHom.coe_rangeRestrict] at hf
obtain ⟨x, hx'⟩ := x
obtain ⟨y, rfl⟩ := RingHom.mem_range.1 hx'
refine Subtype.eq ?_
simp only [RingHom.comp_apply, Quotient.eq_zero_iff_mem, ZeroMemClass.coe_zero]
suffices C (i y) ∈ I.map (Polynomial.mapRingHom i) by
obtain ⟨f, hf⟩ := mem_image_of_mem_map_of_surjective (Polynomial.mapRingHom i)
(Polynomial.map_surjective _ (RingHom.rangeRestrict_surjective ((Quotient.mk I).comp C))) this
refine sub_add_cancel (C y) f ▸ I.add_mem (hi' ?_ : C y - f ∈ I) hf.1
rw [RingHom.mem_ker, RingHom.map_sub, hf.2, sub_eq_zero, coe_mapRingHom, map_C]
exact hx
end
end Ideal
namespace MvPolynomial
variable {R : Type*} {σ : Type*} [CommRing R] {r : R}
theorem quotient_map_C_eq_zero {I : Ideal R} {i : R} (hi : i ∈ I) :
(Ideal.Quotient.mk (Ideal.map (C : R →+* MvPolynomial σ R) I :
Ideal (MvPolynomial σ R))).comp C i = 0 := by
simp only [Function.comp_apply, RingHom.coe_comp, Ideal.Quotient.eq_zero_iff_mem]
exact Ideal.mem_map_of_mem _ hi
theorem eval₂_C_mk_eq_zero {I : Ideal R} {a : MvPolynomial σ R}
(ha : a ∈ (Ideal.map (C : R →+* MvPolynomial σ R) I : Ideal (MvPolynomial σ R))) :
eval₂Hom (C.comp (Ideal.Quotient.mk I)) X a = 0 := by
rw [as_sum a]
rw [coe_eval₂Hom, eval₂_sum]
refine Finset.sum_eq_zero fun n _ => ?_
simp only [eval₂_monomial, Function.comp_apply, RingHom.coe_comp]
refine mul_eq_zero_of_left ?_ _
suffices coeff n a ∈ I by
rw [← @Ideal.mk_ker R _ I, RingHom.mem_ker] at this
simp only [this, C_0]
exact mem_map_C_iff.1 ha n
lemma quotientEquivQuotientMvPolynomial_rightInverse (I : Ideal R) :
Function.RightInverse
(eval₂ (Ideal.Quotient.lift I
((Ideal.Quotient.mk (Ideal.map C I : Ideal (MvPolynomial σ R))).comp C)
fun i hi => quotient_map_C_eq_zero hi)
fun i => Ideal.Quotient.mk (Ideal.map C I : Ideal (MvPolynomial σ R)) (X i))
(Ideal.Quotient.lift (Ideal.map C I : Ideal (MvPolynomial σ R))
(eval₂Hom (C.comp (Ideal.Quotient.mk I)) X) fun a ha => eval₂_C_mk_eq_zero ha) := by
intro f
apply induction_on f
· intro r
obtain ⟨r, rfl⟩ := Ideal.Quotient.mk_surjective r
rw [eval₂_C, Ideal.Quotient.lift_mk, RingHom.comp_apply, Ideal.Quotient.lift_mk, eval₂Hom_C,
RingHom.comp_apply]
· intros p q hp hq
simp only [RingHom.map_add, MvPolynomial.coe_eval₂Hom, coe_eval₂Hom, MvPolynomial.eval₂_add]
at hp hq ⊢
rw [hp, hq]
· intros p i hp
simp only [coe_eval₂Hom] at hp
simp only [hp, coe_eval₂Hom, Ideal.Quotient.lift_mk, eval₂_mul, RingHom.map_mul, eval₂_X]
lemma quotientEquivQuotientMvPolynomial_leftInverse (I : Ideal R) :
Function.LeftInverse
(eval₂ (Ideal.Quotient.lift I
((Ideal.Quotient.mk (Ideal.map C I : Ideal (MvPolynomial σ R))).comp C)
fun i hi => quotient_map_C_eq_zero hi)
fun i => Ideal.Quotient.mk (Ideal.map C I : Ideal (MvPolynomial σ R)) (X i))
(Ideal.Quotient.lift (Ideal.map C I : Ideal (MvPolynomial σ R))
(eval₂Hom (C.comp (Ideal.Quotient.mk I)) X) fun a ha => eval₂_C_mk_eq_zero ha) := by
intro f
obtain ⟨f, rfl⟩ := Ideal.Quotient.mk_surjective f
apply induction_on f
· intro r
rw [Ideal.Quotient.lift_mk, eval₂Hom_C, RingHom.comp_apply, eval₂_C, Ideal.Quotient.lift_mk,
RingHom.comp_apply]
· intros p q hp hq
erw [Ideal.Quotient.lift_mk] at hp hq ⊢
simp only [Submodule.Quotient.quot_mk_eq_mk, eval₂_add, RingHom.map_add, coe_eval₂Hom,
Ideal.Quotient.lift_mk, Ideal.Quotient.mk_eq_mk] at hp hq ⊢
rw [hp, hq]
· intros p i hp
simp only [Submodule.Quotient.quot_mk_eq_mk, coe_eval₂Hom, Ideal.Quotient.lift_mk,
Ideal.Quotient.mk_eq_mk, eval₂_mul, RingHom.map_mul, eval₂_X] at hp ⊢
simp only [hp]
-- Porting note: this definition was split to avoid timeouts.
/-- If `I` is an ideal of `R`, then the ring `MvPolynomial σ I.quotient` is isomorphic as an
`R`-algebra to the quotient of `MvPolynomial σ R` by the ideal generated by `I`. -/
noncomputable def quotientEquivQuotientMvPolynomial (I : Ideal R) :
MvPolynomial σ (R ⧸ I) ≃ₐ[R] MvPolynomial σ R ⧸ (Ideal.map C I : Ideal (MvPolynomial σ R)) :=
let e : MvPolynomial σ (R ⧸ I) →ₐ[R]
MvPolynomial σ R ⧸ (Ideal.map C I : Ideal (MvPolynomial σ R)) :=
{ eval₂Hom
(Ideal.Quotient.lift I ((Ideal.Quotient.mk (Ideal.map C I : Ideal (MvPolynomial σ R))).comp C)
fun _ hi => quotient_map_C_eq_zero hi)
fun i => Ideal.Quotient.mk (Ideal.map C I : Ideal (MvPolynomial σ R)) (X i) with
commutes' := fun r => eval₂Hom_C _ _ (Ideal.Quotient.mk I r) }
{ e with
invFun := Ideal.Quotient.lift (Ideal.map C I : Ideal (MvPolynomial σ R))
(eval₂Hom (C.comp (Ideal.Quotient.mk I)) X) fun _ ha => eval₂_C_mk_eq_zero ha
left_inv := quotientEquivQuotientMvPolynomial_rightInverse I
right_inv := quotientEquivQuotientMvPolynomial_leftInverse I }
end MvPolynomial
|
RingTheory\Polynomial\RationalRoot.lean | /-
Copyright (c) 2020 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen
-/
import Mathlib.RingTheory.IntegralClosure.IntegrallyClosed
import Mathlib.RingTheory.Localization.NumDen
import Mathlib.RingTheory.Polynomial.ScaleRoots
/-!
# Rational root theorem and integral root theorem
This file contains the rational root theorem and integral root theorem.
The rational root theorem for a unique factorization domain `A`
with localization `S`, states that the roots of `p : A[X]` in `A`'s
field of fractions are of the form `x / y` with `x y : A`, `x ∣ p.coeff 0` and
`y ∣ p.leadingCoeff`.
The corollary is the integral root theorem `isInteger_of_is_root_of_monic`:
if `p` is monic, its roots must be integers.
Finally, we use this to show unique factorization domains are integrally closed.
## References
* https://en.wikipedia.org/wiki/Rational_root_theorem
-/
open scoped Polynomial
section ScaleRoots
variable {A K R S : Type*} [CommRing A] [Field K] [CommRing R] [CommRing S]
variable {M : Submonoid A} [Algebra A S] [IsLocalization M S] [Algebra A K] [IsFractionRing A K]
open Finsupp IsFractionRing IsLocalization Polynomial
theorem scaleRoots_aeval_eq_zero_of_aeval_mk'_eq_zero {p : A[X]} {r : A} {s : M}
(hr : aeval (mk' S r s) p = 0) : aeval (algebraMap A S r) (scaleRoots p s) = 0 := by
convert scaleRoots_eval₂_eq_zero (algebraMap A S) hr
-- Porting note: added
funext
rw [aeval_def, mk'_spec' _ r s]
variable [IsDomain A]
theorem num_isRoot_scaleRoots_of_aeval_eq_zero [UniqueFactorizationMonoid A] {p : A[X]} {x : K}
(hr : aeval x p = 0) : IsRoot (scaleRoots p (den A x)) (num A x) := by
apply isRoot_of_eval₂_map_eq_zero (IsFractionRing.injective A K)
refine scaleRoots_aeval_eq_zero_of_aeval_mk'_eq_zero ?_
rw [mk'_num_den]
exact hr
end ScaleRoots
section RationalRootTheorem
variable {A K : Type*} [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A] [Field K]
variable [Algebra A K] [IsFractionRing A K]
open IsFractionRing IsLocalization Polynomial UniqueFactorizationMonoid
/-- **Rational root theorem** part 1:
if `r : f.codomain` is a root of a polynomial over the ufd `A`,
then the numerator of `r` divides the constant coefficient -/
theorem num_dvd_of_is_root {p : A[X]} {r : K} (hr : aeval r p = 0) : num A r ∣ p.coeff 0 := by
suffices num A r ∣ (scaleRoots p (den A r)).coeff 0 by
simp only [coeff_scaleRoots, tsub_zero] at this
haveI inst := Classical.propDecidable
by_cases hr : num A r = 0
· simp_all [nonZeroDivisors.coe_ne_zero]
· refine dvd_of_dvd_mul_left_of_no_prime_factors hr ?_ this
intro q dvd_num dvd_denom_pow hq
apply hq.not_unit
exact num_den_reduced A r dvd_num (hq.dvd_of_dvd_pow dvd_denom_pow)
convert dvd_term_of_isRoot_of_dvd_terms 0 (num_isRoot_scaleRoots_of_aeval_eq_zero hr) _
· rw [pow_zero, mul_one]
intro j hj
apply dvd_mul_of_dvd_right
convert pow_dvd_pow (num A r) (Nat.succ_le_of_lt (bot_lt_iff_ne_bot.mpr hj))
exact (pow_one _).symm
/-- Rational root theorem part 2:
if `r : f.codomain` is a root of a polynomial over the ufd `A`,
then the denominator of `r` divides the leading coefficient -/
theorem den_dvd_of_is_root {p : A[X]} {r : K} (hr : aeval r p = 0) :
(den A r : A) ∣ p.leadingCoeff := by
suffices (den A r : A) ∣ p.leadingCoeff * num A r ^ p.natDegree by
refine
dvd_of_dvd_mul_left_of_no_prime_factors (mem_nonZeroDivisors_iff_ne_zero.mp (den A r).2) ?_
this
intro q dvd_den dvd_num_pow hq
apply hq.not_unit
exact num_den_reduced A r (hq.dvd_of_dvd_pow dvd_num_pow) dvd_den
rw [← coeff_scaleRoots_natDegree]
apply dvd_term_of_isRoot_of_dvd_terms _ (num_isRoot_scaleRoots_of_aeval_eq_zero hr)
intro j hj
by_cases h : j < p.natDegree
· rw [coeff_scaleRoots]
refine (dvd_mul_of_dvd_right ?_ _).mul_right _
convert pow_dvd_pow (den A r : A) (Nat.succ_le_iff.mpr (lt_tsub_iff_left.mpr _))
· exact (pow_one _).symm
simpa using h
rw [← natDegree_scaleRoots p (den A r)] at *
rw [coeff_eq_zero_of_natDegree_lt (lt_of_le_of_ne (le_of_not_gt h) hj.symm),
zero_mul]
exact dvd_zero _
/-- **Integral root theorem**:
if `r : f.codomain` is a root of a monic polynomial over the ufd `A`,
then `r` is an integer -/
theorem isInteger_of_is_root_of_monic {p : A[X]} (hp : Monic p) {r : K} (hr : aeval r p = 0) :
IsInteger A r :=
isInteger_of_isUnit_den (isUnit_of_dvd_one (hp ▸ den_dvd_of_is_root hr))
theorem exists_integer_of_is_root_of_monic {p : A[X]} (hp : Monic p) {r : K} (hr : aeval r p = 0) :
∃ r' : A, r = algebraMap A K r' ∧ r' ∣ p.coeff 0 := by
/- I tried deducing this from above by unwrapping IsInteger,
but the divisibility condition is annoying -/
obtain ⟨inv, h_inv⟩ := hp ▸ den_dvd_of_is_root hr
use num A r * inv, ?_
· have h : inv ∣ 1 := ⟨den A r, by simpa [mul_comm] using h_inv⟩
simpa using mul_dvd_mul (num_dvd_of_is_root hr) h
· have d_ne_zero : algebraMap A K (den A r) ≠ 0 :=
IsFractionRing.to_map_ne_zero_of_mem_nonZeroDivisors (den A r).prop
nth_rw 1 [← mk'_num_den' A r]
rw [div_eq_iff d_ne_zero, map_mul, mul_assoc, mul_comm ((algebraMap A K) inv),
← map_mul, ← h_inv, map_one, mul_one]
namespace UniqueFactorizationMonoid
theorem integer_of_integral {x : K} : IsIntegral A x → IsInteger A x := fun ⟨_, hp, hx⟩ =>
isInteger_of_is_root_of_monic hp hx
-- See library note [lower instance priority]
instance (priority := 100) instIsIntegrallyClosed : IsIntegrallyClosed A :=
(isIntegrallyClosed_iff (FractionRing A)).mpr fun {_} => integer_of_integral
end UniqueFactorizationMonoid
end RationalRootTheorem
|
RingTheory\Polynomial\ScaleRoots.lean | /-
Copyright (c) 2020 Anne Baanen. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Anne Baanen, Devon Tuma
-/
import Mathlib.Algebra.GroupWithZero.NonZeroDivisors
import Mathlib.Algebra.Polynomial.AlgebraMap
import Mathlib.RingTheory.Coprime.Basic
import Mathlib.Tactic.AdaptationNote
/-!
# Scaling the roots of a polynomial
This file defines `scaleRoots p s` for a polynomial `p` in one variable and a ring element `s` to
be the polynomial with root `r * s` for each root `r` of `p` and proves some basic results about it.
-/
variable {R S A K : Type*}
namespace Polynomial
open Polynomial
section Semiring
variable [Semiring R] [Semiring S]
/-- `scaleRoots p s` is a polynomial with root `r * s` for each root `r` of `p`. -/
noncomputable def scaleRoots (p : R[X]) (s : R) : R[X] :=
∑ i ∈ p.support, monomial i (p.coeff i * s ^ (p.natDegree - i))
@[simp]
theorem coeff_scaleRoots (p : R[X]) (s : R) (i : ℕ) :
(scaleRoots p s).coeff i = coeff p i * s ^ (p.natDegree - i) := by
simp (config := { contextual := true }) [scaleRoots, coeff_monomial]
theorem coeff_scaleRoots_natDegree (p : R[X]) (s : R) :
(scaleRoots p s).coeff p.natDegree = p.leadingCoeff := by
rw [leadingCoeff, coeff_scaleRoots, tsub_self, pow_zero, mul_one]
@[simp]
theorem zero_scaleRoots (s : R) : scaleRoots 0 s = 0 := by
ext
simp
theorem scaleRoots_ne_zero {p : R[X]} (hp : p ≠ 0) (s : R) : scaleRoots p s ≠ 0 := by
intro h
have : p.coeff p.natDegree ≠ 0 := mt leadingCoeff_eq_zero.mp hp
have : (scaleRoots p s).coeff p.natDegree = 0 :=
congr_fun (congr_arg (coeff : R[X] → ℕ → R) h) p.natDegree
rw [coeff_scaleRoots_natDegree] at this
contradiction
theorem support_scaleRoots_le (p : R[X]) (s : R) : (scaleRoots p s).support ≤ p.support := by
intro
simpa using left_ne_zero_of_mul
theorem support_scaleRoots_eq (p : R[X]) {s : R} (hs : s ∈ nonZeroDivisors R) :
(scaleRoots p s).support = p.support :=
le_antisymm (support_scaleRoots_le p s)
(by intro i
simp only [coeff_scaleRoots, Polynomial.mem_support_iff]
intro p_ne_zero ps_zero
have := pow_mem hs (p.natDegree - i) _ ps_zero
contradiction)
@[simp]
theorem degree_scaleRoots (p : R[X]) {s : R} : degree (scaleRoots p s) = degree p := by
haveI := Classical.propDecidable
by_cases hp : p = 0
· rw [hp, zero_scaleRoots]
refine le_antisymm (Finset.sup_mono (support_scaleRoots_le p s)) (degree_le_degree ?_)
rw [coeff_scaleRoots_natDegree]
intro h
have := leadingCoeff_eq_zero.mp h
contradiction
@[simp]
theorem natDegree_scaleRoots (p : R[X]) (s : R) : natDegree (scaleRoots p s) = natDegree p := by
simp only [natDegree, degree_scaleRoots]
theorem monic_scaleRoots_iff {p : R[X]} (s : R) : Monic (scaleRoots p s) ↔ Monic p := by
simp only [Monic, leadingCoeff, natDegree_scaleRoots, coeff_scaleRoots_natDegree]
theorem map_scaleRoots (p : R[X]) (x : R) (f : R →+* S) (h : f p.leadingCoeff ≠ 0) :
(p.scaleRoots x).map f = (p.map f).scaleRoots (f x) := by
ext
simp [Polynomial.natDegree_map_of_leadingCoeff_ne_zero _ h]
@[simp]
lemma scaleRoots_C (r c : R) : (C c).scaleRoots r = C c := by
ext; simp
@[simp]
lemma scaleRoots_one (p : R[X]) :
p.scaleRoots 1 = p := by ext; simp
@[simp]
lemma scaleRoots_zero (p : R[X]) :
p.scaleRoots 0 = p.leadingCoeff • X ^ p.natDegree := by
ext n
simp only [coeff_scaleRoots, ne_eq, tsub_eq_zero_iff_le, not_le, zero_pow_eq, mul_ite,
mul_one, mul_zero, coeff_smul, coeff_X_pow, smul_eq_mul]
split_ifs with h₁ h₂ h₂
· subst h₂; rfl
· exact coeff_eq_zero_of_natDegree_lt (lt_of_le_of_ne h₁ (Ne.symm h₂))
· exact (h₁ h₂.ge).elim
· rfl
@[simp]
lemma one_scaleRoots (r : R) :
(1 : R[X]).scaleRoots r = 1 := by ext; simp
end Semiring
section CommSemiring
variable [Semiring S] [CommSemiring R] [Semiring A] [Field K]
theorem scaleRoots_eval₂_mul_of_commute {p : S[X]} (f : S →+* A) (a : A) (s : S)
(hsa : Commute (f s) a) (hf : ∀ s₁ s₂, Commute (f s₁) (f s₂)) :
eval₂ f (f s * a) (scaleRoots p s) = f s ^ p.natDegree * eval₂ f a p := by
calc
_ = (scaleRoots p s).support.sum fun i =>
f (coeff p i * s ^ (p.natDegree - i)) * (f s * a) ^ i := by
simp [eval₂_eq_sum, sum_def]
_ = p.support.sum fun i => f (coeff p i * s ^ (p.natDegree - i)) * (f s * a) ^ i :=
(Finset.sum_subset (support_scaleRoots_le p s) fun i _hi hi' => by
let this : coeff p i * s ^ (p.natDegree - i) = 0 := by simpa using hi'
simp [this])
_ = p.support.sum fun i : ℕ => f (p.coeff i) * f s ^ (p.natDegree - i + i) * a ^ i :=
(Finset.sum_congr rfl fun i _hi => by
simp_rw [f.map_mul, f.map_pow, pow_add, hsa.mul_pow, mul_assoc])
_ = p.support.sum fun i : ℕ => f s ^ p.natDegree * (f (p.coeff i) * a ^ i) :=
Finset.sum_congr rfl fun i hi => by
rw [mul_assoc, ← map_pow, (hf _ _).left_comm, map_pow, tsub_add_cancel_of_le]
exact le_natDegree_of_ne_zero (Polynomial.mem_support_iff.mp hi)
_ = f s ^ p.natDegree * eval₂ f a p := by simp [← Finset.mul_sum, eval₂_eq_sum, sum_def]
theorem scaleRoots_eval₂_mul {p : S[X]} (f : S →+* R) (r : R) (s : S) :
eval₂ f (f s * r) (scaleRoots p s) = f s ^ p.natDegree * eval₂ f r p :=
scaleRoots_eval₂_mul_of_commute f r s (mul_comm _ _) fun _ _ ↦ mul_comm _ _
theorem scaleRoots_eval₂_eq_zero {p : S[X]} (f : S →+* R) {r : R} {s : S} (hr : eval₂ f r p = 0) :
eval₂ f (f s * r) (scaleRoots p s) = 0 := by rw [scaleRoots_eval₂_mul, hr, mul_zero]
theorem scaleRoots_aeval_eq_zero [Algebra R A] {p : R[X]} {a : A} {r : R} (ha : aeval a p = 0) :
aeval (algebraMap R A r * a) (scaleRoots p r) = 0 := by
rw [aeval_def, scaleRoots_eval₂_mul_of_commute, ← aeval_def, ha, mul_zero]
· apply Algebra.commutes
· intros; rw [Commute, SemiconjBy, ← map_mul, ← map_mul, mul_comm]
theorem scaleRoots_eval₂_eq_zero_of_eval₂_div_eq_zero {p : S[X]} {f : S →+* K}
(hf : Function.Injective f) {r s : S} (hr : eval₂ f (f r / f s) p = 0)
(hs : s ∈ nonZeroDivisors S) : eval₂ f (f r) (scaleRoots p s) = 0 := by
-- The proof works without this option, but *much* slower.
set_option tactic.skipAssignedInstances false in
nontriviality S using Subsingleton.eq_zero
convert @scaleRoots_eval₂_eq_zero _ _ _ _ p f _ s hr
rw [← mul_div_assoc, mul_comm, mul_div_cancel_right₀]
exact map_ne_zero_of_mem_nonZeroDivisors _ hf hs
theorem scaleRoots_aeval_eq_zero_of_aeval_div_eq_zero [Algebra R K]
(inj : Function.Injective (algebraMap R K)) {p : R[X]} {r s : R}
(hr : aeval (algebraMap R K r / algebraMap R K s) p = 0) (hs : s ∈ nonZeroDivisors R) :
aeval (algebraMap R K r) (scaleRoots p s) = 0 :=
scaleRoots_eval₂_eq_zero_of_eval₂_div_eq_zero inj hr hs
@[simp]
lemma scaleRoots_mul (p : R[X]) (r s) :
p.scaleRoots (r * s) = (p.scaleRoots r).scaleRoots s := by
ext; simp [mul_pow, mul_assoc]
/-- Multiplication and `scaleRoots` commute up to a power of `r`. The factor disappears if we
assume that the product of the leading coeffs does not vanish. See `Polynomial.mul_scaleRoots'`. -/
lemma mul_scaleRoots (p q : R[X]) (r : R) :
r ^ (natDegree p + natDegree q - natDegree (p * q)) • (p * q).scaleRoots r =
p.scaleRoots r * q.scaleRoots r := by
ext n; simp only [coeff_scaleRoots, coeff_smul, smul_eq_mul]
trans (∑ x ∈ Finset.antidiagonal n, coeff p x.1 * coeff q x.2) *
r ^ (natDegree p + natDegree q - n)
· rw [← coeff_mul]
cases lt_or_le (natDegree (p * q)) n with
| inl h => simp only [coeff_eq_zero_of_natDegree_lt h, zero_mul, mul_zero]
| inr h =>
rw [mul_comm, mul_assoc, ← pow_add, add_comm, tsub_add_tsub_cancel natDegree_mul_le h]
· rw [coeff_mul, Finset.sum_mul]
apply Finset.sum_congr rfl
simp only [Finset.mem_antidiagonal, coeff_scaleRoots, Prod.forall]
intros a b e
cases lt_or_le (natDegree p) a with
| inl h => simp only [coeff_eq_zero_of_natDegree_lt h, zero_mul, mul_zero]
| inr ha =>
cases lt_or_le (natDegree q) b with
| inl h => simp only [coeff_eq_zero_of_natDegree_lt h, zero_mul, mul_zero]
| inr hb =>
simp only [← e, mul_assoc, mul_comm (r ^ (_ - a)), ← pow_add]
rw [add_comm (_ - _), tsub_add_tsub_comm ha hb]
lemma mul_scaleRoots' (p q : R[X]) (r : R) (h : leadingCoeff p * leadingCoeff q ≠ 0) :
(p * q).scaleRoots r = p.scaleRoots r * q.scaleRoots r := by
rw [← mul_scaleRoots, natDegree_mul' h, tsub_self, pow_zero, one_smul]
lemma mul_scaleRoots_of_noZeroDivisors (p q : R[X]) (r : R) [NoZeroDivisors R] :
(p * q).scaleRoots r = p.scaleRoots r * q.scaleRoots r := by
by_cases hp : p = 0; · simp [hp]
by_cases hq : q = 0; · simp [hq]
apply mul_scaleRoots'
simp only [ne_eq, mul_eq_zero, leadingCoeff_eq_zero, hp, hq, or_self, not_false_eq_true]
lemma add_scaleRoots_of_natDegree_eq (p q : R[X]) (r : R) (h : natDegree p = natDegree q) :
r ^ (natDegree p - natDegree (p + q)) • (p + q).scaleRoots r =
p.scaleRoots r + q.scaleRoots r := by
ext n; simp only [coeff_smul, coeff_scaleRoots, coeff_add, smul_eq_mul,
mul_comm (r ^ _), ← pow_add, ← h, ← add_mul, add_comm (_ - n)]
#adaptation_note /-- v4.7.0-rc1
Previously `mul_assoc` was part of the `simp only` above, and this `rw` was not needed.
but this now causes a max rec depth error. -/
rw [mul_assoc, ← pow_add]
cases lt_or_le (natDegree (p + q)) n with
| inl hn => simp only [← coeff_add, coeff_eq_zero_of_natDegree_lt hn, zero_mul]
| inr hn =>
rw [add_comm (_ - n), tsub_add_tsub_cancel (natDegree_add_le_of_degree_le le_rfl h.ge) hn]
lemma scaleRoots_dvd' (p q : R[X]) {r : R} (hr : IsUnit r)
(hpq : p ∣ q) : p.scaleRoots r ∣ q.scaleRoots r := by
obtain ⟨a, rfl⟩ := hpq
rw [← ((hr.pow (natDegree p + natDegree a - natDegree (p * a))).map
(algebraMap R R[X])).dvd_mul_left, ← Algebra.smul_def, mul_scaleRoots]
exact dvd_mul_right (scaleRoots p r) (scaleRoots a r)
lemma scaleRoots_dvd (p q : R[X]) {r : R} [NoZeroDivisors R] (hpq : p ∣ q) :
p.scaleRoots r ∣ q.scaleRoots r := by
obtain ⟨a, rfl⟩ := hpq
rw [mul_scaleRoots_of_noZeroDivisors]
exact dvd_mul_right (scaleRoots p r) (scaleRoots a r)
alias _root_.Dvd.dvd.scaleRoots := scaleRoots_dvd
lemma scaleRoots_dvd_iff (p q : R[X]) {r : R} (hr : IsUnit r) :
p.scaleRoots r ∣ q.scaleRoots r ↔ p ∣ q := by
refine ⟨?_ ∘ scaleRoots_dvd' _ _ (hr.unit⁻¹).isUnit, scaleRoots_dvd' p q hr⟩
simp [← scaleRoots_mul, scaleRoots_one]
alias _root_.IsUnit.scaleRoots_dvd_iff := scaleRoots_dvd_iff
lemma isCoprime_scaleRoots (p q : R[X]) (r : R) (hr : IsUnit r) (h : IsCoprime p q) :
IsCoprime (p.scaleRoots r) (q.scaleRoots r) := by
obtain ⟨a, b, e⟩ := h
let s : R := ↑hr.unit⁻¹
have : natDegree (a * p) = natDegree (b * q) := by
apply natDegree_eq_of_natDegree_add_eq_zero
rw [e, natDegree_one]
use s ^ natDegree (a * p) • s ^ (natDegree a + natDegree p - natDegree (a * p)) • a.scaleRoots r
use s ^ natDegree (a * p) • s ^ (natDegree b + natDegree q - natDegree (b * q)) • b.scaleRoots r
simp only [s, smul_mul_assoc, ← mul_scaleRoots, smul_smul, Units.smul_def, mul_assoc,
← mul_pow, IsUnit.val_inv_mul, one_pow, mul_one, ← smul_add, one_smul, e, natDegree_one,
one_scaleRoots, ← add_scaleRoots_of_natDegree_eq _ _ _ this, tsub_zero]
alias _root_.IsCoprime.scaleRoots := isCoprime_scaleRoots
end CommSemiring
end Polynomial
|
RingTheory\Polynomial\Selmer.lean | /-
Copyright (c) 2022 Thomas Browning. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Thomas Browning
-/
import Mathlib.Analysis.Complex.Polynomial.UnitTrinomial
import Mathlib.RingTheory.Polynomial.GaussLemma
import Mathlib.Tactic.LinearCombination
/-!
# Irreducibility of Selmer Polynomials
This file proves irreducibility of the Selmer polynomials `X ^ n - X - 1`.
## Main results
- `X_pow_sub_X_sub_one_irreducible`: The Selmer polynomials `X ^ n - X - 1` are irreducible.
TODO: Show that the Selmer polynomials have full Galois group.
-/
namespace Polynomial
open scoped Polynomial
variable {n : ℕ}
theorem X_pow_sub_X_sub_one_irreducible_aux (z : ℂ) : ¬(z ^ n = z + 1 ∧ z ^ n + z ^ 2 = 0) := by
rintro ⟨h1, h2⟩
replace h3 : z ^ 3 = 1 := by
linear_combination (1 - z - z ^ 2 - z ^ n) * h1 + (z ^ n - 2) * h2
have key : z ^ n = 1 ∨ z ^ n = z ∨ z ^ n = z ^ 2 := by
rw [← Nat.mod_add_div n 3, pow_add, pow_mul, h3, one_pow, mul_one]
have : n % 3 < 3 := Nat.mod_lt n zero_lt_three
interval_cases n % 3 <;>
simp only [this, pow_zero, pow_one, eq_self_iff_true, or_true_iff, true_or_iff]
have z_ne_zero : z ≠ 0 := fun h =>
zero_ne_one ((zero_pow three_ne_zero).symm.trans (show (0 : ℂ) ^ 3 = 1 from h ▸ h3))
rcases key with (key | key | key)
· exact z_ne_zero (by rwa [key, self_eq_add_left] at h1)
· exact one_ne_zero (by rwa [key, self_eq_add_right] at h1)
· exact z_ne_zero (pow_eq_zero (by rwa [key, add_self_eq_zero] at h2))
theorem X_pow_sub_X_sub_one_irreducible (hn1 : n ≠ 1) : Irreducible (X ^ n - X - 1 : ℤ[X]) := by
by_cases hn0 : n = 0
· rw [hn0, pow_zero, sub_sub, add_comm, ← sub_sub, sub_self, zero_sub]
exact Associated.irreducible ⟨-1, mul_neg_one X⟩ irreducible_X
have hn : 1 < n := Nat.one_lt_iff_ne_zero_and_ne_one.mpr ⟨hn0, hn1⟩
have hp : (X ^ n - X - 1 : ℤ[X]) = trinomial 0 1 n (-1) (-1) 1 := by
simp only [trinomial, C_neg, C_1]; ring
rw [hp]
apply IsUnitTrinomial.irreducible_of_coprime' ⟨0, 1, n, zero_lt_one, hn, -1, -1, 1, rfl⟩
rintro z ⟨h1, h2⟩
apply X_pow_sub_X_sub_one_irreducible_aux (n := n) z
rw [trinomial_mirror zero_lt_one hn (-1 : ℤˣ).ne_zero (1 : ℤˣ).ne_zero] at h2
simp_rw [trinomial, aeval_add, aeval_mul, aeval_X_pow, aeval_C,
Units.val_neg, Units.val_one, map_neg, map_one] at h1 h2
replace h1 : z ^ n = z + 1 := by linear_combination h1
replace h2 := mul_eq_zero_of_left h2 z
rw [add_mul, add_mul, add_zero, mul_assoc (-1 : ℂ), ← pow_succ, Nat.sub_add_cancel hn.le] at h2
rw [h1] at h2 ⊢
exact ⟨rfl, by linear_combination -h2⟩
theorem X_pow_sub_X_sub_one_irreducible_rat (hn1 : n ≠ 1) : Irreducible (X ^ n - X - 1 : ℚ[X]) := by
by_cases hn0 : n = 0
· rw [hn0, pow_zero, sub_sub, add_comm, ← sub_sub, sub_self, zero_sub]
exact Associated.irreducible ⟨-1, mul_neg_one X⟩ irreducible_X
have hp : (X ^ n - X - 1 : ℤ[X]) = trinomial 0 1 n (-1) (-1) 1 := by
simp only [trinomial, C_neg, C_1]; ring
have hn : 1 < n := Nat.one_lt_iff_ne_zero_and_ne_one.mpr ⟨hn0, hn1⟩
have h := (IsPrimitive.Int.irreducible_iff_irreducible_map_cast ?_).mp
(X_pow_sub_X_sub_one_irreducible hn1)
· rwa [Polynomial.map_sub, Polynomial.map_sub, Polynomial.map_pow, Polynomial.map_one,
Polynomial.map_X] at h
· exact hp.symm ▸ (trinomial_monic zero_lt_one hn).isPrimitive
end Polynomial
|
RingTheory\Polynomial\SeparableDegree.lean | /-
Copyright (c) 2021 Jakob Scholbach. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jakob Scholbach
-/
import Mathlib.Algebra.Algebra.Defs
import Mathlib.Algebra.CharP.ExpChar
import Mathlib.FieldTheory.Separable
/-!
# Separable degree
This file contains basics about the separable degree of a polynomial.
## Main results
- `IsSeparableContraction`: is the condition that, for `g` a separable polynomial, we have that
`g(x^(q^m)) = f(x)` for some `m : ℕ`.
- `HasSeparableContraction`: the condition of having a separable contraction
- `HasSeparableContraction.degree`: the separable degree, defined as the degree of some
separable contraction
- `Irreducible.hasSeparableContraction`: any irreducible polynomial can be contracted
to a separable polynomial
- `HasSeparableContraction.dvd_degree'`: the degree of a separable contraction divides the degree,
in function of the exponential characteristic of the field
- `HasSeparableContraction.dvd_degree` and `HasSeparableContraction.eq_degree` specialize the
statement of `separable_degree_dvd_degree`
- `IsSeparableContraction.degree_eq`: the separable degree is well-defined, implemented as the
statement that the degree of any separable contraction equals `HasSeparableContraction.degree`
## Tags
separable degree, degree, polynomial
-/
noncomputable section
namespace Polynomial
open scoped Classical
open Polynomial
section CommSemiring
variable {F : Type*} [CommSemiring F] (q : ℕ)
/-- A separable contraction of a polynomial `f` is a separable polynomial `g` such that
`g(x^(q^m)) = f(x)` for some `m : ℕ`. -/
def IsSeparableContraction (f : F[X]) (g : F[X]) : Prop :=
g.Separable ∧ ∃ m : ℕ, expand F (q ^ m) g = f
/-- The condition of having a separable contraction. -/
def HasSeparableContraction (f : F[X]) : Prop :=
∃ g : F[X], IsSeparableContraction q f g
variable {q} {f : F[X]} (hf : HasSeparableContraction q f)
/-- A choice of a separable contraction. -/
def HasSeparableContraction.contraction : F[X] :=
Classical.choose hf
/-- The separable degree of a polynomial is the degree of a given separable contraction. -/
def HasSeparableContraction.degree : ℕ :=
hf.contraction.natDegree
/-- The `HasSeparableContraction.contraction` is indeed a separable contraction. -/
theorem HasSeparableContraction.isSeparableContraction :
IsSeparableContraction q f hf.contraction := Classical.choose_spec hf
/-- The separable degree divides the degree, in function of the exponential characteristic of F. -/
theorem IsSeparableContraction.dvd_degree' {g} (hf : IsSeparableContraction q f g) :
∃ m : ℕ, g.natDegree * q ^ m = f.natDegree := by
obtain ⟨m, rfl⟩ := hf.2
use m
rw [natDegree_expand]
theorem HasSeparableContraction.dvd_degree' : ∃ m : ℕ, hf.degree * q ^ m = f.natDegree :=
(Classical.choose_spec hf).dvd_degree' hf
/-- The separable degree divides the degree. -/
theorem HasSeparableContraction.dvd_degree : hf.degree ∣ f.natDegree :=
let ⟨a, ha⟩ := hf.dvd_degree'
Dvd.intro (q ^ a) ha
/-- In exponential characteristic one, the separable degree equals the degree. -/
theorem HasSeparableContraction.eq_degree {f : F[X]} (hf : HasSeparableContraction 1 f) :
hf.degree = f.natDegree := by
let ⟨a, ha⟩ := hf.dvd_degree'
rw [← ha, one_pow a, mul_one]
end CommSemiring
section Field
variable {F : Type*} [Field F]
variable (q : ℕ) {f : F[X]} (hf : HasSeparableContraction q f)
/-- Every irreducible polynomial can be contracted to a separable polynomial.
https://stacks.math.columbia.edu/tag/09H0 -/
theorem _root_.Irreducible.hasSeparableContraction (q : ℕ) [hF : ExpChar F q] {f : F[X]}
(irred : Irreducible f) : HasSeparableContraction q f := by
cases hF
· exact ⟨f, irred.separable, ⟨0, by rw [pow_zero, expand_one]⟩⟩
· rcases exists_separable_of_irreducible q irred ‹q.Prime›.ne_zero with ⟨n, g, hgs, hge⟩
exact ⟨g, hgs, n, hge⟩
/-- If two expansions (along the positive characteristic) of two separable polynomials `g` and `g'`
agree, then they have the same degree. -/
theorem contraction_degree_eq_or_insep [hq : NeZero q] [CharP F q] (g g' : F[X]) (m m' : ℕ)
(h_expand : expand F (q ^ m) g = expand F (q ^ m') g') (hg : g.Separable) (hg' : g'.Separable) :
g.natDegree = g'.natDegree := by
wlog hm : m ≤ m'
· exact (this q hf g' g m' m h_expand.symm hg' hg (le_of_not_le hm)).symm
obtain ⟨s, rfl⟩ := exists_add_of_le hm
rw [pow_add, expand_mul, expand_inj (pow_pos (NeZero.pos q) m)] at h_expand
subst h_expand
rcases isUnit_or_eq_zero_of_separable_expand q s (NeZero.pos q) hg with (h | rfl)
· rw [natDegree_expand, natDegree_eq_zero_of_isUnit h, zero_mul]
· rw [natDegree_expand, pow_zero, mul_one]
/-- The separable degree equals the degree of any separable contraction, i.e., it is unique. -/
theorem IsSeparableContraction.degree_eq [hF : ExpChar F q] (g : F[X])
(hg : IsSeparableContraction q f g) : g.natDegree = hf.degree := by
cases hF
· rcases hg with ⟨_, m, hm⟩
rw [one_pow, expand_one] at hm
rw [hf.eq_degree, hm]
· rcases hg with ⟨hg, m, hm⟩
let g' := Classical.choose hf
obtain ⟨hg', m', hm'⟩ := Classical.choose_spec hf
haveI : Fact q.Prime := ⟨by assumption⟩
refine contraction_degree_eq_or_insep q g g' m m' ?_ hg hg'
rw [hm, hm']
end Field
end Polynomial
|
RingTheory\Polynomial\Tower.lean | /-
Copyright (c) 2020 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau, Yuyang Zhao
-/
import Mathlib.Algebra.Algebra.Tower
import Mathlib.Algebra.Polynomial.AlgebraMap
/-!
# Algebra towers for polynomial
This file proves some basic results about the algebra tower structure for the type `R[X]`.
This structure itself is provided elsewhere as `Polynomial.isScalarTower`
When you update this file, you can also try to make a corresponding update in
`RingTheory.MvPolynomial.Tower`.
-/
open Polynomial
variable (R A B : Type*)
namespace Polynomial
section Semiring
variable [CommSemiring R] [CommSemiring A] [Semiring B]
variable [Algebra R A] [Algebra A B] [Algebra R B]
variable [IsScalarTower R A B]
variable {R B}
@[simp]
theorem aeval_map_algebraMap (x : B) (p : R[X]) : aeval x (map (algebraMap R A) p) = aeval x p := by
rw [aeval_def, aeval_def, eval₂_map, IsScalarTower.algebraMap_eq R A B]
@[simp]
lemma eval_map_algebraMap (P : R[X]) (a : A) :
(map (algebraMap R A) P).eval a = aeval a P := by
rw [← aeval_map_algebraMap (A := A), coe_aeval_eq_eval]
end Semiring
section CommSemiring
variable [CommSemiring R] [CommSemiring A] [Semiring B]
variable [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B]
variable {R A}
theorem aeval_algebraMap_apply (x : A) (p : R[X]) :
aeval (algebraMap A B x) p = algebraMap A B (aeval x p) := by
rw [aeval_def, aeval_def, hom_eval₂, ← IsScalarTower.algebraMap_eq]
@[simp]
theorem aeval_algebraMap_eq_zero_iff [NoZeroSMulDivisors A B] [Nontrivial B] (x : A) (p : R[X]) :
aeval (algebraMap A B x) p = 0 ↔ aeval x p = 0 := by
rw [aeval_algebraMap_apply, Algebra.algebraMap_eq_smul_one, smul_eq_zero,
iff_false_intro (one_ne_zero' B), or_false_iff]
variable {B}
theorem aeval_algebraMap_eq_zero_iff_of_injective {x : A} {p : R[X]}
(h : Function.Injective (algebraMap A B)) : aeval (algebraMap A B x) p = 0 ↔ aeval x p = 0 := by
rw [aeval_algebraMap_apply, ← (algebraMap A B).map_zero, h.eq_iff]
end CommSemiring
end Polynomial
namespace Subalgebra
open Polynomial
section CommSemiring
variable {R A} [CommSemiring R] [CommSemiring A] [Algebra R A]
@[simp]
theorem aeval_coe (S : Subalgebra R A) (x : S) (p : R[X]) : aeval (x : A) p = aeval x p :=
aeval_algebraMap_apply A x p
end CommSemiring
end Subalgebra
|
RingTheory\Polynomial\Vieta.lean | /-
Copyright (c) 2020 Hanting Zhang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Hanting Zhang
-/
import Mathlib.Algebra.Polynomial.Splits
import Mathlib.RingTheory.MvPolynomial.Symmetric
/-!
# Vieta's Formula
The main result is `Multiset.prod_X_add_C_eq_sum_esymm`, which shows that the product of
linear terms `X + λ` with `λ` in a `Multiset s` is equal to a linear combination of the
symmetric functions `esymm s`.
From this, we deduce `MvPolynomial.prod_X_add_C_eq_sum_esymm` which is the equivalent formula
for the product of linear terms `X + X i` with `i` in a `Fintype σ` as a linear combination
of the symmetric polynomials `esymm σ R j`.
For `R` be an integral domain (so that `p.roots` is defined for any `p : R[X]` as a multiset),
we derive `Polynomial.coeff_eq_esymm_roots_of_card`, the relationship between the coefficients and
the roots of `p` for a polynomial `p` that splits (i.e. having as many roots as its degree).
-/
open Polynomial
namespace Multiset
open Polynomial
section Semiring
variable {R : Type*} [CommSemiring R]
/-- A sum version of **Vieta's formula** for `Multiset`: the product of the linear terms `X + λ`
where `λ` runs through a multiset `s` is equal to a linear combination of the symmetric functions
`esymm s` of the `λ`'s . -/
theorem prod_X_add_C_eq_sum_esymm (s : Multiset R) :
(s.map fun r => X + C r).prod =
∑ j ∈ Finset.range (Multiset.card s + 1), (C (s.esymm j) * X ^ (Multiset.card s - j)) := by
classical
rw [prod_map_add, antidiagonal_eq_map_powerset, map_map, ← bind_powerset_len,
map_bind, sum_bind, Finset.sum_eq_multiset_sum, Finset.range_val, map_congr (Eq.refl _)]
intro _ _
rw [esymm, ← sum_hom', ← sum_map_mul_right, map_congr (Eq.refl _)]
intro s ht
rw [mem_powersetCard] at ht
dsimp
rw [prod_hom' s (Polynomial.C : R →+* R[X])]
simp [ht, map_const, prod_replicate, prod_hom', map_id', card_sub]
/-- Vieta's formula for the coefficients of the product of linear terms `X + λ` where `λ` runs
through a multiset `s` : the `k`th coefficient is the symmetric function `esymm (card s - k) s`. -/
theorem prod_X_add_C_coeff (s : Multiset R) {k : ℕ} (h : k ≤ Multiset.card s) :
(s.map fun r => X + C r).prod.coeff k = s.esymm (Multiset.card s - k) := by
convert Polynomial.ext_iff.mp (prod_X_add_C_eq_sum_esymm s) k using 1
simp_rw [finset_sum_coeff, coeff_C_mul_X_pow]
rw [Finset.sum_eq_single_of_mem (Multiset.card s - k) _]
· rw [if_pos (Nat.sub_sub_self h).symm]
· intro j hj1 hj2
suffices k ≠ card s - j by rw [if_neg this]
intro hn
rw [hn, Nat.sub_sub_self (Nat.lt_succ_iff.mp (Finset.mem_range.mp hj1))] at hj2
exact Ne.irrefl hj2
· rw [Finset.mem_range]
exact Nat.lt_succ_of_le (Nat.sub_le (Multiset.card s) k)
theorem prod_X_add_C_coeff' {σ} (s : Multiset σ) (r : σ → R) {k : ℕ} (h : k ≤ Multiset.card s) :
(s.map fun i => X + C (r i)).prod.coeff k = (s.map r).esymm (Multiset.card s - k) := by
erw [← map_map (fun r => X + C r) r, prod_X_add_C_coeff] <;> rw [s.card_map r]; assumption
theorem _root_.Finset.prod_X_add_C_coeff {σ} (s : Finset σ) (r : σ → R) {k : ℕ} (h : k ≤ s.card) :
(∏ i ∈ s, (X + C (r i))).coeff k = ∑ t ∈ s.powersetCard (s.card - k), ∏ i ∈ t, r i := by
rw [Finset.prod, prod_X_add_C_coeff' _ r h, Finset.esymm_map_val]
rfl
end Semiring
section Ring
variable {R : Type*} [CommRing R]
theorem esymm_neg (s : Multiset R) (k : ℕ) : (map Neg.neg s).esymm k = (-1) ^ k * esymm s k := by
rw [esymm, esymm, ← Multiset.sum_map_mul_left, Multiset.powersetCard_map, Multiset.map_map,
map_congr rfl]
intro x hx
rw [(mem_powersetCard.mp hx).right.symm, ← prod_replicate, ← Multiset.map_const]
nth_rw 3 [← map_id' x]
rw [← prod_map_mul, map_congr rfl, Function.comp_apply]
exact fun z _ => neg_one_mul z
theorem prod_X_sub_X_eq_sum_esymm (s : Multiset R) :
(s.map fun t => X - C t).prod =
∑ j ∈ Finset.range (Multiset.card s + 1),
(-1) ^ j * (C (s.esymm j) * X ^ (Multiset.card s - j)) := by
conv_lhs =>
congr
congr
ext x
rw [sub_eq_add_neg]
rw [← map_neg C x]
convert prod_X_add_C_eq_sum_esymm (map (fun t => -t) s) using 1
· rw [map_map]; rfl
· simp only [esymm_neg, card_map, mul_assoc, map_mul, map_pow, map_neg, map_one]
theorem prod_X_sub_C_coeff (s : Multiset R) {k : ℕ} (h : k ≤ Multiset.card s) :
(s.map fun t => X - C t).prod.coeff k =
(-1) ^ (Multiset.card s - k) * s.esymm (Multiset.card s - k) := by
conv_lhs =>
congr
congr
congr
ext x
rw [sub_eq_add_neg]
rw [← map_neg C x]
convert prod_X_add_C_coeff (map (fun t => -t) s) _ using 1
· rw [map_map]; rfl
· rw [esymm_neg, card_map]
· rwa [card_map]
/-- Vieta's formula for the coefficients and the roots of a polynomial over an integral domain
with as many roots as its degree. -/
theorem _root_.Polynomial.coeff_eq_esymm_roots_of_card [IsDomain R] {p : R[X]}
(hroots : Multiset.card p.roots = p.natDegree) {k : ℕ} (h : k ≤ p.natDegree) :
p.coeff k = p.leadingCoeff * (-1) ^ (p.natDegree - k) * p.roots.esymm (p.natDegree - k) := by
conv_lhs => rw [← C_leadingCoeff_mul_prod_multiset_X_sub_C hroots]
rw [coeff_C_mul, mul_assoc]; congr
have : k ≤ card (roots p) := by rw [hroots]; exact h
convert p.roots.prod_X_sub_C_coeff this using 3 <;> rw [hroots]
/-- Vieta's formula for split polynomials over a field. -/
theorem _root_.Polynomial.coeff_eq_esymm_roots_of_splits {F} [Field F] {p : F[X]}
(hsplit : p.Splits (RingHom.id F)) {k : ℕ} (h : k ≤ p.natDegree) :
p.coeff k = p.leadingCoeff * (-1) ^ (p.natDegree - k) * p.roots.esymm (p.natDegree - k) :=
Polynomial.coeff_eq_esymm_roots_of_card (splits_iff_card_roots.1 hsplit) h
end Ring
end Multiset
section MvPolynomial
open Finset Polynomial Fintype
variable (R σ : Type*) [CommSemiring R] [Fintype σ]
/-- A sum version of Vieta's formula for `MvPolynomial`: viewing `X i` as variables,
the product of linear terms `λ + X i` is equal to a linear combination of
the symmetric polynomials `esymm σ R j`. -/
theorem MvPolynomial.prod_C_add_X_eq_sum_esymm :
(∏ i : σ, (Polynomial.X + Polynomial.C (MvPolynomial.X i))) =
∑ j ∈ range (card σ + 1), Polynomial.C
(MvPolynomial.esymm σ R j) * Polynomial.X ^ (card σ - j) := by
let s := Finset.univ.val.map fun i : σ => (MvPolynomial.X i : MvPolynomial σ R)
have : Fintype.card σ = Multiset.card s := by
rw [Multiset.card_map, ← Finset.card_univ, Finset.card_def]
simp_rw [this, MvPolynomial.esymm_eq_multiset_esymm σ R, Finset.prod_eq_multiset_prod]
convert Multiset.prod_X_add_C_eq_sum_esymm s
simp_rw [s, Multiset.map_map, Function.comp_apply]
theorem MvPolynomial.prod_X_add_C_coeff (k : ℕ) (h : k ≤ card σ) :
(∏ i : σ, (Polynomial.X + Polynomial.C (MvPolynomial.X i)) : Polynomial _).coeff k =
MvPolynomial.esymm σ R (card σ - k) := by
let s := Finset.univ.val.map fun i => (MvPolynomial.X i : MvPolynomial σ R)
have : Fintype.card σ = Multiset.card s := by
rw [Multiset.card_map, ← Finset.card_univ, Finset.card_def]
rw [this] at h ⊢
rw [MvPolynomial.esymm_eq_multiset_esymm σ R, Finset.prod_eq_multiset_prod]
convert Multiset.prod_X_add_C_coeff s h
dsimp
simp_rw [s, Multiset.map_map, Function.comp_apply]
end MvPolynomial
|
RingTheory\Polynomial\Wronskian.lean | /-
Copyright (c) 2024 Jineon Back and Seewoo Lee. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jineon Baek, Seewoo Lee
-/
import Mathlib.Algebra.Polynomial.AlgebraMap
import Mathlib.Algebra.Polynomial.Derivative
import Mathlib.LinearAlgebra.SesquilinearForm
/-!
# Wronskian of a pair of polynomial
This file defines Wronskian of a pair of polynomials, which is `W(a, b) = ab' - a'b`.
We also prove basic properties of it.
## Main declarations
- `Polynomial.wronskian_eq_of_sum_zero`: We have `W(a, b) = W(b, c)` when `a + b + c = 0`.
- `Polynomial.degree_wronskian_lt_add`: Degree of Wronskian `W(a, b)` is strictly smaller than
the sum of degrees of `a` and `b`
- `Polynomial.natDegree_wronskian_lt_add`: `natDegree` version of the above theorem.
We need to assume that the Wronskian is nonzero. (Otherwise, `a = b = 1` gives a counterexample.)
## TODO
- Define Wronskian for n-tuple of polynomials, not necessarily two.
-/
noncomputable section
open scoped Polynomial
namespace Polynomial
variable {R : Type*} [CommRing R]
/-- Wronskian of a pair of polynomials, `W(a, b) = ab' - a'b`. -/
def wronskian (a b : R[X]) : R[X] :=
a * (derivative b) - (derivative a) * b
variable (R) in
/-- `Polynomial.wronskian` as a bilinear map. -/
def wronskianBilin : R[X] →ₗ[R] R[X] →ₗ[R] R[X] :=
(LinearMap.mul R R[X]).compl₂ derivative - (LinearMap.mul R R[X]).comp derivative
@[simp]
theorem wronskianBilin_apply (a b : R[X]) : wronskianBilin R a b = wronskian a b := rfl
@[simp]
theorem wronskian_zero_left (a : R[X]) : wronskian 0 a = 0 := by
rw [← wronskianBilin_apply 0 a, map_zero]; rfl
@[simp]
theorem wronskian_zero_right (a : R[X]) : wronskian a 0 = 0 := (wronskianBilin R a).map_zero
theorem wronskian_neg_left (a b : R[X]) : wronskian (-a) b = -wronskian a b :=
LinearMap.map_neg₂ (wronskianBilin R) a b
theorem wronskian_neg_right (a b : R[X]) : wronskian a (-b) = -wronskian a b :=
(wronskianBilin R a).map_neg b
theorem wronskian_add_right (a b c : R[X]) : wronskian a (b + c) = wronskian a b + wronskian a c :=
(wronskianBilin R a).map_add b c
theorem wronskian_add_left (a b c : R[X]) : wronskian (a + b) c = wronskian a c + wronskian b c :=
(wronskianBilin R).map_add₂ a b c
theorem wronskian_self_eq_zero (a : R[X]) : wronskian a a = 0 := by
rw [wronskian, mul_comm, sub_self]
theorem isAlt_wronskianBilin : (wronskianBilin R).IsAlt := wronskian_self_eq_zero
theorem wronskian_neg_eq (a b : R[X]) : -wronskian a b = wronskian b a :=
LinearMap.IsAlt.neg isAlt_wronskianBilin a b
theorem wronskian_eq_of_sum_zero {a b c : R[X]} (hAdd : a + b + c = 0) :
wronskian a b = wronskian b c := isAlt_wronskianBilin.eq_of_add_add_eq_zero hAdd
/-- Degree of `W(a,b)` is strictly less than the sum of degrees of `a` and `b` (both nonzero). -/
theorem degree_wronskian_lt_add {a b : R[X]} (ha : a ≠ 0) (hb : b ≠ 0) :
(wronskian a b).degree < a.degree + b.degree := by
calc
(wronskian a b).degree ≤ max (a * derivative b).degree (derivative a * b).degree :=
Polynomial.degree_sub_le _ _
_ < a.degree + b.degree := by
rw [max_lt_iff]
constructor
case left =>
apply lt_of_le_of_lt
· exact degree_mul_le a (derivative b)
· rw [← Polynomial.degree_ne_bot] at ha
rw [WithBot.add_lt_add_iff_left ha]
exact Polynomial.degree_derivative_lt hb
case right =>
apply lt_of_le_of_lt
· exact degree_mul_le (derivative a) b
· rw [← Polynomial.degree_ne_bot] at hb
rw [WithBot.add_lt_add_iff_right hb]
exact Polynomial.degree_derivative_lt ha
/--
`natDegree` version of the above theorem.
Note this would be false with just `(ha : a ≠ 0) (hb : b ≠ 0),
as when `a = b = 1` we have `(wronskian a b).natDegree = a.natDegree = b.natDegree = 0`.
-/
theorem natDegree_wronskian_lt_add {a b : R[X]} (hw : wronskian a b ≠ 0) :
(wronskian a b).natDegree < a.natDegree + b.natDegree := by
have ha : a ≠ 0 := by intro h; subst h; rw [wronskian_zero_left] at hw; exact hw rfl
have hb : b ≠ 0 := by intro h; subst h; rw [wronskian_zero_right] at hw; exact hw rfl
rw [← WithBot.coe_lt_coe, WithBot.coe_add]
convert ← degree_wronskian_lt_add ha hb
· exact Polynomial.degree_eq_natDegree hw
· exact Polynomial.degree_eq_natDegree ha
· exact Polynomial.degree_eq_natDegree hb
end Polynomial
|
RingTheory\Polynomial\Cyclotomic\Basic.lean | /-
Copyright (c) 2020 Riccardo Brasca. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Riccardo Brasca
-/
import Mathlib.Algebra.NeZero
import Mathlib.Algebra.Polynomial.BigOperators
import Mathlib.Algebra.Polynomial.Lifts
import Mathlib.Algebra.Polynomial.Splits
import Mathlib.RingTheory.RootsOfUnity.Complex
import Mathlib.NumberTheory.ArithmeticFunction
import Mathlib.RingTheory.RootsOfUnity.Basic
import Mathlib.FieldTheory.RatFunc.AsPolynomial
/-!
# Cyclotomic polynomials.
For `n : ℕ` and an integral domain `R`, we define a modified version of the `n`-th cyclotomic
polynomial with coefficients in `R`, denoted `cyclotomic' n R`, as `∏ (X - μ)`, where `μ` varies
over the primitive `n`th roots of unity. If there is a primitive `n`th root of unity in `R` then
this the standard definition. We then define the standard cyclotomic polynomial `cyclotomic n R`
with coefficients in any ring `R`.
## Main definition
* `cyclotomic n R` : the `n`-th cyclotomic polynomial with coefficients in `R`.
## Main results
* `Polynomial.degree_cyclotomic` : The degree of `cyclotomic n` is `totient n`.
* `Polynomial.prod_cyclotomic_eq_X_pow_sub_one` : `X ^ n - 1 = ∏ (cyclotomic i)`, where `i`
divides `n`.
* `Polynomial.cyclotomic_eq_prod_X_pow_sub_one_pow_moebius` : The Möbius inversion formula for
`cyclotomic n R` over an abstract fraction field for `R[X]`.
## Implementation details
Our definition of `cyclotomic' n R` makes sense in any integral domain `R`, but the interesting
results hold if there is a primitive `n`-th root of unity in `R`. In particular, our definition is
not the standard one unless there is a primitive `n`th root of unity in `R`. For example,
`cyclotomic' 3 ℤ = 1`, since there are no primitive cube roots of unity in `ℤ`. The main example is
`R = ℂ`, we decided to work in general since the difficulties are essentially the same.
To get the standard cyclotomic polynomials, we use `unique_int_coeff_of_cycl`, with `R = ℂ`,
to get a polynomial with integer coefficients and then we map it to `R[X]`, for any ring `R`.
-/
open scoped Polynomial
noncomputable section
universe u
namespace Polynomial
section Cyclotomic'
section IsDomain
variable {R : Type*} [CommRing R] [IsDomain R]
/-- The modified `n`-th cyclotomic polynomial with coefficients in `R`, it is the usual cyclotomic
polynomial if there is a primitive `n`-th root of unity in `R`. -/
def cyclotomic' (n : ℕ) (R : Type*) [CommRing R] [IsDomain R] : R[X] :=
∏ μ ∈ primitiveRoots n R, (X - C μ)
/-- The zeroth modified cyclotomic polyomial is `1`. -/
@[simp]
theorem cyclotomic'_zero (R : Type*) [CommRing R] [IsDomain R] : cyclotomic' 0 R = 1 := by
simp only [cyclotomic', Finset.prod_empty, primitiveRoots_zero]
/-- The first modified cyclotomic polyomial is `X - 1`. -/
@[simp]
theorem cyclotomic'_one (R : Type*) [CommRing R] [IsDomain R] : cyclotomic' 1 R = X - 1 := by
simp only [cyclotomic', Finset.prod_singleton, RingHom.map_one,
IsPrimitiveRoot.primitiveRoots_one]
/-- The second modified cyclotomic polyomial is `X + 1` if the characteristic of `R` is not `2`. -/
@[simp]
theorem cyclotomic'_two (R : Type*) [CommRing R] [IsDomain R] (p : ℕ) [CharP R p] (hp : p ≠ 2) :
cyclotomic' 2 R = X + 1 := by
rw [cyclotomic']
have prim_root_two : primitiveRoots 2 R = {(-1 : R)} := by
simp only [Finset.eq_singleton_iff_unique_mem, mem_primitiveRoots two_pos]
exact ⟨IsPrimitiveRoot.neg_one p hp, fun x => IsPrimitiveRoot.eq_neg_one_of_two_right⟩
simp only [prim_root_two, Finset.prod_singleton, RingHom.map_neg, RingHom.map_one, sub_neg_eq_add]
/-- `cyclotomic' n R` is monic. -/
theorem cyclotomic'.monic (n : ℕ) (R : Type*) [CommRing R] [IsDomain R] :
(cyclotomic' n R).Monic :=
monic_prod_of_monic _ _ fun _ _ => monic_X_sub_C _
/-- `cyclotomic' n R` is different from `0`. -/
theorem cyclotomic'_ne_zero (n : ℕ) (R : Type*) [CommRing R] [IsDomain R] : cyclotomic' n R ≠ 0 :=
(cyclotomic'.monic n R).ne_zero
/-- The natural degree of `cyclotomic' n R` is `totient n` if there is a primitive root of
unity in `R`. -/
theorem natDegree_cyclotomic' {ζ : R} {n : ℕ} (h : IsPrimitiveRoot ζ n) :
(cyclotomic' n R).natDegree = Nat.totient n := by
rw [cyclotomic']
rw [natDegree_prod (primitiveRoots n R) fun z : R => X - C z]
· simp only [IsPrimitiveRoot.card_primitiveRoots h, mul_one, natDegree_X_sub_C, Nat.cast_id,
Finset.sum_const, nsmul_eq_mul]
intro z _
exact X_sub_C_ne_zero z
/-- The degree of `cyclotomic' n R` is `totient n` if there is a primitive root of unity in `R`. -/
theorem degree_cyclotomic' {ζ : R} {n : ℕ} (h : IsPrimitiveRoot ζ n) :
(cyclotomic' n R).degree = Nat.totient n := by
simp only [degree_eq_natDegree (cyclotomic'_ne_zero n R), natDegree_cyclotomic' h]
/-- The roots of `cyclotomic' n R` are the primitive `n`-th roots of unity. -/
theorem roots_of_cyclotomic (n : ℕ) (R : Type*) [CommRing R] [IsDomain R] :
(cyclotomic' n R).roots = (primitiveRoots n R).val := by
rw [cyclotomic']; exact roots_prod_X_sub_C (primitiveRoots n R)
/-- If there is a primitive `n`th root of unity in `K`, then `X ^ n - 1 = ∏ (X - μ)`, where `μ`
varies over the `n`-th roots of unity. -/
theorem X_pow_sub_one_eq_prod {ζ : R} {n : ℕ} (hpos : 0 < n) (h : IsPrimitiveRoot ζ n) :
X ^ n - 1 = ∏ ζ ∈ nthRootsFinset n R, (X - C ζ) := by
classical
rw [nthRootsFinset, ← Multiset.toFinset_eq (IsPrimitiveRoot.nthRoots_one_nodup h)]
simp only [Finset.prod_mk, RingHom.map_one]
rw [nthRoots]
have hmonic : (X ^ n - C (1 : R)).Monic := monic_X_pow_sub_C (1 : R) (ne_of_lt hpos).symm
symm
apply prod_multiset_X_sub_C_of_monic_of_roots_card_eq hmonic
rw [@natDegree_X_pow_sub_C R _ _ n 1, ← nthRoots]
exact IsPrimitiveRoot.card_nthRoots_one h
end IsDomain
section Field
variable {K : Type*} [Field K]
/-- `cyclotomic' n K` splits. -/
theorem cyclotomic'_splits (n : ℕ) : Splits (RingHom.id K) (cyclotomic' n K) := by
apply splits_prod (RingHom.id K)
intro z _
simp only [splits_X_sub_C (RingHom.id K)]
/-- If there is a primitive `n`-th root of unity in `K`, then `X ^ n - 1` splits. -/
theorem X_pow_sub_one_splits {ζ : K} {n : ℕ} (h : IsPrimitiveRoot ζ n) :
Splits (RingHom.id K) (X ^ n - C (1 : K)) := by
rw [splits_iff_card_roots, ← nthRoots, IsPrimitiveRoot.card_nthRoots_one h, natDegree_X_pow_sub_C]
/-- If there is a primitive `n`-th root of unity in `K`, then
`∏ i ∈ Nat.divisors n, cyclotomic' i K = X ^ n - 1`. -/
theorem prod_cyclotomic'_eq_X_pow_sub_one {K : Type*} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ}
(hpos : 0 < n) (h : IsPrimitiveRoot ζ n) :
∏ i ∈ Nat.divisors n, cyclotomic' i K = X ^ n - 1 := by
classical
have hd : (n.divisors : Set ℕ).PairwiseDisjoint fun k => primitiveRoots k K :=
fun x _ y _ hne => IsPrimitiveRoot.disjoint hne
simp only [X_pow_sub_one_eq_prod hpos h, cyclotomic', ← Finset.prod_biUnion hd,
h.nthRoots_one_eq_biUnion_primitiveRoots]
/-- If there is a primitive `n`-th root of unity in `K`, then
`cyclotomic' n K = (X ^ k - 1) /ₘ (∏ i ∈ Nat.properDivisors k, cyclotomic' i K)`. -/
theorem cyclotomic'_eq_X_pow_sub_one_div {K : Type*} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ}
(hpos : 0 < n) (h : IsPrimitiveRoot ζ n) :
cyclotomic' n K = (X ^ n - 1) /ₘ ∏ i ∈ Nat.properDivisors n, cyclotomic' i K := by
rw [← prod_cyclotomic'_eq_X_pow_sub_one hpos h, ← Nat.cons_self_properDivisors hpos.ne',
Finset.prod_cons]
have prod_monic : (∏ i ∈ Nat.properDivisors n, cyclotomic' i K).Monic := by
apply monic_prod_of_monic
intro i _
exact cyclotomic'.monic i K
rw [(div_modByMonic_unique (cyclotomic' n K) 0 prod_monic _).1]
simp only [degree_zero, zero_add]
refine ⟨by rw [mul_comm], ?_⟩
rw [bot_lt_iff_ne_bot]
intro h
exact Monic.ne_zero prod_monic (degree_eq_bot.1 h)
/-- If there is a primitive `n`-th root of unity in `K`, then `cyclotomic' n K` comes from a
monic polynomial with integer coefficients. -/
theorem int_coeff_of_cyclotomic' {K : Type*} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ}
(h : IsPrimitiveRoot ζ n) : ∃ P : ℤ[X], map (Int.castRingHom K) P =
cyclotomic' n K ∧ P.degree = (cyclotomic' n K).degree ∧ P.Monic := by
refine lifts_and_degree_eq_and_monic ?_ (cyclotomic'.monic n K)
induction' n using Nat.strong_induction_on with k ihk generalizing ζ
rcases k.eq_zero_or_pos with (rfl | hpos)
· use 1
simp only [cyclotomic'_zero, coe_mapRingHom, Polynomial.map_one]
let B : K[X] := ∏ i ∈ Nat.properDivisors k, cyclotomic' i K
have Bmo : B.Monic := by
apply monic_prod_of_monic
intro i _
exact cyclotomic'.monic i K
have Bint : B ∈ lifts (Int.castRingHom K) := by
refine Subsemiring.prod_mem (lifts (Int.castRingHom K)) ?_
intro x hx
have xsmall := (Nat.mem_properDivisors.1 hx).2
obtain ⟨d, hd⟩ := (Nat.mem_properDivisors.1 hx).1
rw [mul_comm] at hd
exact ihk x xsmall (h.pow hpos hd)
replace Bint := lifts_and_degree_eq_and_monic Bint Bmo
obtain ⟨B₁, hB₁, _, hB₁mo⟩ := Bint
let Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
have huniq : 0 + B * cyclotomic' k K = X ^ k - 1 ∧ (0 : K[X]).degree < B.degree := by
constructor
· rw [zero_add, mul_comm, ← prod_cyclotomic'_eq_X_pow_sub_one hpos h, ←
Nat.cons_self_properDivisors hpos.ne', Finset.prod_cons]
· simpa only [degree_zero, bot_lt_iff_ne_bot, Ne, degree_eq_bot] using Bmo.ne_zero
replace huniq := div_modByMonic_unique (cyclotomic' k K) (0 : K[X]) Bmo huniq
simp only [lifts, RingHom.mem_rangeS]
use Q₁
rw [coe_mapRingHom, map_divByMonic (Int.castRingHom K) hB₁mo, hB₁, ← huniq.1]
simp
/-- If `K` is of characteristic `0` and there is a primitive `n`-th root of unity in `K`,
then `cyclotomic n K` comes from a unique polynomial with integer coefficients. -/
theorem unique_int_coeff_of_cycl {K : Type*} [CommRing K] [IsDomain K] [CharZero K] {ζ : K}
{n : ℕ+} (h : IsPrimitiveRoot ζ n) :
∃! P : ℤ[X], map (Int.castRingHom K) P = cyclotomic' n K := by
obtain ⟨P, hP⟩ := int_coeff_of_cyclotomic' h
refine ⟨P, hP.1, fun Q hQ => ?_⟩
apply map_injective (Int.castRingHom K) Int.cast_injective
rw [hP.1, hQ]
end Field
end Cyclotomic'
section Cyclotomic
/-- The `n`-th cyclotomic polynomial with coefficients in `R`. -/
def cyclotomic (n : ℕ) (R : Type*) [Ring R] : R[X] :=
if h : n = 0 then 1
else map (Int.castRingHom R) (int_coeff_of_cyclotomic' (Complex.isPrimitiveRoot_exp n h)).choose
theorem int_cyclotomic_rw {n : ℕ} (h : n ≠ 0) :
cyclotomic n ℤ = (int_coeff_of_cyclotomic' (Complex.isPrimitiveRoot_exp n h)).choose := by
simp only [cyclotomic, h, dif_neg, not_false_iff]
ext i
simp only [coeff_map, Int.cast_id, eq_intCast]
/-- `cyclotomic n R` comes from `cyclotomic n ℤ`. -/
theorem map_cyclotomic_int (n : ℕ) (R : Type*) [Ring R] :
map (Int.castRingHom R) (cyclotomic n ℤ) = cyclotomic n R := by
by_cases hzero : n = 0
· simp only [hzero, cyclotomic, dif_pos, Polynomial.map_one]
simp [cyclotomic, hzero]
theorem int_cyclotomic_spec (n : ℕ) :
map (Int.castRingHom ℂ) (cyclotomic n ℤ) = cyclotomic' n ℂ ∧
(cyclotomic n ℤ).degree = (cyclotomic' n ℂ).degree ∧ (cyclotomic n ℤ).Monic := by
by_cases hzero : n = 0
· simp only [hzero, cyclotomic, degree_one, monic_one, cyclotomic'_zero, dif_pos,
eq_self_iff_true, Polynomial.map_one, and_self_iff]
rw [int_cyclotomic_rw hzero]
exact (int_coeff_of_cyclotomic' (Complex.isPrimitiveRoot_exp n hzero)).choose_spec
theorem int_cyclotomic_unique {n : ℕ} {P : ℤ[X]} (h : map (Int.castRingHom ℂ) P = cyclotomic' n ℂ) :
P = cyclotomic n ℤ := by
apply map_injective (Int.castRingHom ℂ) Int.cast_injective
rw [h, (int_cyclotomic_spec n).1]
/-- The definition of `cyclotomic n R` commutes with any ring homomorphism. -/
@[simp]
theorem map_cyclotomic (n : ℕ) {R S : Type*} [Ring R] [Ring S] (f : R →+* S) :
map f (cyclotomic n R) = cyclotomic n S := by
rw [← map_cyclotomic_int n R, ← map_cyclotomic_int n S, map_map]
have : Subsingleton (ℤ →+* S) := inferInstance
congr!
theorem cyclotomic.eval_apply {R S : Type*} (q : R) (n : ℕ) [Ring R] [Ring S] (f : R →+* S) :
eval (f q) (cyclotomic n S) = f (eval q (cyclotomic n R)) := by
rw [← map_cyclotomic n f, eval_map, eval₂_at_apply]
/-- The zeroth cyclotomic polyomial is `1`. -/
@[simp]
theorem cyclotomic_zero (R : Type*) [Ring R] : cyclotomic 0 R = 1 := by
simp only [cyclotomic, dif_pos]
/-- The first cyclotomic polyomial is `X - 1`. -/
@[simp]
theorem cyclotomic_one (R : Type*) [Ring R] : cyclotomic 1 R = X - 1 := by
have hspec : map (Int.castRingHom ℂ) (X - 1) = cyclotomic' 1 ℂ := by
simp only [cyclotomic'_one, PNat.one_coe, map_X, Polynomial.map_one, Polynomial.map_sub]
symm
rw [← map_cyclotomic_int, ← int_cyclotomic_unique hspec]
simp only [map_X, Polynomial.map_one, Polynomial.map_sub]
/-- `cyclotomic n` is monic. -/
theorem cyclotomic.monic (n : ℕ) (R : Type*) [Ring R] : (cyclotomic n R).Monic := by
rw [← map_cyclotomic_int]
exact (int_cyclotomic_spec n).2.2.map _
/-- `cyclotomic n` is primitive. -/
theorem cyclotomic.isPrimitive (n : ℕ) (R : Type*) [CommRing R] : (cyclotomic n R).IsPrimitive :=
(cyclotomic.monic n R).isPrimitive
/-- `cyclotomic n R` is different from `0`. -/
theorem cyclotomic_ne_zero (n : ℕ) (R : Type*) [Ring R] [Nontrivial R] : cyclotomic n R ≠ 0 :=
(cyclotomic.monic n R).ne_zero
/-- The degree of `cyclotomic n` is `totient n`. -/
theorem degree_cyclotomic (n : ℕ) (R : Type*) [Ring R] [Nontrivial R] :
(cyclotomic n R).degree = Nat.totient n := by
rw [← map_cyclotomic_int]
rw [degree_map_eq_of_leadingCoeff_ne_zero (Int.castRingHom R) _]
· cases' n with k
· simp only [cyclotomic, degree_one, dif_pos, Nat.totient_zero, CharP.cast_eq_zero]
rw [← degree_cyclotomic' (Complex.isPrimitiveRoot_exp k.succ (Nat.succ_ne_zero k))]
exact (int_cyclotomic_spec k.succ).2.1
simp only [(int_cyclotomic_spec n).right.right, eq_intCast, Monic.leadingCoeff, Int.cast_one,
Ne, not_false_iff, one_ne_zero]
/-- The natural degree of `cyclotomic n` is `totient n`. -/
theorem natDegree_cyclotomic (n : ℕ) (R : Type*) [Ring R] [Nontrivial R] :
(cyclotomic n R).natDegree = Nat.totient n := by
rw [natDegree, degree_cyclotomic]; norm_cast
/-- The degree of `cyclotomic n R` is positive. -/
theorem degree_cyclotomic_pos (n : ℕ) (R : Type*) (hpos : 0 < n) [Ring R] [Nontrivial R] :
0 < (cyclotomic n R).degree := by
rwa [degree_cyclotomic n R, Nat.cast_pos, Nat.totient_pos]
open Finset
/-- `∏ i ∈ Nat.divisors n, cyclotomic i R = X ^ n - 1`. -/
theorem prod_cyclotomic_eq_X_pow_sub_one {n : ℕ} (hpos : 0 < n) (R : Type*) [CommRing R] :
∏ i ∈ Nat.divisors n, cyclotomic i R = X ^ n - 1 := by
have integer : ∏ i ∈ Nat.divisors n, cyclotomic i ℤ = X ^ n - 1 := by
apply map_injective (Int.castRingHom ℂ) Int.cast_injective
simp only [Polynomial.map_prod, int_cyclotomic_spec, Polynomial.map_pow, map_X,
Polynomial.map_one, Polynomial.map_sub]
exact prod_cyclotomic'_eq_X_pow_sub_one hpos (Complex.isPrimitiveRoot_exp n hpos.ne')
simpa only [Polynomial.map_prod, map_cyclotomic_int, Polynomial.map_sub, Polynomial.map_one,
Polynomial.map_pow, Polynomial.map_X] using congr_arg (map (Int.castRingHom R)) integer
theorem cyclotomic.dvd_X_pow_sub_one (n : ℕ) (R : Type*) [Ring R] :
cyclotomic n R ∣ X ^ n - 1 := by
suffices cyclotomic n ℤ ∣ X ^ n - 1 by
simpa only [map_cyclotomic_int, Polynomial.map_sub, Polynomial.map_one, Polynomial.map_pow,
Polynomial.map_X] using map_dvd (Int.castRingHom R) this
rcases n.eq_zero_or_pos with (rfl | hn)
· simp
rw [← prod_cyclotomic_eq_X_pow_sub_one hn]
exact Finset.dvd_prod_of_mem _ (n.mem_divisors_self hn.ne')
theorem prod_cyclotomic_eq_geom_sum {n : ℕ} (h : 0 < n) (R) [CommRing R] :
∏ i ∈ n.divisors.erase 1, cyclotomic i R = ∑ i ∈ Finset.range n, X ^ i := by
suffices (∏ i ∈ n.divisors.erase 1, cyclotomic i ℤ) = ∑ i ∈ Finset.range n, X ^ i by
simpa only [Polynomial.map_prod, map_cyclotomic_int, Polynomial.map_sum, Polynomial.map_pow,
Polynomial.map_X] using congr_arg (map (Int.castRingHom R)) this
rw [← mul_left_inj' (cyclotomic_ne_zero 1 ℤ), prod_erase_mul _ _ (Nat.one_mem_divisors.2 h.ne'),
cyclotomic_one, geom_sum_mul, prod_cyclotomic_eq_X_pow_sub_one h]
/-- If `p` is prime, then `cyclotomic p R = ∑ i ∈ range p, X ^ i`. -/
theorem cyclotomic_prime (R : Type*) [Ring R] (p : ℕ) [hp : Fact p.Prime] :
cyclotomic p R = ∑ i ∈ Finset.range p, X ^ i := by
suffices cyclotomic p ℤ = ∑ i ∈ range p, X ^ i by
simpa only [map_cyclotomic_int, Polynomial.map_sum, Polynomial.map_pow, Polynomial.map_X] using
congr_arg (map (Int.castRingHom R)) this
rw [← prod_cyclotomic_eq_geom_sum hp.out.pos, hp.out.divisors,
erase_insert (mem_singleton.not.2 hp.out.ne_one.symm), prod_singleton]
theorem cyclotomic_prime_mul_X_sub_one (R : Type*) [Ring R] (p : ℕ) [hn : Fact (Nat.Prime p)] :
cyclotomic p R * (X - 1) = X ^ p - 1 := by rw [cyclotomic_prime, geom_sum_mul]
@[simp]
theorem cyclotomic_two (R : Type*) [Ring R] : cyclotomic 2 R = X + 1 := by simp [cyclotomic_prime]
@[simp]
theorem cyclotomic_three (R : Type*) [Ring R] : cyclotomic 3 R = X ^ 2 + X + 1 := by
simp [cyclotomic_prime, sum_range_succ']
theorem cyclotomic_dvd_geom_sum_of_dvd (R) [Ring R] {d n : ℕ} (hdn : d ∣ n) (hd : d ≠ 1) :
cyclotomic d R ∣ ∑ i ∈ Finset.range n, X ^ i := by
suffices cyclotomic d ℤ ∣ ∑ i ∈ Finset.range n, X ^ i by
simpa only [map_cyclotomic_int, Polynomial.map_sum, Polynomial.map_pow, Polynomial.map_X] using
map_dvd (Int.castRingHom R) this
rcases n.eq_zero_or_pos with (rfl | hn)
· simp
rw [← prod_cyclotomic_eq_geom_sum hn]
apply Finset.dvd_prod_of_mem
simp [hd, hdn, hn.ne']
theorem X_pow_sub_one_mul_prod_cyclotomic_eq_X_pow_sub_one_of_dvd (R) [CommRing R] {d n : ℕ}
(h : d ∈ n.properDivisors) :
((X ^ d - 1) * ∏ x ∈ n.divisors \ d.divisors, cyclotomic x R) = X ^ n - 1 := by
obtain ⟨hd, hdn⟩ := Nat.mem_properDivisors.mp h
have h0n : 0 < n := pos_of_gt hdn
have h0d : 0 < d := Nat.pos_of_dvd_of_pos hd h0n
rw [← prod_cyclotomic_eq_X_pow_sub_one h0d, ← prod_cyclotomic_eq_X_pow_sub_one h0n, mul_comm,
Finset.prod_sdiff (Nat.divisors_subset_of_dvd h0n.ne' hd)]
theorem X_pow_sub_one_mul_cyclotomic_dvd_X_pow_sub_one_of_dvd (R) [CommRing R] {d n : ℕ}
(h : d ∈ n.properDivisors) : (X ^ d - 1) * cyclotomic n R ∣ X ^ n - 1 := by
have hdn := (Nat.mem_properDivisors.mp h).2
use ∏ x ∈ n.properDivisors \ d.divisors, cyclotomic x R
symm
convert X_pow_sub_one_mul_prod_cyclotomic_eq_X_pow_sub_one_of_dvd R h using 1
rw [mul_assoc]
congr 1
rw [← Nat.insert_self_properDivisors hdn.ne_bot, insert_sdiff_of_not_mem, prod_insert]
· exact Finset.not_mem_sdiff_of_not_mem_left Nat.properDivisors.not_self_mem
· exact fun hk => hdn.not_le <| Nat.divisor_le hk
section ArithmeticFunction
open ArithmeticFunction
open scoped ArithmeticFunction
/-- `cyclotomic n R` can be expressed as a product in a fraction field of `R[X]`
using Möbius inversion. -/
theorem cyclotomic_eq_prod_X_pow_sub_one_pow_moebius {n : ℕ} (R : Type*) [CommRing R]
[IsDomain R] : algebraMap _ (RatFunc R) (cyclotomic n R) =
∏ i ∈ n.divisorsAntidiagonal, algebraMap R[X] _ (X ^ i.snd - 1) ^ μ i.fst := by
rcases n.eq_zero_or_pos with (rfl | hpos)
· simp
have h : ∀ n : ℕ, 0 < n → (∏ i ∈ Nat.divisors n, algebraMap _ (RatFunc R) (cyclotomic i R)) =
algebraMap _ _ (X ^ n - 1 : R[X]) := by
intro n hn
rw [← prod_cyclotomic_eq_X_pow_sub_one hn R, map_prod]
rw [(prod_eq_iff_prod_pow_moebius_eq_of_nonzero (fun n hn => _) fun n hn => _).1 h n hpos] <;>
simp_rw [Ne, IsFractionRing.to_map_eq_zero_iff]
· simp [cyclotomic_ne_zero]
· intro n hn
apply Monic.ne_zero
apply monic_X_pow_sub_C _ (ne_of_gt hn)
end ArithmeticFunction
/-- We have
`cyclotomic n R = (X ^ k - 1) /ₘ (∏ i ∈ Nat.properDivisors k, cyclotomic i K)`. -/
theorem cyclotomic_eq_X_pow_sub_one_div {R : Type*} [CommRing R] {n : ℕ} (hpos : 0 < n) :
cyclotomic n R = (X ^ n - 1) /ₘ ∏ i ∈ Nat.properDivisors n, cyclotomic i R := by
nontriviality R
rw [← prod_cyclotomic_eq_X_pow_sub_one hpos, ← Nat.cons_self_properDivisors hpos.ne',
Finset.prod_cons]
have prod_monic : (∏ i ∈ Nat.properDivisors n, cyclotomic i R).Monic := by
apply monic_prod_of_monic
intro i _
exact cyclotomic.monic i R
rw [(div_modByMonic_unique (cyclotomic n R) 0 prod_monic _).1]
simp only [degree_zero, zero_add]
constructor
· rw [mul_comm]
rw [bot_lt_iff_ne_bot]
intro h
exact Monic.ne_zero prod_monic (degree_eq_bot.1 h)
/-- If `m` is a proper divisor of `n`, then `X ^ m - 1` divides
`∏ i ∈ Nat.properDivisors n, cyclotomic i R`. -/
theorem X_pow_sub_one_dvd_prod_cyclotomic (R : Type*) [CommRing R] {n m : ℕ} (hpos : 0 < n)
(hm : m ∣ n) (hdiff : m ≠ n) : X ^ m - 1 ∣ ∏ i ∈ Nat.properDivisors n, cyclotomic i R := by
replace hm := Nat.mem_properDivisors.2
⟨hm, lt_of_le_of_ne (Nat.divisor_le (Nat.mem_divisors.2 ⟨hm, hpos.ne'⟩)) hdiff⟩
rw [← Finset.sdiff_union_of_subset (Nat.divisors_subset_properDivisors (ne_of_lt hpos).symm
(Nat.mem_properDivisors.1 hm).1 (ne_of_lt (Nat.mem_properDivisors.1 hm).2)),
Finset.prod_union Finset.sdiff_disjoint,
prod_cyclotomic_eq_X_pow_sub_one (Nat.pos_of_mem_properDivisors hm)]
exact ⟨∏ x ∈ n.properDivisors \ m.divisors, cyclotomic x R, by rw [mul_comm]⟩
/-- If there is a primitive `n`-th root of unity in `K`, then
`cyclotomic n K = ∏ μ ∈ primitiveRoots n K, (X - C μ)`. ∈ particular,
`cyclotomic n K = cyclotomic' n K` -/
theorem cyclotomic_eq_prod_X_sub_primitiveRoots {K : Type*} [CommRing K] [IsDomain K] {ζ : K}
{n : ℕ} (hz : IsPrimitiveRoot ζ n) : cyclotomic n K = ∏ μ ∈ primitiveRoots n K, (X - C μ) := by
rw [← cyclotomic']
induction' n using Nat.strong_induction_on with k hk generalizing ζ
obtain hzero | hpos := k.eq_zero_or_pos
· simp only [hzero, cyclotomic'_zero, cyclotomic_zero]
have h : ∀ i ∈ k.properDivisors, cyclotomic i K = cyclotomic' i K := by
intro i hi
obtain ⟨d, hd⟩ := (Nat.mem_properDivisors.1 hi).1
rw [mul_comm] at hd
exact hk i (Nat.mem_properDivisors.1 hi).2 (IsPrimitiveRoot.pow hpos hz hd)
rw [@cyclotomic_eq_X_pow_sub_one_div _ _ _ hpos, cyclotomic'_eq_X_pow_sub_one_div hpos hz,
Finset.prod_congr (refl k.properDivisors) h]
theorem eq_cyclotomic_iff {R : Type*} [CommRing R] {n : ℕ} (hpos : 0 < n) (P : R[X]) :
P = cyclotomic n R ↔
(P * ∏ i ∈ Nat.properDivisors n, Polynomial.cyclotomic i R) = X ^ n - 1 := by
nontriviality R
refine ⟨fun hcycl => ?_, fun hP => ?_⟩
· rw [hcycl, ← prod_cyclotomic_eq_X_pow_sub_one hpos R, ← Nat.cons_self_properDivisors hpos.ne',
Finset.prod_cons]
· have prod_monic : (∏ i ∈ Nat.properDivisors n, cyclotomic i R).Monic := by
apply monic_prod_of_monic
intro i _
exact cyclotomic.monic i R
rw [@cyclotomic_eq_X_pow_sub_one_div R _ _ hpos, (div_modByMonic_unique P 0 prod_monic _).1]
refine ⟨by rwa [zero_add, mul_comm], ?_⟩
rw [degree_zero, bot_lt_iff_ne_bot]
intro h
exact Monic.ne_zero prod_monic (degree_eq_bot.1 h)
/-- If `p ^ k` is a prime power, then
`cyclotomic (p ^ (n + 1)) R = ∑ i ∈ range p, (X ^ (p ^ n)) ^ i`. -/
theorem cyclotomic_prime_pow_eq_geom_sum {R : Type*} [CommRing R] {p n : ℕ} (hp : p.Prime) :
cyclotomic (p ^ (n + 1)) R = ∑ i ∈ Finset.range p, (X ^ p ^ n) ^ i := by
have : ∀ m, (cyclotomic (p ^ (m + 1)) R = ∑ i ∈ Finset.range p, (X ^ p ^ m) ^ i) ↔
((∑ i ∈ Finset.range p, (X ^ p ^ m) ^ i) *
∏ x ∈ Finset.range (m + 1), cyclotomic (p ^ x) R) = X ^ p ^ (m + 1) - 1 := by
intro m
have := eq_cyclotomic_iff (R := R) (P := ∑ i ∈ range p, (X ^ p ^ m) ^ i)
(pow_pos hp.pos (m + 1))
rw [eq_comm] at this
rw [this, Nat.prod_properDivisors_prime_pow hp]
induction' n with n_n n_ih
· haveI := Fact.mk hp; simp [cyclotomic_prime]
rw [((eq_cyclotomic_iff (pow_pos hp.pos (n_n + 1 + 1)) _).mpr _).symm]
rw [Nat.prod_properDivisors_prime_pow hp, Finset.prod_range_succ, n_ih]
rw [this] at n_ih
rw [mul_comm _ (∑ i ∈ _, _), n_ih, geom_sum_mul, sub_left_inj, ← pow_mul]
simp only [pow_add, pow_one]
theorem cyclotomic_prime_pow_mul_X_pow_sub_one (R : Type*) [CommRing R] (p k : ℕ)
[hn : Fact (Nat.Prime p)] :
cyclotomic (p ^ (k + 1)) R * (X ^ p ^ k - 1) = X ^ p ^ (k + 1) - 1 := by
rw [cyclotomic_prime_pow_eq_geom_sum hn.out, geom_sum_mul, ← pow_mul, pow_succ, mul_comm]
/-- The constant term of `cyclotomic n R` is `1` if `2 ≤ n`. -/
theorem cyclotomic_coeff_zero (R : Type*) [CommRing R] {n : ℕ} (hn : 1 < n) :
(cyclotomic n R).coeff 0 = 1 := by
induction' n using Nat.strong_induction_on with n hi
have hprod : (∏ i ∈ Nat.properDivisors n, (Polynomial.cyclotomic i R).coeff 0) = -1 := by
rw [← Finset.insert_erase (Nat.one_mem_properDivisors_iff_one_lt.2
(lt_of_lt_of_le one_lt_two hn)), Finset.prod_insert (Finset.not_mem_erase 1 _),
cyclotomic_one R]
have hleq : ∀ j ∈ n.properDivisors.erase 1, 2 ≤ j := by
intro j hj
apply Nat.succ_le_of_lt
exact (Ne.le_iff_lt (Finset.mem_erase.1 hj).1.symm).mp
(Nat.succ_le_of_lt (Nat.pos_of_mem_properDivisors (Finset.mem_erase.1 hj).2))
have hcongr : ∀ j ∈ n.properDivisors.erase 1, (cyclotomic j R).coeff 0 = 1 := by
intro j hj
exact hi j (Nat.mem_properDivisors.1 (Finset.mem_erase.1 hj).2).2 (hleq j hj)
have hrw : (∏ x ∈ n.properDivisors.erase 1, (cyclotomic x R).coeff 0) = 1 := by
rw [Finset.prod_congr (refl (n.properDivisors.erase 1)) hcongr]
simp only [Finset.prod_const_one]
simp only [hrw, mul_one, zero_sub, coeff_one_zero, coeff_X_zero, coeff_sub]
have heq : (X ^ n - 1 : R[X]).coeff 0 = -(cyclotomic n R).coeff 0 := by
rw [← prod_cyclotomic_eq_X_pow_sub_one (zero_le_one.trans_lt hn), ←
Nat.cons_self_properDivisors hn.ne_bot, Finset.prod_cons, mul_coeff_zero, coeff_zero_prod,
hprod, mul_neg, mul_one]
have hzero : (X ^ n - 1 : R[X]).coeff 0 = (-1 : R) := by
rw [coeff_zero_eq_eval_zero _]
simp only [zero_pow (by positivity : n ≠ 0), eval_X, eval_one, zero_sub, eval_pow, eval_sub]
rw [hzero] at heq
exact neg_inj.mp (Eq.symm heq)
/-- If `(a : ℕ)` is a root of `cyclotomic n (ZMod p)`, where `p` is a prime, then `a` and `p` are
coprime. -/
theorem coprime_of_root_cyclotomic {n : ℕ} (hpos : 0 < n) {p : ℕ} [hprime : Fact p.Prime] {a : ℕ}
(hroot : IsRoot (cyclotomic n (ZMod p)) (Nat.castRingHom (ZMod p) a)) : a.Coprime p := by
apply Nat.Coprime.symm
rw [hprime.1.coprime_iff_not_dvd]
intro h
replace h := (ZMod.natCast_zmod_eq_zero_iff_dvd a p).2 h
rw [IsRoot.def, eq_natCast, h, ← coeff_zero_eq_eval_zero] at hroot
by_cases hone : n = 1
· simp only [hone, cyclotomic_one, zero_sub, coeff_one_zero, coeff_X_zero, neg_eq_zero,
one_ne_zero, coeff_sub] at hroot
rw [cyclotomic_coeff_zero (ZMod p) (Nat.succ_le_of_lt
(lt_of_le_of_ne (Nat.succ_le_of_lt hpos) (Ne.symm hone)))] at hroot
exact one_ne_zero hroot
end Cyclotomic
section Order
/-- If `(a : ℕ)` is a root of `cyclotomic n (ZMod p)`, then the multiplicative order of `a` modulo
`p` divides `n`. -/
theorem orderOf_root_cyclotomic_dvd {n : ℕ} (hpos : 0 < n) {p : ℕ} [Fact p.Prime] {a : ℕ}
(hroot : IsRoot (cyclotomic n (ZMod p)) (Nat.castRingHom (ZMod p) a)) :
orderOf (ZMod.unitOfCoprime a (coprime_of_root_cyclotomic hpos hroot)) ∣ n := by
apply orderOf_dvd_of_pow_eq_one
suffices hpow : eval (Nat.castRingHom (ZMod p) a) (X ^ n - 1 : (ZMod p)[X]) = 0 by
simp only [eval_X, eval_one, eval_pow, eval_sub, eq_natCast] at hpow
apply Units.val_eq_one.1
simp only [sub_eq_zero.mp hpow, ZMod.coe_unitOfCoprime, Units.val_pow_eq_pow_val]
rw [IsRoot.def] at hroot
rw [← prod_cyclotomic_eq_X_pow_sub_one hpos (ZMod p), ← Nat.cons_self_properDivisors hpos.ne',
Finset.prod_cons, eval_mul, hroot, zero_mul]
end Order
end Polynomial
|
RingTheory\Polynomial\Cyclotomic\Eval.lean | /-
Copyright (c) 2021 Eric Rodriguez. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Eric Rodriguez
-/
import Mathlib.RingTheory.Polynomial.Cyclotomic.Roots
import Mathlib.Tactic.ByContra
import Mathlib.Topology.Algebra.Polynomial
import Mathlib.NumberTheory.Padics.PadicVal
import Mathlib.Analysis.Complex.Arg
/-!
# Evaluating cyclotomic polynomials
This file states some results about evaluating cyclotomic polynomials in various different ways.
## Main definitions
* `Polynomial.eval(₂)_one_cyclotomic_prime(_pow)`: `eval 1 (cyclotomic p^k R) = p`.
* `Polynomial.eval_one_cyclotomic_not_prime_pow`: Otherwise, `eval 1 (cyclotomic n R) = 1`.
* `Polynomial.cyclotomic_pos` : `∀ x, 0 < eval x (cyclotomic n R)` if `2 < n`.
-/
namespace Polynomial
open Finset Nat
@[simp]
theorem eval_one_cyclotomic_prime {R : Type*} [CommRing R] {p : ℕ} [hn : Fact p.Prime] :
eval 1 (cyclotomic p R) = p := by
simp only [cyclotomic_prime, eval_X, one_pow, Finset.sum_const, eval_pow, eval_finset_sum,
Finset.card_range, smul_one_eq_cast]
-- @[simp] -- Porting note (#10618): simp already proves this
theorem eval₂_one_cyclotomic_prime {R S : Type*} [CommRing R] [Semiring S] (f : R →+* S) {p : ℕ}
[Fact p.Prime] : eval₂ f 1 (cyclotomic p R) = p := by simp
@[simp]
theorem eval_one_cyclotomic_prime_pow {R : Type*} [CommRing R] {p : ℕ} (k : ℕ)
[hn : Fact p.Prime] : eval 1 (cyclotomic (p ^ (k + 1)) R) = p := by
simp only [cyclotomic_prime_pow_eq_geom_sum hn.out, eval_X, one_pow, Finset.sum_const, eval_pow,
eval_finset_sum, Finset.card_range, smul_one_eq_cast]
-- @[simp] -- Porting note (#10618): simp already proves this
theorem eval₂_one_cyclotomic_prime_pow {R S : Type*} [CommRing R] [Semiring S] (f : R →+* S)
{p : ℕ} (k : ℕ) [Fact p.Prime] : eval₂ f 1 (cyclotomic (p ^ (k + 1)) R) = p := by simp
private theorem cyclotomic_neg_one_pos {n : ℕ} (hn : 2 < n) {R} [LinearOrderedCommRing R] :
0 < eval (-1 : R) (cyclotomic n R) := by
haveI := NeZero.of_gt hn
rw [← map_cyclotomic_int, ← Int.cast_one, ← Int.cast_neg, eval_intCast_map, Int.coe_castRingHom,
Int.cast_pos]
suffices 0 < eval (↑(-1 : ℤ)) (cyclotomic n ℝ) by
rw [← map_cyclotomic_int n ℝ, eval_intCast_map, Int.coe_castRingHom] at this
simpa only [Int.cast_pos] using this
simp only [Int.cast_one, Int.cast_neg]
have h0 := cyclotomic_coeff_zero ℝ hn.le
rw [coeff_zero_eq_eval_zero] at h0
by_contra! hx
have := intermediate_value_univ (-1) 0 (cyclotomic n ℝ).continuous
obtain ⟨y, hy : IsRoot _ y⟩ := this (show (0 : ℝ) ∈ Set.Icc _ _ by simpa [h0] using hx)
rw [@isRoot_cyclotomic_iff] at hy
rw [hy.eq_orderOf] at hn
exact hn.not_le LinearOrderedRing.orderOf_le_two
theorem cyclotomic_pos {n : ℕ} (hn : 2 < n) {R} [LinearOrderedCommRing R] (x : R) :
0 < eval x (cyclotomic n R) := by
induction' n using Nat.strong_induction_on with n ih
have hn' : 0 < n := pos_of_gt hn
have hn'' : 1 < n := one_lt_two.trans hn
have := prod_cyclotomic_eq_geom_sum hn' R
apply_fun eval x at this
rw [← cons_self_properDivisors hn'.ne', Finset.erase_cons_of_ne _ hn''.ne', Finset.prod_cons,
eval_mul, eval_geom_sum] at this
rcases lt_trichotomy 0 (∑ i ∈ Finset.range n, x ^ i) with (h | h | h)
· apply pos_of_mul_pos_left
· rwa [this]
rw [eval_prod]
refine Finset.prod_nonneg fun i hi => ?_
simp only [Finset.mem_erase, mem_properDivisors] at hi
rw [geom_sum_pos_iff hn'.ne'] at h
cases' h with hk hx
· refine (ih _ hi.2.2 (Nat.two_lt_of_ne ?_ hi.1 ?_)).le <;> rintro rfl
· exact hn'.ne' (zero_dvd_iff.mp hi.2.1)
· exact even_iff_not_odd.mp (even_iff_two_dvd.mpr hi.2.1) hk
· rcases eq_or_ne i 2 with (rfl | hk)
· simpa only [eval_X, eval_one, cyclotomic_two, eval_add] using hx.le
refine (ih _ hi.2.2 (Nat.two_lt_of_ne ?_ hi.1 hk)).le
rintro rfl
exact hn'.ne' <| zero_dvd_iff.mp hi.2.1
· rw [eq_comm, geom_sum_eq_zero_iff_neg_one hn'.ne'] at h
exact h.1.symm ▸ cyclotomic_neg_one_pos hn
· apply pos_of_mul_neg_left
· rwa [this]
rw [geom_sum_neg_iff hn'.ne'] at h
have h2 : 2 ∈ n.properDivisors.erase 1 := by
rw [Finset.mem_erase, mem_properDivisors]
exact ⟨by decide, even_iff_two_dvd.mp h.1, hn⟩
rw [eval_prod, ← Finset.prod_erase_mul _ _ h2]
apply mul_nonpos_of_nonneg_of_nonpos
· refine Finset.prod_nonneg fun i hi => le_of_lt ?_
simp only [Finset.mem_erase, mem_properDivisors] at hi
refine ih _ hi.2.2.2 (Nat.two_lt_of_ne ?_ hi.2.1 hi.1)
rintro rfl
rw [zero_dvd_iff] at hi
exact hn'.ne' hi.2.2.1
· simpa only [eval_X, eval_one, cyclotomic_two, eval_add] using h.right.le
theorem cyclotomic_pos_and_nonneg (n : ℕ) {R} [LinearOrderedCommRing R] (x : R) :
(1 < x → 0 < eval x (cyclotomic n R)) ∧ (1 ≤ x → 0 ≤ eval x (cyclotomic n R)) := by
rcases n with (_ | _ | _ | n)
· simp only [cyclotomic_zero, eval_one, zero_lt_one, implies_true, zero_le_one, and_self]
· simp only [zero_add, cyclotomic_one, eval_sub, eval_X, eval_one, sub_pos, imp_self, sub_nonneg,
and_self]
· simp only [zero_add, reduceAdd, cyclotomic_two, eval_add, eval_X, eval_one]
constructor <;> intro <;> linarith
· constructor <;> intro <;> [skip; apply le_of_lt] <;> apply cyclotomic_pos (by omega)
/-- Cyclotomic polynomials are always positive on inputs larger than one.
Similar to `cyclotomic_pos` but with the condition on the input rather than index of the
cyclotomic polynomial. -/
theorem cyclotomic_pos' (n : ℕ) {R} [LinearOrderedCommRing R] {x : R} (hx : 1 < x) :
0 < eval x (cyclotomic n R) :=
(cyclotomic_pos_and_nonneg n x).1 hx
/-- Cyclotomic polynomials are always nonnegative on inputs one or more. -/
theorem cyclotomic_nonneg (n : ℕ) {R} [LinearOrderedCommRing R] {x : R} (hx : 1 ≤ x) :
0 ≤ eval x (cyclotomic n R) :=
(cyclotomic_pos_and_nonneg n x).2 hx
theorem eval_one_cyclotomic_not_prime_pow {R : Type*} [Ring R] {n : ℕ}
(h : ∀ {p : ℕ}, p.Prime → ∀ k : ℕ, p ^ k ≠ n) : eval 1 (cyclotomic n R) = 1 := by
rcases n.eq_zero_or_pos with (rfl | hn')
· simp
have hn : 1 < n := one_lt_iff_ne_zero_and_ne_one.mpr ⟨hn'.ne', (h Nat.prime_two 0).symm⟩
rsuffices h | h : eval 1 (cyclotomic n ℤ) = 1 ∨ eval 1 (cyclotomic n ℤ) = -1
· have := eval_intCast_map (Int.castRingHom R) (cyclotomic n ℤ) 1
simpa only [map_cyclotomic, Int.cast_one, h, eq_intCast] using this
· exfalso
linarith [cyclotomic_nonneg n (le_refl (1 : ℤ))]
rw [← Int.natAbs_eq_natAbs_iff, Int.natAbs_one, Nat.eq_one_iff_not_exists_prime_dvd]
intro p hp hpe
haveI := Fact.mk hp
have := prod_cyclotomic_eq_geom_sum hn' ℤ
apply_fun eval 1 at this
rw [eval_geom_sum, one_geom_sum, eval_prod, eq_comm, ←
Finset.prod_sdiff <| @range_pow_padicValNat_subset_divisors' p _ _, Finset.prod_image] at this
· simp_rw [eval_one_cyclotomic_prime_pow, Finset.prod_const, Finset.card_range, mul_comm] at this
rw [← Finset.prod_sdiff <| show {n} ⊆ _ from _] at this
swap
· simp only [singleton_subset_iff, mem_sdiff, mem_erase, Ne, mem_divisors, dvd_refl,
true_and_iff, mem_image, mem_range, exists_prop, not_exists, not_and]
exact ⟨⟨hn.ne', hn'.ne'⟩, fun t _ => h hp _⟩
rw [← Int.natAbs_ofNat p, Int.natAbs_dvd_natAbs] at hpe
obtain ⟨t, ht⟩ := hpe
rw [Finset.prod_singleton, ht, mul_left_comm, mul_comm, ← mul_assoc, mul_assoc] at this
have : (p : ℤ) ^ padicValNat p n * p ∣ n := ⟨_, this⟩
simp only [← _root_.pow_succ, ← Int.natAbs_dvd_natAbs, Int.natAbs_ofNat, Int.natAbs_pow] at this
exact pow_succ_padicValNat_not_dvd hn'.ne' this
· rintro x - y - hxy
apply Nat.succ_injective
exact Nat.pow_right_injective hp.two_le hxy
theorem sub_one_pow_totient_lt_cyclotomic_eval {n : ℕ} {q : ℝ} (hn' : 2 ≤ n) (hq' : 1 < q) :
(q - 1) ^ totient n < (cyclotomic n ℝ).eval q := by
have hn : 0 < n := pos_of_gt hn'
have hq := zero_lt_one.trans hq'
have hfor : ∀ ζ' ∈ primitiveRoots n ℂ, q - 1 ≤ ‖↑q - ζ'‖ := by
intro ζ' hζ'
rw [mem_primitiveRoots hn] at hζ'
convert norm_sub_norm_le (↑q) ζ'
· rw [Complex.norm_real, Real.norm_of_nonneg hq.le]
· rw [hζ'.norm'_eq_one hn.ne']
let ζ := Complex.exp (2 * ↑Real.pi * Complex.I / ↑n)
have hζ : IsPrimitiveRoot ζ n := Complex.isPrimitiveRoot_exp n hn.ne'
have hex : ∃ ζ' ∈ primitiveRoots n ℂ, q - 1 < ‖↑q - ζ'‖ := by
refine ⟨ζ, (mem_primitiveRoots hn).mpr hζ, ?_⟩
suffices ¬SameRay ℝ (q : ℂ) ζ by
convert lt_norm_sub_of_not_sameRay this <;>
simp only [hζ.norm'_eq_one hn.ne', Real.norm_of_nonneg hq.le, Complex.norm_real]
rw [Complex.sameRay_iff]
push_neg
refine ⟨mod_cast hq.ne', hζ.ne_zero hn.ne', ?_⟩
rw [Complex.arg_ofReal_of_nonneg hq.le, Ne, eq_comm, hζ.arg_eq_zero_iff hn.ne']
clear_value ζ
rintro rfl
linarith [hζ.unique IsPrimitiveRoot.one]
have : ¬eval (↑q) (cyclotomic n ℂ) = 0 := by
erw [cyclotomic.eval_apply q n (algebraMap ℝ ℂ)]
simpa only [Complex.coe_algebraMap, Complex.ofReal_eq_zero] using (cyclotomic_pos' n hq').ne'
suffices Units.mk0 (Real.toNNReal (q - 1)) (by simp [hq']) ^ totient n <
Units.mk0 ‖(cyclotomic n ℂ).eval ↑q‖₊ (by simp [this]) by
simp only [← Units.val_lt_val, Units.val_pow_eq_pow_val, Units.val_mk0, ← NNReal.coe_lt_coe,
hq'.le, Real.toNNReal_lt_toNNReal_iff_of_nonneg, coe_nnnorm, Complex.norm_eq_abs,
NNReal.coe_pow, Real.coe_toNNReal', max_eq_left, sub_nonneg] at this
convert this
erw [cyclotomic.eval_apply q n (algebraMap ℝ ℂ), eq_comm]
simp only [cyclotomic_nonneg n hq'.le, Complex.coe_algebraMap, Complex.abs_ofReal, abs_eq_self]
simp only [cyclotomic_eq_prod_X_sub_primitiveRoots hζ, eval_prod, eval_C, eval_X, eval_sub,
nnnorm_prod, Units.mk0_prod]
convert Finset.prod_lt_prod' (M := NNRealˣ) _ _
swap; · exact fun _ => Units.mk0 (Real.toNNReal (q - 1)) (by simp [hq'])
· simp only [Complex.card_primitiveRoots, prod_const, card_attach]
· simp only [Subtype.coe_mk, Finset.mem_attach, forall_true_left, Subtype.forall, ←
Units.val_le_val, ← NNReal.coe_le_coe, Complex.abs.nonneg, hq'.le, Units.val_mk0,
Real.coe_toNNReal', coe_nnnorm, Complex.norm_eq_abs, max_le_iff, tsub_le_iff_right]
intro x hx
simpa only [and_true_iff, tsub_le_iff_right] using hfor x hx
· simp only [Subtype.coe_mk, Finset.mem_attach, exists_true_left, Subtype.exists, ←
NNReal.coe_lt_coe, ← Units.val_lt_val, Units.val_mk0 _, coe_nnnorm]
simpa [hq'.le, Real.coe_toNNReal', max_eq_left, sub_nonneg] using hex
theorem sub_one_pow_totient_le_cyclotomic_eval {q : ℝ} (hq' : 1 < q) :
∀ n, (q - 1) ^ totient n ≤ (cyclotomic n ℝ).eval q
| 0 => by simp only [totient_zero, _root_.pow_zero, cyclotomic_zero, eval_one, le_refl]
| 1 => by simp only [totient_one, pow_one, cyclotomic_one, eval_sub, eval_X, eval_one, le_refl]
| n + 2 => (sub_one_pow_totient_lt_cyclotomic_eval le_add_self hq').le
theorem cyclotomic_eval_lt_add_one_pow_totient {n : ℕ} {q : ℝ} (hn' : 3 ≤ n) (hq' : 1 < q) :
(cyclotomic n ℝ).eval q < (q + 1) ^ totient n := by
have hn : 0 < n := pos_of_gt hn'
have hq := zero_lt_one.trans hq'
have hfor : ∀ ζ' ∈ primitiveRoots n ℂ, ‖↑q - ζ'‖ ≤ q + 1 := by
intro ζ' hζ'
rw [mem_primitiveRoots hn] at hζ'
convert norm_sub_le (↑q) ζ'
· rw [Complex.norm_real, Real.norm_of_nonneg (zero_le_one.trans_lt hq').le]
· rw [hζ'.norm'_eq_one hn.ne']
let ζ := Complex.exp (2 * ↑Real.pi * Complex.I / ↑n)
have hζ : IsPrimitiveRoot ζ n := Complex.isPrimitiveRoot_exp n hn.ne'
have hex : ∃ ζ' ∈ primitiveRoots n ℂ, ‖↑q - ζ'‖ < q + 1 := by
refine ⟨ζ, (mem_primitiveRoots hn).mpr hζ, ?_⟩
suffices ¬SameRay ℝ (q : ℂ) (-ζ) by
convert norm_add_lt_of_not_sameRay this using 2
· rw [Complex.norm_eq_abs, Complex.abs_ofReal]
symm
exact abs_eq_self.mpr hq.le
· simp [abs_of_pos hq, hζ.norm'_eq_one hn.ne', -Complex.norm_eq_abs]
rw [Complex.sameRay_iff]
push_neg
refine ⟨mod_cast hq.ne', neg_ne_zero.mpr <| hζ.ne_zero hn.ne', ?_⟩
rw [Complex.arg_ofReal_of_nonneg hq.le, Ne, eq_comm]
intro h
rw [Complex.arg_eq_zero_iff, Complex.neg_re, neg_nonneg, Complex.neg_im, neg_eq_zero] at h
have hζ₀ : ζ ≠ 0 := by
clear_value ζ
rintro rfl
exact hn.ne' (hζ.unique IsPrimitiveRoot.zero)
have : ζ.re < 0 ∧ ζ.im = 0 := ⟨h.1.lt_of_ne ?_, h.2⟩
· rw [← Complex.arg_eq_pi_iff, hζ.arg_eq_pi_iff hn.ne'] at this
rw [this] at hζ
linarith [hζ.unique <| IsPrimitiveRoot.neg_one 0 two_ne_zero.symm]
· contrapose! hζ₀
apply Complex.ext <;> simp [hζ₀, h.2]
have : ¬eval (↑q) (cyclotomic n ℂ) = 0 := by
erw [cyclotomic.eval_apply q n (algebraMap ℝ ℂ)]
simp only [Complex.coe_algebraMap, Complex.ofReal_eq_zero]
exact (cyclotomic_pos' n hq').ne.symm
suffices Units.mk0 ‖(cyclotomic n ℂ).eval ↑q‖₊ (by simp [this]) <
Units.mk0 (Real.toNNReal (q + 1)) (by simp; linarith) ^ totient n by
simp only [← Units.val_lt_val, Units.val_pow_eq_pow_val, Units.val_mk0, ← NNReal.coe_lt_coe,
hq'.le, Real.toNNReal_lt_toNNReal_iff_of_nonneg, coe_nnnorm, Complex.norm_eq_abs,
NNReal.coe_pow, Real.coe_toNNReal', max_eq_left, sub_nonneg] at this
convert this using 2
· erw [cyclotomic.eval_apply q n (algebraMap ℝ ℂ), eq_comm]
simp [cyclotomic_nonneg n hq'.le]
rw [eq_comm, max_eq_left_iff]
linarith
simp only [cyclotomic_eq_prod_X_sub_primitiveRoots hζ, eval_prod, eval_C, eval_X, eval_sub,
nnnorm_prod, Units.mk0_prod]
convert Finset.prod_lt_prod' (M := NNRealˣ) _ _
swap; · exact fun _ => Units.mk0 (Real.toNNReal (q + 1)) (by simp; linarith only [hq'])
· simp [Complex.card_primitiveRoots]
· simp only [Subtype.coe_mk, Finset.mem_attach, forall_true_left, Subtype.forall, ←
Units.val_le_val, ← NNReal.coe_le_coe, Complex.abs.nonneg, hq'.le, Units.val_mk0,
Real.coe_toNNReal, coe_nnnorm, Complex.norm_eq_abs, max_le_iff]
intro x hx
have : Complex.abs _ ≤ _ := hfor x hx
simp [this]
· simp only [Subtype.coe_mk, Finset.mem_attach, exists_true_left, Subtype.exists, ←
NNReal.coe_lt_coe, ← Units.val_lt_val, Units.val_mk0 _, coe_nnnorm]
obtain ⟨ζ, hζ, hhζ : Complex.abs _ < _⟩ := hex
exact ⟨ζ, hζ, by simp [hhζ]⟩
theorem cyclotomic_eval_le_add_one_pow_totient {q : ℝ} (hq' : 1 < q) :
∀ n, (cyclotomic n ℝ).eval q ≤ (q + 1) ^ totient n
| 0 => by simp
| 1 => by simp [add_assoc, add_nonneg, zero_le_one]
| 2 => by simp
| n + 3 => (cyclotomic_eval_lt_add_one_pow_totient le_add_self hq').le
theorem sub_one_pow_totient_lt_natAbs_cyclotomic_eval {n : ℕ} {q : ℕ} (hn' : 1 < n) (hq : q ≠ 1) :
(q - 1) ^ totient n < ((cyclotomic n ℤ).eval ↑q).natAbs := by
rcases hq.lt_or_lt.imp_left Nat.lt_one_iff.mp with (rfl | hq')
· rw [zero_tsub, zero_pow (Nat.totient_pos.2 (pos_of_gt hn')).ne', pos_iff_ne_zero,
Int.natAbs_ne_zero, Nat.cast_zero, ← coeff_zero_eq_eval_zero, cyclotomic_coeff_zero _ hn']
exact one_ne_zero
rw [← @Nat.cast_lt ℝ, Nat.cast_pow, Nat.cast_sub hq'.le, Nat.cast_one, Int.cast_natAbs]
refine (sub_one_pow_totient_lt_cyclotomic_eval hn' (Nat.one_lt_cast.2 hq')).trans_le ?_
convert (cyclotomic.eval_apply (q : ℤ) n (algebraMap ℤ ℝ)).trans_le (le_abs_self _)
simp
theorem sub_one_lt_natAbs_cyclotomic_eval {n : ℕ} {q : ℕ} (hn' : 1 < n) (hq : q ≠ 1) :
q - 1 < ((cyclotomic n ℤ).eval ↑q).natAbs :=
calc
q - 1 ≤ (q - 1) ^ totient n := Nat.le_self_pow (Nat.totient_pos.2 <| pos_of_gt hn').ne' _
_ < ((cyclotomic n ℤ).eval ↑q).natAbs := sub_one_pow_totient_lt_natAbs_cyclotomic_eval hn' hq
end Polynomial
|
RingTheory\Polynomial\Cyclotomic\Expand.lean | /-
Copyright (c) 2020 Riccardo Brasca. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Riccardo Brasca
-/
import Mathlib.RingTheory.Polynomial.Cyclotomic.Roots
import Mathlib.Data.ZMod.Algebra
/-!
# Cyclotomic polynomials and `expand`.
We gather results relating cyclotomic polynomials and `expand`.
## Main results
* `Polynomial.cyclotomic_expand_eq_cyclotomic_mul` : If `p` is a prime such that `¬ p ∣ n`, then
`expand R p (cyclotomic n R) = (cyclotomic (n * p) R) * (cyclotomic n R)`.
* `Polynomial.cyclotomic_expand_eq_cyclotomic` : If `p` is a prime such that `p ∣ n`, then
`expand R p (cyclotomic n R) = cyclotomic (p * n) R`.
* `Polynomial.cyclotomic_mul_prime_eq_pow_of_not_dvd` : If `R` is of characteristic `p` and
`¬p ∣ n`, then `cyclotomic (n * p) R = (cyclotomic n R) ^ (p - 1)`.
* `Polynomial.cyclotomic_mul_prime_dvd_eq_pow` : If `R` is of characteristic `p` and `p ∣ n`, then
`cyclotomic (n * p) R = (cyclotomic n R) ^ p`.
* `Polynomial.cyclotomic_mul_prime_pow_eq` : If `R` is of characteristic `p` and `¬p ∣ m`, then
`cyclotomic (p ^ k * m) R = (cyclotomic m R) ^ (p ^ k - p ^ (k - 1))`.
-/
namespace Polynomial
/-- If `p` is a prime such that `¬ p ∣ n`, then
`expand R p (cyclotomic n R) = (cyclotomic (n * p) R) * (cyclotomic n R)`. -/
@[simp]
theorem cyclotomic_expand_eq_cyclotomic_mul {p n : ℕ} (hp : Nat.Prime p) (hdiv : ¬p ∣ n)
(R : Type*) [CommRing R] :
expand R p (cyclotomic n R) = cyclotomic (n * p) R * cyclotomic n R := by
rcases Nat.eq_zero_or_pos n with (rfl | hnpos)
· simp
haveI := NeZero.of_pos hnpos
suffices expand ℤ p (cyclotomic n ℤ) = cyclotomic (n * p) ℤ * cyclotomic n ℤ by
rw [← map_cyclotomic_int, ← map_expand, this, Polynomial.map_mul, map_cyclotomic_int,
map_cyclotomic]
refine eq_of_monic_of_dvd_of_natDegree_le ((cyclotomic.monic _ ℤ).mul (cyclotomic.monic _ ℤ))
((cyclotomic.monic n ℤ).expand hp.pos) ?_ ?_
· refine (IsPrimitive.Int.dvd_iff_map_cast_dvd_map_cast _ _
(IsPrimitive.mul (cyclotomic.isPrimitive (n * p) ℤ) (cyclotomic.isPrimitive n ℤ))
((cyclotomic.monic n ℤ).expand hp.pos).isPrimitive).2 ?_
rw [Polynomial.map_mul, map_cyclotomic_int, map_cyclotomic_int, map_expand, map_cyclotomic_int]
refine IsCoprime.mul_dvd (cyclotomic.isCoprime_rat fun h => ?_) ?_ ?_
· replace h : n * p = n * 1 := by simp [h]
exact Nat.Prime.ne_one hp (mul_left_cancel₀ hnpos.ne' h)
· have hpos : 0 < n * p := mul_pos hnpos hp.pos
have hprim := Complex.isPrimitiveRoot_exp _ hpos.ne'
rw [cyclotomic_eq_minpoly_rat hprim hpos]
refine minpoly.dvd ℚ _ ?_
rw [aeval_def, ← eval_map, map_expand, map_cyclotomic, expand_eval, ← IsRoot.def,
@isRoot_cyclotomic_iff]
convert IsPrimitiveRoot.pow_of_dvd hprim hp.ne_zero (dvd_mul_left p n)
rw [Nat.mul_div_cancel _ (Nat.Prime.pos hp)]
· have hprim := Complex.isPrimitiveRoot_exp _ hnpos.ne.symm
rw [cyclotomic_eq_minpoly_rat hprim hnpos]
refine minpoly.dvd ℚ _ ?_
rw [aeval_def, ← eval_map, map_expand, expand_eval, ← IsRoot.def, ←
cyclotomic_eq_minpoly_rat hprim hnpos, map_cyclotomic, @isRoot_cyclotomic_iff]
exact IsPrimitiveRoot.pow_of_prime hprim hp hdiv
· rw [natDegree_expand, natDegree_cyclotomic,
natDegree_mul (cyclotomic_ne_zero _ ℤ) (cyclotomic_ne_zero _ ℤ), natDegree_cyclotomic,
natDegree_cyclotomic, mul_comm n,
Nat.totient_mul ((Nat.Prime.coprime_iff_not_dvd hp).2 hdiv), Nat.totient_prime hp,
mul_comm (p - 1), ← Nat.mul_succ, Nat.sub_one, Nat.succ_pred_eq_of_pos hp.pos]
/-- If `p` is a prime such that `p ∣ n`, then
`expand R p (cyclotomic n R) = cyclotomic (p * n) R`. -/
@[simp]
theorem cyclotomic_expand_eq_cyclotomic {p n : ℕ} (hp : Nat.Prime p) (hdiv : p ∣ n) (R : Type*)
[CommRing R] : expand R p (cyclotomic n R) = cyclotomic (n * p) R := by
rcases n.eq_zero_or_pos with (rfl | hzero)
· simp
haveI := NeZero.of_pos hzero
suffices expand ℤ p (cyclotomic n ℤ) = cyclotomic (n * p) ℤ by
rw [← map_cyclotomic_int, ← map_expand, this, map_cyclotomic_int]
refine eq_of_monic_of_dvd_of_natDegree_le (cyclotomic.monic _ ℤ)
((cyclotomic.monic n ℤ).expand hp.pos) ?_ ?_
· have hpos := Nat.mul_pos hzero hp.pos
have hprim := Complex.isPrimitiveRoot_exp _ hpos.ne.symm
rw [cyclotomic_eq_minpoly hprim hpos]
refine minpoly.isIntegrallyClosed_dvd (hprim.isIntegral hpos) ?_
rw [aeval_def, ← eval_map, map_expand, map_cyclotomic, expand_eval, ← IsRoot.def,
@isRoot_cyclotomic_iff]
convert IsPrimitiveRoot.pow_of_dvd hprim hp.ne_zero (dvd_mul_left p n)
rw [Nat.mul_div_cancel _ hp.pos]
· rw [natDegree_expand, natDegree_cyclotomic, natDegree_cyclotomic, mul_comm n,
Nat.totient_mul_of_prime_of_dvd hp hdiv, mul_comm]
/-- If the `p ^ n`th cyclotomic polynomial is irreducible, so is the `p ^ m`th, for `m ≤ n`. -/
theorem cyclotomic_irreducible_pow_of_irreducible_pow {p : ℕ} (hp : Nat.Prime p) {R} [CommRing R]
[IsDomain R] {n m : ℕ} (hmn : m ≤ n) (h : Irreducible (cyclotomic (p ^ n) R)) :
Irreducible (cyclotomic (p ^ m) R) := by
rcases m.eq_zero_or_pos with (rfl | hm)
· simpa using irreducible_X_sub_C (1 : R)
obtain ⟨k, rfl⟩ := Nat.exists_eq_add_of_le hmn
induction' k with k hk
· simpa using h
have : m + k ≠ 0 := (add_pos_of_pos_of_nonneg hm k.zero_le).ne'
rw [Nat.add_succ, pow_succ, ← cyclotomic_expand_eq_cyclotomic hp <| dvd_pow_self p this] at h
exact hk (by omega) (of_irreducible_expand hp.ne_zero h)
/-- If `Irreducible (cyclotomic (p ^ n) R)` then `Irreducible (cyclotomic p R).` -/
theorem cyclotomic_irreducible_of_irreducible_pow {p : ℕ} (hp : Nat.Prime p) {R} [CommRing R]
[IsDomain R] {n : ℕ} (hn : n ≠ 0) (h : Irreducible (cyclotomic (p ^ n) R)) :
Irreducible (cyclotomic p R) :=
pow_one p ▸ cyclotomic_irreducible_pow_of_irreducible_pow hp hn.bot_lt h
section CharP
/-- If `R` is of characteristic `p` and `¬p ∣ n`, then
`cyclotomic (n * p) R = (cyclotomic n R) ^ (p - 1)`. -/
theorem cyclotomic_mul_prime_eq_pow_of_not_dvd (R : Type*) {p n : ℕ} [hp : Fact (Nat.Prime p)]
[Ring R] [CharP R p] (hn : ¬p ∣ n) : cyclotomic (n * p) R = cyclotomic n R ^ (p - 1) := by
letI : Algebra (ZMod p) R := ZMod.algebra _ _
suffices cyclotomic (n * p) (ZMod p) = cyclotomic n (ZMod p) ^ (p - 1) by
rw [← map_cyclotomic _ (algebraMap (ZMod p) R), ← map_cyclotomic _ (algebraMap (ZMod p) R),
this, Polynomial.map_pow]
apply mul_right_injective₀ (cyclotomic_ne_zero n <| ZMod p); dsimp
rw [← pow_succ', tsub_add_cancel_of_le hp.out.one_lt.le, mul_comm, ← ZMod.expand_card]
conv_rhs => rw [← map_cyclotomic_int]
rw [← map_expand, cyclotomic_expand_eq_cyclotomic_mul hp.out hn, Polynomial.map_mul,
map_cyclotomic, map_cyclotomic]
/-- If `R` is of characteristic `p` and `p ∣ n`, then
`cyclotomic (n * p) R = (cyclotomic n R) ^ p`. -/
theorem cyclotomic_mul_prime_dvd_eq_pow (R : Type*) {p n : ℕ} [hp : Fact (Nat.Prime p)] [Ring R]
[CharP R p] (hn : p ∣ n) : cyclotomic (n * p) R = cyclotomic n R ^ p := by
letI : Algebra (ZMod p) R := ZMod.algebra _ _
suffices cyclotomic (n * p) (ZMod p) = cyclotomic n (ZMod p) ^ p by
rw [← map_cyclotomic _ (algebraMap (ZMod p) R), ← map_cyclotomic _ (algebraMap (ZMod p) R),
this, Polynomial.map_pow]
rw [← ZMod.expand_card, ← map_cyclotomic_int n, ← map_expand,
cyclotomic_expand_eq_cyclotomic hp.out hn, map_cyclotomic]
/-- If `R` is of characteristic `p` and `¬p ∣ m`, then
`cyclotomic (p ^ k * m) R = (cyclotomic m R) ^ (p ^ k - p ^ (k - 1))`. -/
theorem cyclotomic_mul_prime_pow_eq (R : Type*) {p m : ℕ} [Fact (Nat.Prime p)] [Ring R] [CharP R p]
(hm : ¬p ∣ m) : ∀ {k}, 0 < k → cyclotomic (p ^ k * m) R = cyclotomic m R ^ (p ^ k - p ^ (k - 1))
| 1, _ => by
rw [pow_one, Nat.sub_self, pow_zero, mul_comm, cyclotomic_mul_prime_eq_pow_of_not_dvd R hm]
| a + 2, _ => by
have hdiv : p ∣ p ^ a.succ * m := ⟨p ^ a * m, by rw [← mul_assoc, pow_succ']⟩
rw [pow_succ', mul_assoc, mul_comm, cyclotomic_mul_prime_dvd_eq_pow R hdiv,
cyclotomic_mul_prime_pow_eq _ _ a.succ_pos, ← pow_mul]
· simp only [tsub_zero, Nat.succ_sub_succ_eq_sub]
rw [Nat.mul_sub_right_distrib, mul_comm, pow_succ]
· assumption
/-- If `R` is of characteristic `p` and `¬p ∣ m`, then `ζ` is a root of `cyclotomic (p ^ k * m) R`
if and only if it is a primitive `m`-th root of unity. -/
theorem isRoot_cyclotomic_prime_pow_mul_iff_of_charP {m k p : ℕ} {R : Type*} [CommRing R]
[IsDomain R] [hp : Fact (Nat.Prime p)] [hchar : CharP R p] {μ : R} [NeZero (m : R)] :
(Polynomial.cyclotomic (p ^ k * m) R).IsRoot μ ↔ IsPrimitiveRoot μ m := by
rcases k.eq_zero_or_pos with (rfl | hk)
· rw [pow_zero, one_mul, isRoot_cyclotomic_iff]
refine ⟨fun h => ?_, fun h => ?_⟩
· rw [IsRoot.def, cyclotomic_mul_prime_pow_eq R (NeZero.not_char_dvd R p m) hk, eval_pow]
at h
replace h := pow_eq_zero h
rwa [← IsRoot.def, isRoot_cyclotomic_iff] at h
· rw [← isRoot_cyclotomic_iff, IsRoot.def] at h
rw [cyclotomic_mul_prime_pow_eq R (NeZero.not_char_dvd R p m) hk, IsRoot.def, eval_pow,
h, zero_pow]
exact Nat.sub_ne_zero_of_lt $ pow_right_strictMono hp.out.one_lt $ Nat.pred_lt hk.ne'
end CharP
end Polynomial
|
RingTheory\Polynomial\Cyclotomic\Roots.lean | /-
Copyright (c) 2020 Riccardo Brasca. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Riccardo Brasca
-/
import Mathlib.RingTheory.Polynomial.Cyclotomic.Basic
import Mathlib.RingTheory.RootsOfUnity.Minpoly
/-!
# Roots of cyclotomic polynomials.
We gather results about roots of cyclotomic polynomials. In particular we show in
`Polynomial.cyclotomic_eq_minpoly` that `cyclotomic n R` is the minimal polynomial of a primitive
root of unity.
## Main results
* `IsPrimitiveRoot.isRoot_cyclotomic` : Any `n`-th primitive root of unity is a root of
`cyclotomic n R`.
* `isRoot_cyclotomic_iff` : if `NeZero (n : R)`, then `μ` is a root of `cyclotomic n R`
if and only if `μ` is a primitive root of unity.
* `Polynomial.cyclotomic_eq_minpoly` : `cyclotomic n ℤ` is the minimal polynomial of a primitive
`n`-th root of unity `μ`.
* `Polynomial.cyclotomic.irreducible` : `cyclotomic n ℤ` is irreducible.
## Implementation details
To prove `Polynomial.cyclotomic.irreducible`, the irreducibility of `cyclotomic n ℤ`, we show in
`Polynomial.cyclotomic_eq_minpoly` that `cyclotomic n ℤ` is the minimal polynomial of any `n`-th
primitive root of unity `μ : K`, where `K` is a field of characteristic `0`.
-/
namespace Polynomial
variable {R : Type*} [CommRing R] {n : ℕ}
theorem isRoot_of_unity_of_root_cyclotomic {ζ : R} {i : ℕ} (hi : i ∈ n.divisors)
(h : (cyclotomic i R).IsRoot ζ) : ζ ^ n = 1 := by
rcases n.eq_zero_or_pos with (rfl | hn)
· exact pow_zero _
have := congr_arg (eval ζ) (prod_cyclotomic_eq_X_pow_sub_one hn R).symm
rw [eval_sub, eval_pow, eval_X, eval_one] at this
convert eq_add_of_sub_eq' this
convert (add_zero (M := R) _).symm
apply eval_eq_zero_of_dvd_of_eval_eq_zero _ h
exact Finset.dvd_prod_of_mem _ hi
section IsDomain
variable [IsDomain R]
theorem _root_.isRoot_of_unity_iff (h : 0 < n) (R : Type*) [CommRing R] [IsDomain R] {ζ : R} :
ζ ^ n = 1 ↔ ∃ i ∈ n.divisors, (cyclotomic i R).IsRoot ζ := by
rw [← mem_nthRoots h, nthRoots, mem_roots <| X_pow_sub_C_ne_zero h _, C_1, ←
prod_cyclotomic_eq_X_pow_sub_one h, isRoot_prod]
/-- Any `n`-th primitive root of unity is a root of `cyclotomic n R`. -/
theorem _root_.IsPrimitiveRoot.isRoot_cyclotomic (hpos : 0 < n) {μ : R} (h : IsPrimitiveRoot μ n) :
IsRoot (cyclotomic n R) μ := by
rw [← mem_roots (cyclotomic_ne_zero n R), cyclotomic_eq_prod_X_sub_primitiveRoots h,
roots_prod_X_sub_C, ← Finset.mem_def]
rwa [← mem_primitiveRoots hpos] at h
private theorem isRoot_cyclotomic_iff' {n : ℕ} {K : Type*} [Field K] {μ : K} [NeZero (n : K)] :
IsRoot (cyclotomic n K) μ ↔ IsPrimitiveRoot μ n := by
-- in this proof, `o` stands for `orderOf μ`
have hnpos : 0 < n := (NeZero.of_neZero_natCast K).out.bot_lt
refine ⟨fun hμ => ?_, IsPrimitiveRoot.isRoot_cyclotomic hnpos⟩
have hμn : μ ^ n = 1 := by
rw [isRoot_of_unity_iff hnpos _]
exact ⟨n, n.mem_divisors_self hnpos.ne', hμ⟩
by_contra hnμ
have ho : 0 < orderOf μ := (isOfFinOrder_iff_pow_eq_one.2 <| ⟨n, hnpos, hμn⟩).orderOf_pos
have := pow_orderOf_eq_one μ
rw [isRoot_of_unity_iff ho] at this
obtain ⟨i, hio, hiμ⟩ := this
replace hio := Nat.dvd_of_mem_divisors hio
rw [IsPrimitiveRoot.not_iff] at hnμ
rw [← orderOf_dvd_iff_pow_eq_one] at hμn
have key : i < n := (Nat.le_of_dvd ho hio).trans_lt ((Nat.le_of_dvd hnpos hμn).lt_of_ne hnμ)
have key' : i ∣ n := hio.trans hμn
rw [← Polynomial.dvd_iff_isRoot] at hμ hiμ
have hni : {i, n} ⊆ n.divisors := by simpa [Finset.insert_subset_iff, key'] using hnpos.ne'
obtain ⟨k, hk⟩ := hiμ
obtain ⟨j, hj⟩ := hμ
have := prod_cyclotomic_eq_X_pow_sub_one hnpos K
rw [← Finset.prod_sdiff hni, Finset.prod_pair key.ne, hk, hj] at this
have hn := (X_pow_sub_one_separable_iff.mpr <| NeZero.natCast_ne n K).squarefree
rw [← this, Squarefree] at hn
specialize hn (X - C μ) ⟨(∏ x ∈ n.divisors \ {i, n}, cyclotomic x K) * k * j, by ring⟩
simp [Polynomial.isUnit_iff_degree_eq_zero] at hn
theorem isRoot_cyclotomic_iff [NeZero (n : R)] {μ : R} :
IsRoot (cyclotomic n R) μ ↔ IsPrimitiveRoot μ n := by
have hf : Function.Injective _ := IsFractionRing.injective R (FractionRing R)
haveI : NeZero (n : FractionRing R) := NeZero.nat_of_injective hf
rw [← isRoot_map_iff hf, ← IsPrimitiveRoot.map_iff_of_injective hf, map_cyclotomic, ←
isRoot_cyclotomic_iff']
theorem roots_cyclotomic_nodup [NeZero (n : R)] : (cyclotomic n R).roots.Nodup := by
obtain h | ⟨ζ, hζ⟩ := (cyclotomic n R).roots.empty_or_exists_mem
· exact h.symm ▸ Multiset.nodup_zero
rw [mem_roots <| cyclotomic_ne_zero n R, isRoot_cyclotomic_iff] at hζ
refine Multiset.nodup_of_le
(roots.le_of_dvd (X_pow_sub_C_ne_zero (NeZero.pos_of_neZero_natCast R) 1) <|
cyclotomic.dvd_X_pow_sub_one n R) hζ.nthRoots_one_nodup
theorem cyclotomic.roots_to_finset_eq_primitiveRoots [NeZero (n : R)] :
(⟨(cyclotomic n R).roots, roots_cyclotomic_nodup⟩ : Finset _) = primitiveRoots n R := by
ext a
-- Porting note: was
-- `simp [cyclotomic_ne_zero n R, isRoot_cyclotomic_iff, mem_primitiveRoots,`
-- ` NeZero.pos_of_neZero_natCast R]`
simp only [mem_primitiveRoots, NeZero.pos_of_neZero_natCast R]
convert isRoot_cyclotomic_iff (n := n) (μ := a)
simp [cyclotomic_ne_zero n R]
theorem cyclotomic.roots_eq_primitiveRoots_val [NeZero (n : R)] :
(cyclotomic n R).roots = (primitiveRoots n R).val := by
rw [← cyclotomic.roots_to_finset_eq_primitiveRoots]
/-- If `R` is of characteristic zero, then `ζ` is a root of `cyclotomic n R` if and only if it is a
primitive `n`-th root of unity. -/
theorem isRoot_cyclotomic_iff_charZero {n : ℕ} {R : Type*} [CommRing R] [IsDomain R] [CharZero R]
{μ : R} (hn : 0 < n) : (Polynomial.cyclotomic n R).IsRoot μ ↔ IsPrimitiveRoot μ n :=
letI := NeZero.of_gt hn
isRoot_cyclotomic_iff
end IsDomain
/-- Over a ring `R` of characteristic zero, `fun n => cyclotomic n R` is injective. -/
theorem cyclotomic_injective [CharZero R] : Function.Injective fun n => cyclotomic n R := by
intro n m hnm
simp only at hnm
rcases eq_or_ne n 0 with (rfl | hzero)
· rw [cyclotomic_zero] at hnm
replace hnm := congr_arg natDegree hnm
rwa [natDegree_one, natDegree_cyclotomic, eq_comm, Nat.totient_eq_zero, eq_comm] at hnm
· haveI := NeZero.mk hzero
rw [← map_cyclotomic_int _ R, ← map_cyclotomic_int _ R] at hnm
replace hnm := map_injective (Int.castRingHom R) Int.cast_injective hnm
replace hnm := congr_arg (map (Int.castRingHom ℂ)) hnm
rw [map_cyclotomic_int, map_cyclotomic_int] at hnm
have hprim := Complex.isPrimitiveRoot_exp _ hzero
have hroot := isRoot_cyclotomic_iff (R := ℂ).2 hprim
rw [hnm] at hroot
haveI hmzero : NeZero m := ⟨fun h => by simp [h] at hroot⟩
rw [isRoot_cyclotomic_iff (R := ℂ)] at hroot
replace hprim := hprim.eq_orderOf
rwa [← IsPrimitiveRoot.eq_orderOf hroot] at hprim
/-- The minimal polynomial of a primitive `n`-th root of unity `μ` divides `cyclotomic n ℤ`. -/
theorem _root_.IsPrimitiveRoot.minpoly_dvd_cyclotomic {n : ℕ} {K : Type*} [Field K] {μ : K}
(h : IsPrimitiveRoot μ n) (hpos : 0 < n) [CharZero K] : minpoly ℤ μ ∣ cyclotomic n ℤ := by
apply minpoly.isIntegrallyClosed_dvd (h.isIntegral hpos)
simpa [aeval_def, eval₂_eq_eval_map, IsRoot.def] using h.isRoot_cyclotomic hpos
section minpoly
open IsPrimitiveRoot Complex
theorem _root_.IsPrimitiveRoot.minpoly_eq_cyclotomic_of_irreducible {K : Type*} [Field K]
{R : Type*} [CommRing R] [IsDomain R] {μ : R} {n : ℕ} [Algebra K R] (hμ : IsPrimitiveRoot μ n)
(h : Irreducible <| cyclotomic n K) [NeZero (n : K)] : cyclotomic n K = minpoly K μ := by
haveI := NeZero.of_noZeroSMulDivisors K R n
refine minpoly.eq_of_irreducible_of_monic h ?_ (cyclotomic.monic n K)
rwa [aeval_def, eval₂_eq_eval_map, map_cyclotomic, ← IsRoot.def, isRoot_cyclotomic_iff]
/-- `cyclotomic n ℤ` is the minimal polynomial of a primitive `n`-th root of unity `μ`. -/
theorem cyclotomic_eq_minpoly {n : ℕ} {K : Type*} [Field K] {μ : K} (h : IsPrimitiveRoot μ n)
(hpos : 0 < n) [CharZero K] : cyclotomic n ℤ = minpoly ℤ μ := by
refine eq_of_monic_of_dvd_of_natDegree_le (minpoly.monic (IsPrimitiveRoot.isIntegral h hpos))
(cyclotomic.monic n ℤ) (h.minpoly_dvd_cyclotomic hpos) ?_
simpa [natDegree_cyclotomic n ℤ] using totient_le_degree_minpoly h
/-- `cyclotomic n ℚ` is the minimal polynomial of a primitive `n`-th root of unity `μ`. -/
theorem cyclotomic_eq_minpoly_rat {n : ℕ} {K : Type*} [Field K] {μ : K} (h : IsPrimitiveRoot μ n)
(hpos : 0 < n) [CharZero K] : cyclotomic n ℚ = minpoly ℚ μ := by
rw [← map_cyclotomic_int, cyclotomic_eq_minpoly h hpos]
exact (minpoly.isIntegrallyClosed_eq_field_fractions' _ (IsPrimitiveRoot.isIntegral h hpos)).symm
/-- `cyclotomic n ℤ` is irreducible. -/
theorem cyclotomic.irreducible {n : ℕ} (hpos : 0 < n) : Irreducible (cyclotomic n ℤ) := by
rw [cyclotomic_eq_minpoly (isPrimitiveRoot_exp n hpos.ne') hpos]
apply minpoly.irreducible
exact (isPrimitiveRoot_exp n hpos.ne').isIntegral hpos
/-- `cyclotomic n ℚ` is irreducible. -/
theorem cyclotomic.irreducible_rat {n : ℕ} (hpos : 0 < n) : Irreducible (cyclotomic n ℚ) := by
rw [← map_cyclotomic_int]
exact (IsPrimitive.irreducible_iff_irreducible_map_fraction_map (cyclotomic.isPrimitive n ℤ)).1
(cyclotomic.irreducible hpos)
/-- If `n ≠ m`, then `(cyclotomic n ℚ)` and `(cyclotomic m ℚ)` are coprime. -/
theorem cyclotomic.isCoprime_rat {n m : ℕ} (h : n ≠ m) :
IsCoprime (cyclotomic n ℚ) (cyclotomic m ℚ) := by
rcases n.eq_zero_or_pos with (rfl | hnzero)
· exact isCoprime_one_left
rcases m.eq_zero_or_pos with (rfl | hmzero)
· exact isCoprime_one_right
rw [Irreducible.coprime_iff_not_dvd <| cyclotomic.irreducible_rat <| hnzero]
exact fun hdiv => h <| cyclotomic_injective <|
eq_of_monic_of_associated (cyclotomic.monic n ℚ) (cyclotomic.monic m ℚ) <|
Irreducible.associated_of_dvd (cyclotomic.irreducible_rat hnzero)
(cyclotomic.irreducible_rat hmzero) hdiv
end minpoly
end Polynomial
|
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