import Mathlib
import OAI.Geometry.CubicTorus.Regularity.SmoothApproxLip

namespace OAI

section
section
noncomputable section
open MeasureTheory Set
open scoped ENNReal
namespace CubicTorus
open Filter
open scoped Topology RealInnerProductSpace

lemma cutoff_integration_by_parts {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) (hb : ∀ x, f x ∈ Icc 0 1)
    {F : Cover → Cover} (hF : AdmissibleField F) :
    (∫ x in fundamentalCube, f x * divergence F x) =
      -(∫ x in fundamentalCube, ⟪gradient f x, F x⟫) := by
  have hi₁ : IntegrableOn (fun x ↦ f x * divergence F x) fundamentalCube :=
    integrableOn_periodic_continuous (hf.continuous.mul (continuous_divergence hF.1))
      (fun n x ↦ by rw [hp, divergence_periodic hF.2.1])
  have hi₂ : IntegrableOn (fun x ↦ ⟪gradient f x, F x⟫) fundamentalCube :=
    integrableOn_periodic_continuous ((gradient_contDiff hf).continuous.inner hF.1.continuous)
      (fun n x ↦ by rw [gradient_periodic hp, hF.2.1])
  have hz := integral_divergence_fundamentalCube (admissible_cutoff hf hp hb hF)
  simp_rw [divergence_smul_function (hf.differentiable (by simp)).differentiableAt
    (hF.1.differentiable (by simp)).differentiableAt] at hz
  rw [integral_add hi₁ hi₂] at hz
  linarith

def gradientCalibration (f : Cover → ℝ) (ε : ℝ) (x : Cover) : Cover :=
  -(Real.sqrt (⟪gradient f x, gradient f x⟫ + ε ^ 2))⁻¹ • gradient f x

lemma gradientCalibration_admissible {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) {ε : ℝ} (hε : 0 < ε) :
    AdmissibleField (gradientCalibration f ε) := by
  have hg := gradient_contDiff hf
  have hpos (x : Cover) : 0 < ⟪gradient f x, gradient f x⟫ + ε ^ 2 :=
    add_pos_of_nonneg_of_pos (real_inner_self_nonneg) (sq_pos_of_pos hε)
  refine ⟨?_, ?_, ?_⟩
  · exact (((ContDiff.inner ℝ hg hg).add contDiff_const).sqrt (fun x ↦ (hpos x).ne')).inv
      (fun x ↦ (Real.sqrt_pos.2 (hpos x)).ne') |>.neg |>.smul hg
  · intro n x
    dsimp only [gradientCalibration]
    rw [gradient_periodic hp]
  · intro x
    rw [gradientCalibration, norm_smul, Real.norm_eq_abs, abs_neg,
      abs_of_pos (inv_pos.2 (Real.sqrt_pos.2 (hpos x)))]
    rw [inv_mul_le_iff₀ (Real.sqrt_pos.2 (hpos x)), mul_one]
    apply Real.le_sqrt_of_sq_le
    rw [real_inner_self_eq_norm_sq]
    linarith [sq_nonneg ε]

lemma inner_gradientCalibration (f : Cover → ℝ) (ε : ℝ) (x : Cover) :
    ⟪gradient f x, gradientCalibration f ε x⟫ =
      -(‖gradient f x‖ ^ 2 / Real.sqrt (‖gradient f x‖ ^ 2 + ε ^ 2)) := by
  simp [gradientCalibration, inner_smul_right, div_eq_mul_inv, mul_comm]

end CubicTorus

namespace CubicTorus
open Filter
open scoped Topology RealInnerProductSpace

theorem integral_norm_gradient_le {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) (hb : ∀ x, f x ∈ Icc 0 1)
    {K : ℝ} (hK : ∀ F : Cover → Cover, AdmissibleField F →
      (∫ x in fundamentalCube, f x * divergence F x) ≤ K) :
    (∫ x in fundamentalCube, ‖gradient f x‖) ≤ K := by
  let ε : ℕ → ℝ := fun n ↦ 1 / ((n : ℝ) + 1)
  have hε (n) : 0 < ε n := by dsimp [ε]; positivity
  have hεlim : Tendsto ε atTop (𝓝 0) := tendsto_one_div_add_atTop_nhds_zero_nat
  let g : ℕ → Cover → ℝ := fun n x ↦
    ‖gradient f x‖ ^ 2 / Real.sqrt (‖gradient f x‖ ^ 2 + ε n ^ 2)
  have hgcont (n) : Continuous (g n) := by
    apply Continuous.div ((gradient_contDiff hf).continuous.norm.pow 2)
      (((gradient_contDiff hf).continuous.norm.pow 2).add continuous_const).sqrt
    intro x
    exact (Real.sqrt_pos.2 (add_pos_of_nonneg_of_pos (sq_nonneg _) (sq_pos_of_pos (hε n)))).ne'
  have hgbd (n) (x) : ‖g n x‖ ≤ ‖gradient f x‖ := by
    have hsq : 0 < ‖gradient f x‖ ^ 2 + ε n ^ 2 :=
      add_pos_of_nonneg_of_pos (sq_nonneg _) (sq_pos_of_pos (hε n))
    rw [Real.norm_eq_abs, abs_of_nonneg (by dsimp [g]; positivity)]
    dsimp only [g]
    apply (div_le_iff₀ (Real.sqrt_pos.2 hsq)).2
    calc
      _ = ‖gradient f x‖ * ‖gradient f x‖ := sq _
      _ ≤ _ := mul_le_mul_of_nonneg_left
        (Real.le_sqrt_of_sq_le (by linarith [sq_nonneg (ε n)])) (norm_nonneg _)
  have hglim (x) : Tendsto (fun n ↦ g n x) atTop (𝓝 ‖gradient f x‖) := by
    by_cases hz : gradient f x = 0
    · simp [g, hz]
    have hn : ‖gradient f x‖ ≠ 0 := norm_ne_zero_iff.2 hz
    have hl := (tendsto_const_nhds (x := ‖gradient f x‖ ^ 2)).div
      (Real.continuous_sqrt.tendsto _ |>.comp ((tendsto_const_nhds (x := ‖gradient f x‖ ^ 2)).add (hεlim.pow 2)))
      (by simpa only [zero_pow (by norm_num : 2 ≠ 0), add_zero, Real.sqrt_sq (norm_nonneg _)] using hn : Real.sqrt (‖gradient f x‖ ^ 2 + 0 ^ 2) ≠ 0)
    convert hl using 1
    · ext n
      rfl
    · simp [pow_two, hn]
  have hI : IntegrableOn (fun x ↦ ‖gradient f x‖) fundamentalCube :=
    integrableOn_periodic_continuous (gradient_contDiff hf).continuous.norm
      (fun n x ↦ by rw [gradient_periodic hp])
  have hconv := tendsto_integral_of_dominated_convergence (fun x ↦ ‖gradient f x‖)
    (fun n ↦ (hgcont n).aestronglyMeasurable) hI
    (fun n ↦ Eventually.of_forall (hgbd n)) (Eventually.of_forall hglim)
  apply le_of_tendsto hconv
  apply Eventually.of_forall
  intro n
  have hc := gradientCalibration_admissible hf hp (hε n)
  have hi := cutoff_integration_by_parts hf hp hb hc
  simp_rw [inner_gradientCalibration, integral_neg, neg_neg] at hi
  rw [← hi]
  exact hK _ hc

end CubicTorus

namespace CubicTorus
open Filter
open scoped Topology Convolution RealInnerProductSpace

lemma characteristic_nonneg (E : Set Torus) (x : Torus) : 0 ≤ characteristic E x := by
  rcases characteristic_dichotomy E x with h | h <;> simp [h]

lemma smoothApprox_characteristic_range (E : Set Torus) (n : ℕ) (x : Cover) :
    smoothApprox (characteristic E ∘ quotientMap) n x ∈ Icc (0 : ℝ) 1 := by
  refine ⟨?_, (le_abs_self _).trans (smoothApprox_bound
    (fun y ↦ characteristic_bound E (quotientMap y)) n x)⟩
  unfold smoothApprox
  rw [convolution_def]
  apply integral_nonneg
  intro y
  exact mul_nonneg ((mollifier n).nonneg_normed y) (characteristic_nonneg E _)

lemma characteristic_lift_periodic (E : Set Torus) (n : Fin 3 → ℤ) (x : Cover) :
    (characteristic E ∘ quotientMap) (x + latticeVector n) =
      (characteristic E ∘ quotientMap) x := by
  simp only [Function.comp_def, quotientMap_add, quotientMap_latticeVector, add_zero]

lemma torusApprox_eq_integral (E : Set Torus) (n : ℕ) (x : Torus) :
    torusApprox E n x = ∫ y : Cover, (mollifier n).normed volume y *
      characteristic E (x - quotientMap y) := by
  simp only [torusApprox, smoothApprox, convolution_def, ContinuousLinearMap.lsmul_apply,
    smul_eq_mul, Function.comp_def, quotientMap_sub, quotientMap_representative]

theorem smoothApprox_flux_le {E : Set Torus} (hE : MeasurableSet E)
    (hP : perimeter E ≠ ⊤) (n : ℕ) {F : Cover → Cover} (hF : AdmissibleField F) :
    (∫ x in fundamentalCube, smoothApprox (characteristic E ∘ quotientMap) n x *
      divergence F x) ≤ (perimeter E).toReal := by
  let φ : Cover → ℝ := (mollifier n).normed volume
  let D : Torus → ℝ := fun x ↦ divergence F (representative x)
  let d : Torus × Cover → ℝ := fun z ↦ characteristic E (z.1 - quotientMap z.2) * D z.1
  have hDm : Measurable D := (continuous_divergence hF.1).measurable.comp measurable_representative
  obtain ⟨M, _, hDb⟩ := periodic_bounded (continuous_divergence hF.1) (divergence_periodic hF.2.1)
  have hd : Measurable d :=
    (((measurable_characteristic hE).comp
      (measurable_fst.sub (measurable_quotientMap.comp measurable_snd))).mul
        (hDm.comp measurable_fst))
  have hdm (z : Torus × Cover) : |d z| ≤ M := by
    dsimp only [d]
    rw [abs_mul]
    calc
      _ ≤ 1 * |D z.1| := mul_le_mul_of_nonneg_right (characteristic_bound E _) (abs_nonneg _)
      _ ≤ M := by simpa [D, Real.norm_eq_abs] using hDb (representative z.1)
  have hφ : Integrable φ volume := (mollifier n).integrable_normed
  have hbase : Integrable (fun z : Torus × Cover ↦ φ z.2) (torusVolume.prod volume) := by
    simpa only [one_mul] using (integrable_const (1 : ℝ) (μ := torusVolume)).mul_prod hφ
  have hprod : Integrable (fun z : Torus × Cover ↦ φ z.2 * d z) (torusVolume.prod volume) :=
    hbase.mul_bdd hd.aestronglyMeasurable (Eventually.of_forall hdm)
  have hinner (y : Cover) : (∫ x, d (x, y) ∂torusVolume) ≤ (perimeter E).toReal := by
    let B : Set Torus := (fun x : Torus ↦ x + -(quotientMap y)) ⁻¹' E
    have hmB : MeasurableSet B := (measurable_id.add_const _) hE
    have hPB : perimeter B = perimeter E := perimeter_translation hE.nullMeasurableSet _
    have hEq : (fun x ↦ d (x, y)) = B.indicator D := by
      funext x
      classical
      by_cases hx : x ∈ B
      · have hx' : x - quotientMap y ∈ E := by simpa [B, sub_eq_add_neg] using hx
        simp [d, characteristic, hx, hx']
      · have hx' : x - quotientMap y ∉ E := by simpa [B, sub_eq_add_neg] using hx
        simp [d, characteristic, hx, hx']
    rw [hEq, integral_indicator hmB,
      setIntegral_lift (continuous_divergence hF.1) (divergence_periodic hF.2.1) hmB]
    have hh := abs_flux_le_perimeter (hPB ▸ hP) hF
    rw [hPB] at hh
    exact (le_abs_self _).trans hh
  have hSwap : (∫ x, torusApprox E n x * D x ∂torusVolume) =
      ∫ y : Cover, φ y * (∫ x, d (x, y) ∂torusVolume) := by
    simp_rw [torusApprox_eq_integral, ← integral_mul_const, mul_assoc]
    rw [integral_integral_swap hprod]
    congr 1
    funext y
    exact integral_const_mul _ _
  have hI : (∫ x, torusApprox E n x * D x ∂torusVolume) ≤ (perimeter E).toReal := by
    rw [hSwap]
    calc
      _ ≤ ∫ y : Cover, φ y * (perimeter E).toReal := by
        apply integral_mono
        · simpa only [Function.comp_def, Prod.swap_prod_mk, integral_const_mul] using hprod.swap.integral_prod_left
        · exact hφ.mul_const _
        · intro y
          exact mul_le_mul_of_nonneg_left (hinner y) ((mollifier n).nonneg_normed y)
      _ = _ := by rw [integral_mul_const, (mollifier n).integral_normed, one_mul]
  have hsc := smoothApprox_contDiff ((measurable_characteristic hE).comp measurable_quotientMap)
    (fun x ↦ characteristic_bound E (quotientMap x)) n
  have hsp := smoothApprox_periodic (characteristic_lift_periodic E) n
  have heq := setIntegral_lift (hsc.continuous.mul (continuous_divergence hF.1))
    (fun k x ↦ by simp only [Pi.mul_apply]; rw [hsp, divergence_periodic hF.2.1]) MeasurableSet.univ
  simp only [setIntegral_univ, preimage_univ, inter_univ] at heq
  exact heq ▸ hI

theorem smoothApprox_gradient_bound {E : Set Torus} (hE : MeasurableSet E)
    (hP : perimeter E ≠ ⊤) (n : ℕ) :
    (∫ x in fundamentalCube, ‖gradient (smoothApprox (characteristic E ∘ quotientMap) n) x‖)
      ≤ (perimeter E).toReal := by
  apply integral_norm_gradient_le
    (smoothApprox_contDiff ((measurable_characteristic hE).comp measurable_quotientMap)
      (fun x ↦ characteristic_bound E (quotientMap x)) n)
    (smoothApprox_periodic (characteristic_lift_periodic E) n)
    (smoothApprox_characteristic_range E n)
  intro F hF
  exact smoothApprox_flux_le hE hP n hF

end CubicTorus

namespace CubicTorus
open Filter
open scoped Topology RealInnerProductSpace

lemma integrableOn_characteristic_mul {E : Set Torus}
    (hE : NullMeasurableSet E torusVolume) {g : Cover → ℝ}
    (hg : IntegrableOn g fundamentalCube) :
    IntegrableOn (fun x ↦ characteristic E (quotientMap x) * g x) fundamentalCube := by
  simp_rw [characteristic_mul]
  exact hg.indicator₀ (hE.preimage quotientMap_measurePreserving.quasiMeasurePreserving)

lemma gradient_const_sub (f : Cover → ℝ) (c : ℝ) (x : Cover) :
    gradient (fun y ↦ c - f y) x = -gradient f x := by
  simp only [gradient, fderiv_const_sub, map_neg]

lemma characteristic_union (E F : Set Torus) (x : Torus) :
    characteristic (E ∪ F) x = characteristic E x +
      (1 - characteristic E x) * characteristic F x := by
  classical
  by_cases he : x ∈ E <;> by_cases hf : x ∈ F <;> simp [characteristic, he, hf]

lemma cutoff_union_flux_le {E : Set Torus} (hE : NullMeasurableSet E torusVolume)
    (hP : perimeter E ≠ ⊤) {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) (hb : ∀ x, f x ∈ Icc 0 1)
    {G : Cover → Cover} (hG : AdmissibleField G) :
    (∫ x in fundamentalCube, (characteristic E (quotientMap x) +
      (1 - characteristic E (quotientMap x)) * f x) * divergence G x) ≤
        (perimeter E).toReal + ∫ x in fundamentalCube, ‖gradient f x‖ := by
  let H : Cover → Cover := fun x ↦ (1 - f x) • G x
  have hH : AdmissibleField H := admissible_cutoff (contDiff_const.sub hf)
    (fun n x ↦ by rw [hp]) (fun x ↦ by constructor <;> linarith [(hb x).1, (hb x).2]) hG
  have hdH (x) : divergence H x = (1 - f x) * divergence G x -
      ⟪gradient f x, G x⟫ := by
    rw [divergence_smul_function ((contDiff_const.sub hf).differentiable (by simp)).differentiableAt
      (hG.1.differentiable (by simp)).differentiableAt, gradient_const_sub, inner_neg_left]
    rfl
  let a : Cover → ℝ := fun x ↦ characteristic E (quotientMap x) * divergence H x
  let b : Cover → ℝ := fun x ↦ (1 - characteristic E (quotientMap x)) * ⟪gradient f x, G x⟫
  let c : Cover → ℝ := fun x ↦ divergence (fun y ↦ f y • G y) x
  have hia : IntegrableOn a fundamentalCube := integrableOn_characteristic_mul hE
    (integrableOn_divergence hH.1)
  have hic : IntegrableOn c fundamentalCube :=
    integrableOn_divergence (admissible_cutoff hf hp hb hG).1
  have hii : IntegrableOn (fun x ↦ ⟪gradient f x, G x⟫) fundamentalCube :=
    integrableOn_periodic_continuous ((gradient_contDiff hf).continuous.inner hG.1.continuous)
      (fun n x ↦ by rw [gradient_periodic hp, hG.2.1])
  have hib : IntegrableOn b fundamentalCube := by
    have he := hii.sub (integrableOn_characteristic_mul hE hii)
    simpa only [b, Pi.sub_def, one_sub_mul] using he
  have hign : IntegrableOn (fun x ↦ ‖gradient f x‖) fundamentalCube :=
    integrableOn_periodic_continuous (gradient_contDiff hf).continuous.norm
      (fun n x ↦ by rw [gradient_periodic hp])
  have heq (x) : (characteristic E (quotientMap x) +
      (1 - characteristic E (quotientMap x)) * f x) * divergence G x =
        a x - b x + c x := by
    dsimp only [a, b, c]
    rw [hdH, divergence_smul_function (hf.differentiable (by simp)).differentiableAt
      (hG.1.differentiable (by simp)).differentiableAt]
    ring
  simp_rw [heq]
  have hiab : IntegrableOn (fun x ↦ a x - b x) fundamentalCube := by
    simpa only [Pi.sub_def] using hia.sub hib
  rw [integral_add hiab hic, integral_sub hia hib]
  have hc : (∫ x in fundamentalCube, c x) = 0 :=
    integral_divergence_fundamentalCube (admissible_cutoff hf hp hb hG)
  rw [hc, add_zero]
  have ha : (∫ x in fundamentalCube, a x) ≤ (perimeter E).toReal := by
    dsimp only [a]
    simp_rw [characteristic_mul]
    rw [lift_indicator_integral hE]
    exact (le_abs_self _).trans (abs_flux_le_perimeter hP hH)
  have hb' : -(∫ x in fundamentalCube, b x) ≤ ∫ x in fundamentalCube, ‖gradient f x‖ := by
    rw [← integral_neg]
    apply integral_mono hib.neg hign
    intro x
    have he : 0 ≤ 1 - characteristic E (quotientMap x) := by
      linarith [(le_abs_self _).trans (characteristic_bound E (quotientMap x))]
    have he' : 1 - characteristic E (quotientMap x) ≤ 1 := by
      linarith [characteristic_nonneg E (quotientMap x)]
    calc
      -b x ≤ |b x| := neg_le_abs _
      _ = (1 - characteristic E (quotientMap x)) * |⟪gradient f x, G x⟫| := by
        dsimp only [b]
        rw [abs_mul, abs_of_nonneg he]
      _ ≤ 1 * (‖gradient f x‖ * ‖G x‖) := mul_le_mul he'
        (abs_real_inner_le_norm _ _) (abs_nonneg _) (by norm_num)
      _ ≤ ‖gradient f x‖ := by simpa using mul_le_of_le_one_right (norm_nonneg _) (hG.2.2 x)
  linarith

theorem perimeter_union_le_of_measurable {E F : Set Torus}
    (hE : MeasurableSet E) (hF : MeasurableSet F) :
    perimeter (E ∪ F) ≤ perimeter E + perimeter F := by
  by_cases hPE : perimeter E = ⊤
  · simp [hPE]
  by_cases hPF : perimeter F = ⊤
  · simp [hPF]
  have hg := (measurable_characteristic hF).comp measurable_quotientMap
  have hb := fun x ↦ characteristic_bound F (quotientMap x)
  let f : ℕ → Cover → ℝ := smoothApprox (characteristic F ∘ quotientMap)
  have hf (n) := smoothApprox_contDiff hg hb n
  have hp (n) := smoothApprox_periodic (characteristic_lift_periodic F) n
  have hfb (n) := smoothApprox_characteristic_range F n
  apply iSup_le
  intro G
  apply iSup_le
  intro hG
  rw [← ENNReal.ofReal_toReal hPE, ← ENNReal.ofReal_toReal hPF, ← ENNReal.ofReal_add
    ENNReal.toReal_nonneg ENNReal.toReal_nonneg]
  apply ENNReal.ofReal_le_ofReal
  have hiG := integrableOn_divergence hG.1
  let u : ℕ → Cover → ℝ := fun n x ↦ (characteristic E (quotientMap x) +
    (1 - characteristic E (quotientMap x)) * f n x) * divergence G x
  have hm (n) : AEStronglyMeasurable (u n) (volume.restrict fundamentalCube) := by
    exact (((((measurable_characteristic hE).comp measurable_quotientMap).add
      ((measurable_const.sub ((measurable_characteristic hE).comp measurable_quotientMap)).mul
        (hf n).continuous.measurable)).mul (continuous_divergence hG.1).measurable)).aestronglyMeasurable
  have hbound (n) : ∀ᵐ x ∂volume.restrict fundamentalCube, ‖u n x‖ ≤ |divergence G x| := by
    apply Eventually.of_forall
    intro x
    have hx := characteristic_dichotomy E (quotientMap x)
    rcases hx with hx | hx
    · simpa [u, f, hx, Real.norm_eq_abs, abs_mul, Function.comp_def] using
        mul_le_of_le_one_left (abs_nonneg (divergence G x)) (smoothApprox_bound hb n x)
    · simp [u, hx, Real.norm_eq_abs]
  have hlim : ∀ᵐ x ∂volume.restrict fundamentalCube,
      Tendsto (fun n ↦ u n x) atTop (𝓝 (characteristic (E ∪ F) (quotientMap x) * divergence G x)) := by
    filter_upwards [ae_restrict_of_ae (smoothApprox_ae_tendsto hg hb)] with x hx
    rw [characteristic_union]
    exact (tendsto_const_nhds.add (tendsto_const_nhds.mul hx)).mul_const _
  have ht := tendsto_integral_of_dominated_convergence _ hm hiG.abs hbound hlim
  have hh : (∫ x in fundamentalCube, characteristic (E ∪ F) (quotientMap x) * divergence G x) ≤
      (perimeter E).toReal + (perimeter F).toReal := by
    apply le_of_tendsto ht
    apply Eventually.of_forall
    intro n
    exact (cutoff_union_flux_le hE.nullMeasurableSet hPE (hf n) (hp n) (hfb n) hG).trans
      (add_le_add_right (smoothApprox_gradient_bound hF hPF n) _)
  simpa only [characteristic_mul, lift_indicator_integral (hE.union hF).nullMeasurableSet] using hh

end CubicTorus

namespace CubicTorus

theorem perimeter_union_le {E F : Set Torus}
    (hE : NullMeasurableSet E torusVolume) (hF : NullMeasurableSet F torusVolume) :
    perimeter (E ∪ F) ≤ perimeter E + perimeter F := by
  have he := hE.toMeasurable_ae_eq
  have hf := hF.toMeasurable_ae_eq
  have hef : (toMeasurable torusVolume E ∪ toMeasurable torusVolume F : Set Torus) =ᵐ[torusVolume] (E ∪ F : Set Torus) := by
    filter_upwards [he, hf] with x hx hy
    change (x ∈ toMeasurable torusVolume E ∨ x ∈ toMeasurable torusVolume F) = (x ∈ E ∨ x ∈ F)
    rw [show (x ∈ toMeasurable torusVolume E) = (x ∈ E) from hx,
      show (x ∈ toMeasurable torusVolume F) = (x ∈ F) from hy]
  rw [← perimeter_congr_ae hef, ← perimeter_congr_ae he, ← perimeter_congr_ae hf]
  exact perimeter_union_le_of_measurable (measurableSet_toMeasurable _ _) (measurableSet_toMeasurable _ _)

theorem perimeter_diff_le {E F : Set Torus}
    (hE : NullMeasurableSet E torusVolume) (hF : NullMeasurableSet F torusVolume) :
    perimeter (E \ F) ≤ perimeter E + perimeter F := by
  have heq : E \ F = (Eᶜ ∪ F)ᶜ := by ext x; simp
  rw [heq, perimeter_compl (hE.compl.union hF)]
  exact (perimeter_union_le hE.compl hF).trans_eq (by rw [perimeter_compl hE])

theorem FinitePerimeter.union {E F : Set Torus} (hE : FinitePerimeter E) (hF : FinitePerimeter F) :
    FinitePerimeter (E ∪ F) :=
  ⟨hE.1.union hF.1, (perimeter_union_le hE.1 hF.1).trans_lt (ENNReal.add_lt_top.2 ⟨hE.2, hF.2⟩)⟩

theorem FinitePerimeter.diff {E F : Set Torus} (hE : FinitePerimeter E) (hF : FinitePerimeter F) :
    FinitePerimeter (E \ F) :=
  ⟨hE.1.diff hF.1, (perimeter_diff_le hE.1 hF.1).trans_lt (ENNReal.add_lt_top.2 ⟨hE.2, hF.2⟩)⟩

end CubicTorus

namespace CubicTorus
open Filter
open scoped Topology RealInnerProductSpace

lemma smoothApprox_flux_tendsto {E : Set Torus} (hE : MeasurableSet E)
    {F : Cover → Cover} (hF : AdmissibleField F) :
    Tendsto (fun n ↦ ∫ x in fundamentalCube,
      smoothApprox (characteristic E ∘ quotientMap) n x * divergence F x) atTop
      (𝓝 (∫ x in fundamentalCube ∩ quotientMap ⁻¹' E, divergence F x)) := by
  have hg := (measurable_characteristic hE).comp measurable_quotientMap
  have hb := fun x ↦ characteristic_bound E (quotientMap x)
  have hf n := smoothApprox_contDiff hg hb n
  have ht := tendsto_integral_of_dominated_convergence (fun x ↦ |divergence F x|)
    (fun n ↦ ((hf n).continuous.mul (continuous_divergence hF.1)).aestronglyMeasurable)
    (integrableOn_divergence hF.1).abs
    (fun n ↦ Eventually.of_forall (fun x ↦ by
      change ‖smoothApprox (characteristic E ∘ quotientMap) n x * divergence F x‖ ≤ _
      rw [norm_mul, Real.norm_eq_abs, Real.norm_eq_abs]
      exact mul_le_of_le_one_left (abs_nonneg _) (smoothApprox_bound hb n x)))
    (by
      filter_upwards [ae_restrict_of_ae (smoothApprox_ae_tendsto hg hb)] with x hx
      exact hx.mul_const (divergence F x))
  simpa! only [Pi.mul_apply, Function.comp_def, characteristic_mul,
    lift_indicator_integral hE.nullMeasurableSet] using ht

lemma smooth_flux_le_gradient {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) (hb : ∀ x, f x ∈ Icc 0 1)
    {F : Cover → Cover} (hF : AdmissibleField F) :
    (∫ x in fundamentalCube, f x * divergence F x) ≤
      ∫ x in fundamentalCube, ‖gradient f x‖ := by
  rw [cutoff_integration_by_parts hf hp hb hF, ← integral_neg]
  apply integral_mono
  · exact (integrableOn_periodic_continuous
      ((gradient_contDiff hf).continuous.inner hF.1.continuous)
      (fun n x ↦ by rw [gradient_periodic hp, hF.2.1])).neg
  · exact integrableOn_periodic_continuous (gradient_contDiff hf).continuous.norm
      (fun n x ↦ by rw [gradient_periodic hp])
  · intro x
    exact (neg_le_abs _).trans ((abs_real_inner_le_norm _ _).trans
      (mul_le_of_le_one_right (norm_nonneg _) (hF.2.2 x)))

theorem smoothApprox_gradient_tendsto {E : Set Torus} (hE : MeasurableSet E)
    (hP : perimeter E ≠ ⊤) :
    Tendsto (fun n ↦ ∫ x in fundamentalCube,
      ‖gradient (smoothApprox (characteristic E ∘ quotientMap) n) x‖) atTop
      (𝓝 (perimeter E).toReal) := by
  apply tendsto_order.2
  constructor
  · intro a ha
    by_cases ha0 : a < 0
    · exact Eventually.of_forall fun n ↦ ha0.trans_le (integral_nonneg fun x ↦ norm_nonneg _)
    have ha0' : 0 ≤ a := le_of_not_gt ha0
    have hh : ENNReal.ofReal a < perimeter E := by
      rw [← ENNReal.ofReal_toReal hP]
      exact (ENNReal.ofReal_lt_ofReal_iff_of_nonneg ha0').2 ha
    obtain ⟨F, hF⟩ := lt_iSup_iff.1 hh
    obtain ⟨hF, hflux⟩ := lt_iSup_iff.1 hF
    have hflux' : a < ∫ x in fundamentalCube ∩ quotientMap ⁻¹' E, divergence F x :=
      (ENNReal.ofReal_lt_ofReal_iff_of_nonneg ha0').1 hflux
    filter_upwards [(smoothApprox_flux_tendsto hE hF).eventually (eventually_gt_nhds hflux')] with n hn
    exact hn.trans_le (smooth_flux_le_gradient
      (smoothApprox_contDiff ((measurable_characteristic hE).comp measurable_quotientMap)
        (fun x ↦ characteristic_bound E (quotientMap x)) n)
      (smoothApprox_periodic (characteristic_lift_periodic E) n)
      (smoothApprox_characteristic_range E n) hF)
  · intro a ha
    exact Eventually.of_forall fun n ↦ (smoothApprox_gradient_bound hE hP n).trans_lt ha

end CubicTorus

namespace CubicTorus
open Filter
open scoped Topology RealInnerProductSpace

abbrev NormalSphere := {v : Cover // ‖v‖ = 1}

instance : CompactSpace NormalSphere := by
  apply isCompact_iff_compactSpace.mp
  have hK : IsCompact ((WithLp.toLp 2) '' (Icc (-1 : Fin 3 → ℝ) 1)) :=
    isCompact_Icc.image (EuclideanSpace.equiv (Fin 3) ℝ).symm.continuous
  apply hK.of_isClosed_subset (isClosed_eq continuous_norm continuous_const)
  intro v hv
  change ‖v‖ = 1 at hv
  refine ⟨WithLp.ofLp v, ⟨?_, ?_⟩, rfl⟩
  · intro i
    have hh : |v i| ≤ 1 := by simpa only [Real.norm_eq_abs, hv] using PiLp.norm_apply_le v i
    exact (abs_le.mp hh).1
  · intro i
    have hh : |v i| ≤ 1 := by simpa only [Real.norm_eq_abs, hv] using PiLp.norm_apply_le v i
    exact (abs_le.mp hh).2

abbrev NormalBundle := CompactCell × NormalSphere

def normalizedVector (v : Cover) : Cover :=
  if v = 0 then EuclideanSpace.single 0 1 else ‖v‖⁻¹ • v

lemma normalizedVector_norm (v : Cover) : ‖normalizedVector v‖ = 1 := by
  unfold normalizedVector
  split_ifs with hv
  · simp
  · rw [norm_smul, Real.norm_of_nonneg (inv_nonneg.mpr (norm_nonneg _)),
      inv_mul_cancel₀ (norm_ne_zero_iff.mpr hv)]

lemma measurable_normalizedVector : Measurable normalizedVector := by
  exact Measurable.ite (measurableSet_singleton (0 : Cover)) measurable_const
    (measurable_id.norm.inv.smul measurable_id)

def unitNormal (v : Cover) : NormalSphere := ⟨normalizedVector v, normalizedVector_norm v⟩

lemma measurable_unitNormal : Measurable unitNormal := measurable_normalizedVector.subtype_mk

lemma norm_smul_normalizedVector (v : Cover) : ‖v‖ • normalizedVector v = v := by
  unfold normalizedVector
  split_ifs with hv
  · simp [hv]
  · rw [smul_smul, mul_inv_cancel₀ (norm_ne_zero_iff.mpr hv), one_smul]

def smoothNormalMap (f : Cover → ℝ) (x : Cover) : NormalBundle :=
  (cubeRepresentative (quotientMap x), unitNormal (-gradient f x))

lemma measurable_smoothNormalMap {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f) :
    Measurable (smoothNormalMap f) :=
  (measurable_cubeRepresentative.comp measurable_quotientMap).prodMk
    (measurable_unitNormal.comp (gradient_contDiff hf).continuous.measurable.neg)

def smoothNormalMeasure (f : Cover → ℝ) (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) : FiniteMeasure NormalBundle := by
  let μ := (volume.restrict fundamentalCube).withDensity (fun x ↦ ENNReal.ofReal ‖gradient f x‖)
  haveI : IsFiniteMeasure μ := isFiniteMeasure_withDensity_ofReal
    (integrableOn_periodic_continuous (gradient_contDiff hf).continuous.norm
      (fun n x ↦ by rw [gradient_periodic hp])).hasFiniteIntegral
  exact ⟨Measure.map (smoothNormalMap f) μ, inferInstance⟩

lemma integral_smoothNormalMeasure {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) (g : C(NormalBundle, ℝ)) :
    (∫ z, g z ∂(smoothNormalMeasure f hf hp : Measure NormalBundle)) =
      ∫ x in fundamentalCube, ‖gradient f x‖ * g (smoothNormalMap f x) := by
  change (∫ z, g z ∂Measure.map (smoothNormalMap f) _) = _
  rw [integral_map (measurable_smoothNormalMap hf).aemeasurable g.continuous.aestronglyMeasurable]
  rw [integral_withDensity_eq_integral_toReal_smul
    (gradient_contDiff hf).continuous.norm.measurable.ennreal_ofReal
    (Eventually.of_forall fun x ↦ ENNReal.ofReal_lt_top)]
  simp only [ENNReal.toReal_ofReal (norm_nonneg _), smul_eq_mul]

lemma smoothNormalMeasure_mass {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) :
    ((smoothNormalMeasure f hf hp).mass : ℝ) =
      ∫ x in fundamentalCube, ‖gradient f x‖ := by
  have hh := integral_smoothNormalMeasure hf hp (ContinuousMap.const _ (1 : ℝ))
  simpa only [ContinuousMap.const_apply, integral_const, smul_eq_mul, mul_one,
    FiniteMeasure.mass, FiniteMeasure.coeFn_def, Measure.real, ENNReal.toReal] using hh

def normalTest (F : Cover → Cover) (hF : Continuous F) : C(NormalBundle, ℝ) :=
  ⟨fun z ↦ ⟪(z.2 : Cover), F (z.1 : Cover)⟫,
    (continuous_subtype_val.comp continuous_snd).inner
      (hF.comp (continuous_subtype_val.comp continuous_fst))⟩

lemma smoothNormalMeasure_flux {f : Cover → ℝ} (hf : ContDiff ℝ (⊤ : ℕ∞) f)
    (hp : ∀ n x, f (x + latticeVector n) = f x) (hb : ∀ x, f x ∈ Icc 0 1)
    {F : Cover → Cover} (hF : AdmissibleField F) :
    (∫ z, normalTest F hF.1.continuous z ∂(smoothNormalMeasure f hf hp : Measure NormalBundle)) =
      ∫ x in fundamentalCube, f x * divergence F x := by
  rw [integral_smoothNormalMeasure hf hp, cutoff_integration_by_parts hf hp hb hF,
    ← integral_neg]
  apply integral_congr_ae
  exact Eventually.of_forall fun x ↦ by
    change ‖gradient f x‖ * ⟪normalizedVector (-gradient f x),
      F (representative (quotientMap x))⟫ = -⟪gradient f x, F x⟫
    rw [periodic_representative hF.2.1, ← real_inner_smul_left,
      ← norm_neg (gradient f x), norm_smul_normalizedVector, inner_neg_left]

end CubicTorus

namespace CubicTorus
open Filter
open scoped Topology RealInnerProductSpace

theorem exists_normalMeasure {E : Set Torus} (hE : MeasurableSet E)
    (hP : perimeter E ≠ ⊤) :
    ∃ μ : FiniteMeasure NormalBundle,
      (μ.mass : ℝ) = (perimeter E).toReal ∧
      ∀ (F : Cover → Cover) (hF : AdmissibleField F),
        (∫ z, normalTest F hF.1.continuous z ∂(μ : Measure NormalBundle)) =
          ∫ x in fundamentalCube ∩ quotientMap ⁻¹' E, divergence F x := by
  let f n := smoothApprox (characteristic E ∘ quotientMap) n
  have hf n : ContDiff ℝ (⊤ : ℕ∞) (f n) := smoothApprox_contDiff
    ((measurable_characteristic hE).comp measurable_quotientMap)
    (fun x ↦ characteristic_bound E (quotientMap x)) n
  have hp n : ∀ k x, f n (x + latticeVector k) = f n x :=
    smoothApprox_periodic (characteristic_lift_periodic E) n
  let μn n := smoothNormalMeasure (f n) (hf n) (hp n)
  let P : NNReal := ⟨(perimeter E).toReal, ENNReal.toReal_nonneg⟩
  have hbound n : (μn n).mass ≤ P := by
    rw [← NNReal.coe_le_coe]
    change ((smoothNormalMeasure (f n) (hf n) (hp n)).mass : ℝ) ≤ (perimeter E).toReal
    rw [smoothNormalMeasure_mass]
    exact smoothApprox_gradient_bound hE hP n
  obtain ⟨μ, _, hμ⟩ := (isCompact_setOfPred_finiteMeasure_le_of_compactSpace NormalBundle P).exists_mapClusterPt (show Filter.map μn atTop ≤ 𝓟 {μ | μ.mass ≤ P} from
      tendsto_principal.mpr (Eventually.of_forall hbound))
  obtain ⟨U, hUa, hU⟩ := mapClusterPt_iff_ultrafilter.mp hμ
  refine ⟨μ, ?_, ?_⟩
  · have hmass : Tendsto (fun n ↦ ((μn n).mass : ℝ)) U (𝓝 (μ.mass : ℝ)) :=
      (continuous_subtype_val.comp FiniteMeasure.continuous_mass).continuousAt.tendsto.comp hU
    have henergy : Tendsto (fun n ↦ ((μn n).mass : ℝ)) U (𝓝 (perimeter E).toReal) := by
      simpa only [μn, smoothNormalMeasure_mass, f] using
        (smoothApprox_gradient_tendsto hE hP).mono_left hUa
    exact tendsto_nhds_unique hmass henergy
  · intro F hF
    have hinter : Tendsto (fun n ↦ ∫ z, normalTest F hF.1.continuous z
        ∂(μn n : Measure NormalBundle)) U
        (𝓝 (∫ z, normalTest F hF.1.continuous z ∂(μ : Measure NormalBundle))) :=
      (FiniteMeasure.continuous_integral_continuousMap (normalTest F hF.1.continuous)).continuousAt.tendsto.comp hU
    have hflux : Tendsto (fun n ↦ ∫ z, normalTest F hF.1.continuous z
        ∂(μn n : Measure NormalBundle)) U
        (𝓝 (∫ x in fundamentalCube ∩ quotientMap ⁻¹' E, divergence F x)) := by
      have heq n := smoothNormalMeasure_flux (hf n) (hp n)
        (smoothApprox_characteristic_range E n) hF
      simp_rw [show (fun n ↦ ∫ z, normalTest F hF.1.continuous z
          ∂(μn n : Measure NormalBundle)) = (fun n ↦ ∫ x in fundamentalCube, f n x * divergence F x)
        from funext heq]
      exact (smoothApprox_flux_tendsto hE hF).mono_left hUa
    exact tendsto_nhds_unique hinter hflux

end CubicTorus

namespace CubicTorus
open scoped RealInnerProductSpace

def IsNormalMeasure (E : Set Torus) (μ : FiniteMeasure NormalBundle) : Prop :=
  (μ.mass : ℝ) = (perimeter E).toReal ∧
  ∀ (F : Cover → Cover) (hF : AdmissibleField F),
    (∫ z, normalTest F hF.1.continuous z ∂(μ : Measure NormalBundle)) =
      ∫ x in fundamentalCube ∩ quotientMap ⁻¹' E, divergence F x

lemma normalMeasure_flux {E : Set Torus} {μ : FiniteMeasure NormalBundle}
    (hμ : IsNormalMeasure E μ) {F : Cover → Cover}
    (hF : ContDiff ℝ (⊤ : ℕ∞) F) (hp : ∀ n x, F (x + latticeVector n) = F x)
    {k : ℝ} (hk : 0 < k) (hb : ∀ x, ‖F x‖ ≤ k) :
    (∫ z, normalTest F hF.continuous z ∂(μ : Measure NormalBundle)) =
      ∫ x in fundamentalCube ∩ quotientMap ⁻¹' E, divergence F x := by
  have hG : AdmissibleField (fun x ↦ k⁻¹ • F x) := by
    refine ⟨hF.const_smul (k⁻¹ : ℝ), ?_, ?_⟩
    · intro n x
      change k⁻¹ • F (x + latticeVector n) = k⁻¹ • F x
      rw [hp]
    · intro x
      rw [norm_smul, Real.norm_eq_abs, abs_of_pos (inv_pos.mpr hk)]
      exact (mul_le_mul_of_nonneg_left (hb x) (inv_nonneg.mpr hk.le)).trans_eq
        (inv_mul_cancel₀ hk.ne')
  have hh := hμ.2 _ hG
  simp only [normalTest, ContinuousMap.coe_mk, real_inner_smul_right,
    integral_const_mul] at hh ⊢
  simp_rw [divergence_const_smul (hF.differentiable (by simp)).differentiableAt,
    integral_const_mul] at hh
  exact mul_left_cancel₀ (inv_ne_zero hk.ne') hh

end CubicTorus

open scoped RealInnerProductSpace

namespace CubicTorus
open Filter
open scoped Topology

def coordinateMove (i : Fin 3) (f : Cover → ℝ) (t : ℝ) (x : Cover) : Cover :=
  x + (t * f x) • EuclideanSpace.single i 1

def coordinateMoveDeriv (i : Fin 3) (f : Cover → ℝ) (t : ℝ) (x : Cover) :
    Cover →L[ℝ] Cover :=
  ContinuousLinearMap.id ℝ Cover + (fderiv ℝ f x).smulRight (t • EuclideanSpace.single i 1)

def coordinateJacobian (i : Fin 3) (f : Cover → ℝ) (t : ℝ) (x : Cover) : ℝ :=
  1 + t * fderiv ℝ f x (EuclideanSpace.single i 1)

lemma coordinateMove_contDiff (i : Fin 3) {f : Cover → ℝ}
    (hf : ContDiff ℝ (⊤ : ℕ∞) f) (t : ℝ) :
    ContDiff ℝ (⊤ : ℕ∞) (coordinateMove i f t) := by
  unfold coordinateMove
  fun_prop

lemma coordinateMove_hasFDerivAt (i : Fin 3) {f : Cover → ℝ} {point : Cover}
    (hf : DifferentiableAt ℝ f point) (t : ℝ) :
    HasFDerivAt (coordinateMove i f t) (coordinateMoveDeriv i f t point) point := by
  have h₁ : HasFDerivAt (fun y : Cover ↦ y) (ContinuousLinearMap.id ℝ Cover) point :=
    hasFDerivAt_id point
  have h₂ := (hf.hasFDerivAt.const_mul t).smul_const
    (EuclideanSpace.single i (1 : ℝ) : Cover)
  have he : coordinateMoveDeriv i f t point = ContinuousLinearMap.id ℝ Cover +
      (t • fderiv ℝ f point).smulRight (EuclideanSpace.single i (1 : ℝ) : Cover) := by
    ext y j
    simp [coordinateMoveDeriv, mul_comm]
  rw [he]
  exact h₁.add h₂

lemma coordinateMove_periodic (i : Fin 3) {f : Cover → ℝ}
    (hp : ∀ n x, f (x + latticeVector n) = f x) (t : ℝ) (n : Fin 3 → ℤ) (x : Cover) :
    coordinateMove i f t (x + latticeVector n) = coordinateMove i f t x + latticeVector n := by
  simp only [coordinateMove, hp]
  abel

lemma coordinateMove_apply (i j : Fin 3) (f : Cover → ℝ) (t : ℝ) (x : Cover) :
    coordinateMove i f t x j = x j + if i = j then t * f x else 0 := by
  by_cases h : i = j <;> simp [coordinateMove, h, eq_comm]

lemma coordinateMoveDeriv_det (i : Fin 3) (f : Cover → ℝ) (t : ℝ) (x : Cover) :
    (coordinateMoveDeriv i f t x).det = coordinateJacobian i f t x := by
  let B := (EuclideanSpace.basisFun (Fin 3) ℝ).toBasis
  have he : LinearMap.toMatrix B B (coordinateMoveDeriv i f t x).toLinearMap =
      Matrix.of (fun j k ↦ (if j = k then 1 else 0) +
        (if j = i then t * fderiv ℝ f x (EuclideanSpace.single k 1) else 0)) := by
    ext j k
    rw [LinearMap.toMatrix_apply]
    change (EuclideanSpace.basisFun (Fin 3) ℝ).toBasis.repr
      (coordinateMoveDeriv i f t x ((EuclideanSpace.basisFun (Fin 3) ℝ) k)) j = _
    rw [OrthonormalBasis.coe_toBasis_repr_apply, EuclideanSpace.basisFun_repr,
      EuclideanSpace.basisFun_apply]
    by_cases h : j = i <;> simp [coordinateMoveDeriv, h, mul_comm]
  rw [ContinuousLinearMap.det, ← LinearMap.det_toMatrix B, he]
  rw [Matrix.det_fin_three]
  fin_cases i <;> simp [coordinateJacobian]

lemma coordinateMove_injective (i : Fin 3) {f : Cover → ℝ}
    (hf : Differentiable ℝ f) {M : NNReal} (hM : ∀ x, ‖fderiv ℝ f x‖₊ ≤ M)
    {t : ℝ} (ht : |t| * M < 1) : Function.Injective (coordinateMove i f t) := by
  have hLip := lipschitzWith_of_nnnorm_fderiv_le hf hM
  intro x y hxy
  have he : x - y = -(t * (f x - f y)) • EuclideanSpace.single i 1 := by
    calc
      x - y = coordinateMove i f t x - coordinateMove i f t y -
          (t * (f x - f y)) • EuclideanSpace.single i 1 := by
        unfold coordinateMove
        module
      _ = _ := by rw [hxy, sub_self, zero_sub, neg_smul]
  have hn : ‖x - y‖ = |t| * ‖f x - f y‖ := by
    rw [he, norm_smul, norm_neg, Real.norm_eq_abs, abs_mul]
    simp [Real.norm_eq_abs]
  have hh : ‖x - y‖ ≤ |t| * M * ‖x - y‖ := by
    have hz := hLip.norm_sub_le x y
    calc
      ‖x - y‖ = |t| * ‖f x - f y‖ := hn
      _ ≤ |t| * (M * ‖x - y‖) :=
        mul_le_mul_of_nonneg_left hz (abs_nonneg _)
      _ = _ := by ring
  have : ‖x - y‖ = 0 := by nlinarith [norm_nonneg (x - y)]
  exact sub_eq_zero.1 (norm_eq_zero.1 this)

end CubicTorus

namespace CubicTorus
open scoped RealInnerProductSpace

lemma divergence_sub_fields {F G : Cover → Cover} {point : Cover}
    (hF : DifferentiableAt ℝ F point) (hG : DifferentiableAt ℝ G point) :
    divergence (fun y ↦ F y - G y) point = divergence F point - divergence G point := by
  change (∑ i : Fin 3, (fderiv ℝ (F - G) point (EuclideanSpace.single i 1)) i) = _
  rw [fderiv_sub hF hG]
  simp [divergence, Finset.sum_sub_distrib]

lemma linearForm_apply_basis (L : Cover →L[ℝ] ℝ) (v : Cover) :
    L v = ∑ j : Fin 3, v j * L (EuclideanSpace.single j 1) := by
  have hv : (∑ j : Fin 3, v j • EuclideanSpace.single j 1) = v := by
    ext k
    simp [Pi.single_apply]
  conv_lhs => rw [← hv, map_sum]
  simp

lemma divergence_comp_coordinateMove (i : Fin 3) {f : Cover → ℝ} {point : Cover}
    (hf : DifferentiableAt ℝ f point) {G : Cover → Cover} {time : ℝ}
    (hG : DifferentiableAt ℝ G (coordinateMove i f time point)) :
    divergence (fun y ↦ G (coordinateMove i f time y)) point =
      divergence G (coordinateMove i f time point) +
        time * fderiv ℝ f point
          (fderiv ℝ G (coordinateMove i f time point) (EuclideanSpace.single i 1)) := by
  have hm := coordinateMove_hasFDerivAt i hf time
  have hd := (hG.hasFDerivAt.comp point hm).fderiv
  change fderiv ℝ (fun y ↦ G (coordinateMove i f time y)) point = _ at hd
  simp only [divergence, hd,
    coordinateMoveDeriv, ContinuousLinearMap.comp_apply, add_apply,
    ContinuousLinearMap.id_apply, ContinuousLinearMap.smulRight_apply,
    map_add, map_smul, PiLp.add_apply, PiLp.smul_apply, smul_eq_mul,
    Finset.sum_add_distrib]
  congr 1
  rw [linearForm_apply_basis, Finset.mul_sum]
  apply Finset.sum_congr rfl
  intro j _
  ring

def coordinatePullback (i : Fin 3) (f : Cover → ℝ) (t : ℝ) (G : Cover → Cover)
    (x : Cover) : Cover :=
  coordinateJacobian i f t x • G (coordinateMove i f t x) -
    (t * fderiv ℝ f x (G (coordinateMove i f t x))) • EuclideanSpace.single i 1

lemma coordinateJacobian_contDiff (i : Fin 3) {f : Cover → ℝ}
    (hf : ContDiff ℝ (⊤ : ℕ∞) f) (t : ℝ) :
    ContDiff ℝ (⊤ : ℕ∞) (coordinateJacobian i f t) := by
  have hd := hf.fderiv_right (m := (⊤ : ℕ∞)) (by simp)
  unfold coordinateJacobian
  fun_prop

lemma coordinatePullback_contDiff (i : Fin 3) {f : Cover → ℝ}
    (hf : ContDiff ℝ (⊤ : ℕ∞) f) (t : ℝ) {G : Cover → Cover}
    (hG : ContDiff ℝ (⊤ : ℕ∞) G) :
    ContDiff ℝ (⊤ : ℕ∞) (coordinatePullback i f t G) := by
  have hd := hf.fderiv_right (m := (⊤ : ℕ∞)) (by simp)
  have hm := coordinateMove_contDiff i hf t
  have hj := coordinateJacobian_contDiff i hf t
  unfold coordinatePullback
  exact (hj.smul (hG.comp hm)).sub
    ((contDiff_const.mul (hd.clm_apply (hG.comp hm))).smul contDiff_const)

lemma coordinatePullback_periodic (i : Fin 3) {f : Cover → ℝ}
    (hp : ∀ n x, f (x + latticeVector n) = f x) (t : ℝ) {G : Cover → Cover}
    (hG : ∀ n x, G (x + latticeVector n) = G x) (n : Fin 3 → ℤ) (x : Cover) :
    coordinatePullback i f t G (x + latticeVector n) = coordinatePullback i f t G x := by
  simp only [coordinatePullback, coordinateJacobian, fderiv_periodic hp,
    coordinateMove_periodic i hp, hG]

lemma coordinatePullback_divergence (i : Fin 3) {f : Cover → ℝ}
    (hf : ContDiff ℝ (⊤ : ℕ∞) f) (t : ℝ) {G : Cover → Cover}
    (hG : ContDiff ℝ (⊤ : ℕ∞) G) (x : Cover) :
    divergence (coordinatePullback i f t G) x =
      coordinateJacobian i f t x * divergence G (coordinateMove i f t x) := by
  have hfd := (hf.differentiable (by simp)).differentiableAt (x := x)
  have hdf := ((hf.fderiv_right (m := (⊤ : ℕ∞)) (by simp)).differentiable
    (by simp)).differentiableAt (x := x)
  have hm := coordinateMove_hasFDerivAt i hfd t
  have hg := (hG.differentiable (by simp)).differentiableAt (x := coordinateMove i f t x)
  have hu := hg.hasFDerivAt.comp x hm
  have hj := ((hdf.hasFDerivAt.clm_apply (hasFDerivAt_const
    (EuclideanSpace.single i (1 : ℝ) : Cover) x)).const_mul t).const_add 1
  have hb := (hdf.hasFDerivAt.clm_apply hu).const_mul t
  have he := ((hf.contDiffAt (x := x)).isSymmSndFDerivAt (by
    simp only [minSmoothness_of_isRCLikeNormedField]
    change ((2 : ℕ∞) : WithTop ℕ∞) ≤ ((⊤ : ℕ∞) : WithTop ℕ∞)
    exact WithTop.coe_le_coe.mpr le_top))
    (EuclideanSpace.single i 1) (G (coordinateMove i f t x))
  change divergence (fun y ↦ coordinateJacobian i f t y • G (coordinateMove i f t y) -
    (t * fderiv ℝ f y (G (coordinateMove i f t y))) • EuclideanSpace.single i 1) x = _
  have hjf := hj.fderiv
  change fderiv ℝ (coordinateJacobian i f t) x = _ at hjf
  dsimp only [Function.comp_def] at hb
  have hJ : DifferentiableAt ℝ (coordinateJacobian i f t) x := hj.differentiableAt
  have hU : DifferentiableAt ℝ (fun y ↦ G (coordinateMove i f t y)) x := hu.differentiableAt
  have hB : DifferentiableAt ℝ
      (fun y ↦ t * fderiv ℝ f y (G (coordinateMove i f t y))) x := hb.differentiableAt
  have hA : DifferentiableAt ℝ
      (fun y ↦ coordinateJacobian i f t y • G (coordinateMove i f t y)) x := hJ.smul hU
  have hC : DifferentiableAt ℝ
      (fun y ↦ (t * fderiv ℝ f y (G (coordinateMove i f t y))) •
        (EuclideanSpace.single i 1 : Cover)) x := hB.smul_const _
  rw [divergence_sub_fields hA hC,
    divergence_smul_function hJ hU,
    divergence_smul_const hB, inner_gradient_left,
    hjf, hb.fderiv, divergence_comp_coordinateMove i hfd hg]
  simp only [smul_apply, add_apply,
    ContinuousLinearMap.comp_apply, zero_apply,
    ContinuousLinearMap.flip_apply, smul_eq_mul, zero_add,
    coordinateMoveDeriv, ContinuousLinearMap.id_apply, ContinuousLinearMap.smulRight_apply,
    map_add, map_smul, map_zero]
  dsimp only [coordinateJacobian, Function.comp_def]
  rw [← he]
  ring

end CubicTorus

namespace CubicTorus

lemma coordinateJacobian_norm_le (i : Fin 3) {f : Cover → ℝ} {M : ℝ}
    (hM : ∀ x, ‖fderiv ℝ f x‖ ≤ M) (t : ℝ) (x : Cover) :
    ‖coordinateJacobian i f t x‖ ≤ 1 + |t| * M := by
  have hd : ‖fderiv ℝ f x (EuclideanSpace.single i 1)‖ ≤ M := by
    simpa using (ContinuousLinearMap.le_opNorm (fderiv ℝ f x) (EuclideanSpace.single i 1)).trans
      (mul_le_mul_of_nonneg_right (hM x) (norm_nonneg _))
  calc
    _ ≤ ‖(1 : ℝ)‖ + ‖t * fderiv ℝ f x (EuclideanSpace.single i 1)‖ := norm_add_le _ _
    _ ≤ 1 + |t| * M := by
      rw [norm_one, norm_mul, Real.norm_eq_abs t]
      exact add_le_add_right (mul_le_mul_of_nonneg_left hd (abs_nonneg _)) 1

lemma coordinateJacobian_pos (i : Fin 3) {f : Cover → ℝ} {M : ℝ}
    (hM : ∀ x, ‖fderiv ℝ f x‖ ≤ M) {t : ℝ} (ht : |t| * M < 1) (x : Cover) :
    0 < coordinateJacobian i f t x := by
  have hd : |fderiv ℝ f x (EuclideanSpace.single i 1)| ≤ M := by
    simpa [Real.norm_eq_abs] using
      (ContinuousLinearMap.le_opNorm (fderiv ℝ f x) (EuclideanSpace.single i 1)).trans
        (mul_le_mul_of_nonneg_right (hM x) (norm_nonneg _))
  have hb := mul_le_mul_of_nonneg_left hd (abs_nonneg t)
  rw [← abs_mul] at hb
  have := (abs_le.1 hb).1
  unfold coordinateJacobian
  linarith

end CubicTorus
end
end
end

end OAI