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695bd70 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 | import Mathlib
namespace OAI
noncomputable section
open Set Filter Manifold Bundle MeasureTheory
open scoped Topology ContDiff ENNReal
namespace YauCounterexamples
abbrev Euclidean (n : ℕ) := EuclideanSpace ℝ (Fin n)
abbrev Sphere (n : ℕ) := Metric.sphere (0 : Euclidean (n + 1)) 1
abbrev FourManifold := Sphere 2 × (Circle × Circle)
abbrev FourModel := Euclidean 2 × (Euclidean 1 × Euclidean 1)
instance torusChartedSpace : ChartedSpace (Euclidean 1 × Euclidean 1) (Circle × Circle) :=
prodChartedSpace (Euclidean 1) Circle (Euclidean 1) Circle
instance torusIsManifold : IsManifold 𝓘(ℝ, Euclidean 1 × Euclidean 1) ∞ (Circle × Circle) := by
rw [modelWithCornersSelf_prod]
exact IsManifold.prod Circle Circle
instance fourChartedSpace : ChartedSpace FourModel FourManifold :=
prodChartedSpace (Euclidean 2) (Sphere 2) (Euclidean 1 × Euclidean 1) (Circle × Circle)
instance fourIsManifold : IsManifold 𝓘(ℝ, FourModel) ∞ FourManifold := by
rw [show 𝓘(ℝ, FourModel) = 𝓘(ℝ, Euclidean 2).prod
𝓘(ℝ, Euclidean 1 × Euclidean 1) from modelWithCornersSelf_prod]
exact IsManifold.prod (Sphere 2) (Circle × Circle)
section Definitions
variable {E : Type*} [NormedAddCommGroup E] [NormedSpace ℝ E]
[FiniteDimensional ℝ E]
variable {M : Type*} [TopologicalSpace M] [ChartedSpace E M]
[IsManifold 𝓘(ℝ, E) ∞ M]
/-- Ordinary smooth positive-definite Riemannian metrics, with the existing tangent topology. -/
abbrev SmoothMetric (E : Type*) [NormedAddCommGroup E] [NormedSpace ℝ E]
(M : Type*) [TopologicalSpace M] [ChartedSpace E M] [IsManifold 𝓘(ℝ, E) ∞ M] :=
ContMDiffRiemannianMetric 𝓘(ℝ, E) ∞ E (fun x : M => TangentSpace 𝓘(ℝ, E) x)
abbrev CoordIndex (E : Type*) [NormedAddCommGroup E] [NormedSpace ℝ E]
[FiniteDimensional ℝ E] := Fin (Module.finrank ℝ E)
/-- A coordinate vector, pushed from a chart back into the tangent bundle. -/
def coordinateVector (p : M) (y : E) (i : CoordIndex E) :
TangentSpace 𝓘(ℝ, E) ((chartAt E p).symm y) :=
mfderiv 𝓘(ℝ, E) 𝓘(ℝ, E) (chartAt E p).symm y (Module.finBasis ℝ E i)
/-- Metric coefficients in a chart. Only values in the chart target are used. -/
def metricCoefficients (g : SmoothMetric E M) (p : M) (y : E) :
Matrix (CoordIndex E) (CoordIndex E) ℝ :=
fun i j => g.inner ((chartAt E p).symm y)
(coordinateVector p y i) (coordinateVector p y j)
/-- The local coordinate formula det(g)^(-1/2) d_i(det(g)^(1/2) g^{ij} d_j u).
The chart at the evaluation point is used, so all derivatives are taken in
its open target. -/
def laplaceBeltrami (g : SmoothMetric E M) (u : M → ℝ) (x : M) : ℝ :=
let c := chartAt E x
let a := metricCoefficients g x
let density := fun y => Real.sqrt (Matrix.det (a y))
let u' := u ∘ c.symm
(density (c x))⁻¹ * ∑ i, fderiv ℝ
(fun y => density y * ∑ j, (a y)⁻¹ i j *
fderiv ℝ u' y (Module.finBasis ℝ E j))
(c x) (Module.finBasis ℝ E i)
/-- One compact-chart seminorm test for the usual smooth topology of metrics. -/
structure CoordinateTest (E : Type*) [NormedAddCommGroup E] [NormedSpace ℝ E]
[FiniteDimensional ℝ E] (M : Type*) [TopologicalSpace M] [ChartedSpace E M] where
center : M
compactSet : Set E
isCompact : IsCompact compactSet
inTarget : compactSet ⊆ (chartAt E center).target
order : ℕ
row : CoordIndex E
column : CoordIndex E
/-- The finite compact-chart derivative tests defining a C∞ neighborhood.
A single positive tolerance suffices for the finitely many tests. -/
def IsSmoothNeighborhood (g₀ : SmoothMetric E M) (N : Set (SmoothMetric E M)) : Prop :=
∃ tests : List (CoordinateTest E M), ∃ ε : ℝ, 0 < ε ∧
∀ g : SmoothMetric E M,
(∀ t ∈ tests, ∀ y ∈ t.compactSet,
‖iteratedFDeriv ℝ t.order
(fun z => metricCoefficients g t.center z t.row t.column -
metricCoefficients g₀ t.center z t.row t.column) y‖ < ε) → g ∈ N
/-- Hausdorff measure for the path distance of the actual Riemannian metric,
not the metric inherited from any ambient embedding. -/
def nodalMeasure [T3Space M] (g : SmoothMetric E M) (d : ℝ) (u : M → ℝ) : ℝ≥0∞ :=
letI : RiemannianBundle (fun x : M => TangentSpace 𝓘(ℝ, E) x) :=
⟨g.toRiemannianMetric⟩
letI : IsContinuousRiemannianBundle E (fun x : M => TangentSpace 𝓘(ℝ, E) x) :=
⟨⟨g.inner, g.contMDiff.continuous, fun _ _ _ => rfl⟩⟩
letI : EMetricSpace M := EMetricSpace.ofRiemannianMetric 𝓘(ℝ, E) M
letI : MeasurableSpace M := borel M
letI : BorelSpace M := ⟨rfl⟩
Measure.hausdorffMeasure d {x | u x = 0}
/-- The eigensequence belongs to one fixed metric and has finite nodal measure. -/
def HasUnboundedNodalRatio [T3Space M] (g : SmoothMetric E M) (d : ℕ) : Prop :=
∃ (u : ℕ → M → ℝ) (eigenvalue : ℕ → ℝ),
(∀ j, ContMDiff 𝓘(ℝ, E) 𝓘(ℝ, ℝ) ∞ (u j) ∧ u j ≠ 0 ∧ 0 < eigenvalue j ∧
(∀ x, -laplaceBeltrami g (u j) x = eigenvalue j * u j x) ∧
nodalMeasure g (d - 1 : ℕ) (u j) ≠ (∞ : ℝ≥0∞)) ∧
Tendsto eigenvalue atTop atTop ∧
Tendsto (fun j => (nodalMeasure g (d - 1 : ℕ) (u j)).toReal / Real.sqrt (eigenvalue j))
atTop atTop
end Definitions
/-- The round metric is the pullback of the Euclidean scalar product under
inclusion of the unit sphere. This predicate specifies exactly that tensor. -/
def IsRound (g : SmoothMetric (Euclidean 3) (Sphere 3)) : Prop :=
∀ (x : Sphere 3) (v w : TangentSpace 𝓘(ℝ, Euclidean 3) x),
g.inner x v w = inner ℝ
(mfderiv 𝓘(ℝ, Euclidean 3) 𝓘(ℝ, Euclidean 4)
(fun y : Sphere 3 => (y : Euclidean 4)) x v)
(mfderiv 𝓘(ℝ, Euclidean 3) 𝓘(ℝ, Euclidean 4)
(fun y : Sphere 3 => (y : Euclidean 4)) x w)
end YauCounterexamples
end
end OAI
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