The Yamabe problem and conformal scalar curvature

The declarations the status of Tau Ceti's roadmap names, and the other results named in the modules where only this status names declarations, or in folders below the areas where only it names 2 or more, as Tau Ceti has them at commit ace60277.

TheoremScalar curvature of a conformal metric#

In dimension n ≠ 2, let u be a positive function such that ĝ = u ^ (4 / (n - 2)) g is a smooth metric. Then the scalar curvature of ĝ is Scal_ĝ = u ^ (-(n + 2) / (n - 2)) L_g u, where L_g is the conformal Laplacian of g. Such a u is automatically smooth.

TauCeti.Yamabe.scalarCurvature_rescale_eq_rpow_mul_conformalLaplacian.{u_1, u_2, u_3} {E : Type u_1}
  [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2}
  [TopologicalSpace H] {I : ModelWithCorners ℝ E H} {M : Type u_3} [TopologicalSpace M]
  [ChartedSpace H M] [T2Space M] [IsManifold I (↑⊤) M]
  (g : Bundle.ContMDiffRiemannianMetric I (↑⊤) E fun x => TangentSpace I x)
  (hn : Module.finrank ℝ E ≠ 2) {u : M → ℝ} (hu_pos : ∀ (x : M), 0 < u x)
  {hf : ContMDiff I (modelWithCornersSelf ℝ ℝ) ↑⊤ fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))}
  {hf_pos : ∀ (x : M), 0 < u x ^ (4 / (↑(Module.finrank ℝ E) - 2))} (x : M) :
  (g.rescale (fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))) hf hf_pos).scalarCurvature x =
    u x ^ (-(↑(Module.finrank ℝ E) + 2) / (↑(Module.finrank ℝ E) - 2)) *
      TauCeti.Yamabe.conformalLaplacian I u x
Uses 3 · Used by 2

TheoremThe Yamabe equation#

In dimension n ≠ 2, let u be a positive function such that ĝ = u ^ (4 / (n - 2)) g is a smooth metric. Then ĝ has scalar curvature c at a point exactly when u solves the Yamabe equation L_g u = c u ^ (pₙ - 1) there, where pₙ = 2 n / (n - 2).

TauCeti.Yamabe.scalarCurvature_rescale_eq_iff_conformalLaplacian_eq.{u_1, u_2, u_3} {E : Type u_1}
  [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2}
  [TopologicalSpace H] {I : ModelWithCorners ℝ E H} {M : Type u_3} [TopologicalSpace M]
  [ChartedSpace H M] [T2Space M] [IsManifold I (↑⊤) M]
  (g : Bundle.ContMDiffRiemannianMetric I (↑⊤) E fun x => TangentSpace I x)
  (hn : Module.finrank ℝ E ≠ 2) {u : M → ℝ} (hu_pos : ∀ (x : M), 0 < u x)
  {hf : ContMDiff I (modelWithCornersSelf ℝ ℝ) ↑⊤ fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))}
  {hf_pos : ∀ (x : M), 0 < u x ^ (4 / (↑(Module.finrank ℝ E) - 2))} (c : ℝ) (x : M) :
  (g.rescale (fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))) hf hf_pos).scalarCurvature x = c ↔
    TauCeti.Yamabe.conformalLaplacian I u x =
      c * u x ^ (TauCeti.Yamabe.criticalExponent (Module.finrank ℝ E) - 1)
Uses 3

TheoremVolume-normalized total scalar curvature of a conformal metric#

On a compact manifold of dimension n ≠ 2, let u be a positive function such that ĝ = u ^ (4 / (n - 2)) g is a smooth metric, and let pₙ = 2 n / (n - 2). Then (∫ Scal_ĝ dV_ĝ) / Vol(ĝ) ^ (2 / pₙ) = (∫ u L_g u dV_g) / (∫ u ^ pₙ dV_g) ^ (2 / pₙ), where L_g is the conformal Laplacian of g.

TauCeti.Yamabe.integral_scalarCurvature_rescale_div_riemannianTotalVolume_rpow.{u_1, u_2, u_3}
  {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2}
  [TopologicalSpace H] {I : ModelWithCorners ℝ E H} {M : Type u_3} [TopologicalSpace M]
  [ChartedSpace H M] [T2Space M] [IsManifold I (↑⊤) M] [LindelofSpace M] [CompactSpace M]
  (g : Bundle.ContMDiffRiemannianMetric I (↑⊤) E fun x => TangentSpace I x)
  (hn : Module.finrank ℝ E ≠ 2) {u : M → ℝ} (hu_pos : ∀ (x : M), 0 < u x)
  {hf : ContMDiff I (modelWithCornersSelf ℝ ℝ) ↑⊤ fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))}
  {hf_pos : ∀ (x : M), 0 < u x ^ (4 / (↑(Module.finrank ℝ E) - 2))} :
  (∫ (x :
        TauCeti.WithMetric
          (g.rescale (fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))) hf hf_pos)),
        (g.rescale (fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))) hf hf_pos).scalarCurvature
          x.ofWithMetric ∂TauCeti.riemannianVolume I
          (TauCeti.WithMetric
            (g.rescale (fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))) hf hf_pos))) /
      TauCeti.riemannianTotalVolume I
          (TauCeti.WithMetric
            (g.rescale (fun x => u x ^ (4 / (↑(Module.finrank ℝ E) - 2))) hf hf_pos)) ^
        (2 / TauCeti.Yamabe.criticalExponent (Module.finrank ℝ E)) =
    (∫ (x : TauCeti.WithMetric g),
        u x.ofWithMetric *
          TauCeti.Yamabe.conformalLaplacian I u
            x.ofWithMetric ∂TauCeti.riemannianVolume I (TauCeti.WithMetric g)) /
      (∫ (x : TauCeti.WithMetric g),
          u x.ofWithMetric ^
            TauCeti.Yamabe.criticalExponent
              (Module.finrank ℝ E) ∂TauCeti.riemannianVolume I (TauCeti.WithMetric g)) ^
        (2 / TauCeti.Yamabe.criticalExponent (Module.finrank ℝ E))
Uses 5

TheoremWeyl tensor under conformal change#

Away from dimension 2, the fully covariant Weyl tensor of a conformally rescaled smooth metric f g is f times that of g: W^(f g)(u, v, w, z) = f W^g(u, v, w, z). Equivalently, the Weyl tensor of type (1, 3) is conformally invariant.

Bundle.ContMDiffRiemannianMetric.weylTensor_leviCivitaConnection_rescale.{u_1, u_2, u_3}
  {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2}
  [TopologicalSpace H] {I : ModelWithCorners ℝ E H} {M : Type u_3} [TopologicalSpace M]
  [ChartedSpace H M] [T2Space M] [IsManifold I (↑⊤) M]
  (g : Bundle.ContMDiffRiemannianMetric I (↑⊤) E fun x => TangentSpace I x) {f : M → ℝ}
  (hf : ContMDiff I (modelWithCornersSelf ℝ ℝ) (↑⊤) f) (hf_pos : ∀ (x : M), 0 < f x)
  (hn : Module.finrank ℝ E ≠ 2) (x : M) (u v w z : TangentSpace I x) :
  (((((CovariantDerivative.leviCivitaConnection I M).weylTensor x) u) v) w) z =
    f x * (((((CovariantDerivative.leviCivitaConnection I M).weylTensor x) u) v) w) z
Uses 2 · Used by 2

TheoremLocal conformal flatness forces the Weyl tensor to vanish#

Away from dimension 2, the Weyl tensor of a locally conformally flat metric vanishes at every point. This is the easy half of the Weyl–Schouten theorem.

Bundle.ContMDiffRiemannianMetric.IsLocallyConformallyFlat.weylTensor_eq_zero.{u_1, u_2, u_3}
  {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2}
  [TopologicalSpace H] {I : ModelWithCorners ℝ E H} {M : Type u_3} [TopologicalSpace M]
  [ChartedSpace H M] [T2Space M] [IsManifold I (↑⊤) M]
  {g : Bundle.ContMDiffRiemannianMetric I (↑⊤) E fun x => TangentSpace I x}
  (hg : g.IsLocallyConformallyFlat) (hn : Module.finrank ℝ E ≠ 2) (x : M) :
  (CovariantDerivative.leviCivitaConnection I M).weylTensor x = 0
Uses 3 · Used by 1

TheoremThe bubbles attain the Yamabe constant of the sphere#

in dimension n > 2, for Lebesgue measure, aₙ ∫ ‖dU_{t,b}‖² = Y(Sⁿ) (∫ U_{t,b} ^ pₙ) ^ (2 / pₙ). This is the equality case of the sharp Euclidean Sobolev inequality.

TauCeti.Yamabe.conformalCoefficient_mul_integral_norm_fderiv_bubble_sq_eq_sphereYamabeConstant.{u_1}
  {E : Type u_1} [NormedAddCommGroup E] [InnerProductSpace ℝ E] [FiniteDimensional ℝ E]
  [MeasurableSpace E] [BorelSpace E] (hn : 2 < Module.finrank ℝ E) {t : ℝ} (ht : 0 < t) (b : E) :
  TauCeti.Yamabe.conformalCoefficient (Module.finrank ℝ E) *
      ∫ (x : E), ‖fderiv ℝ (TauCeti.Yamabe.bubble t b) x‖ ^ 2 =
    TauCeti.Yamabe.sphereYamabeConstant (Module.finrank ℝ E) *
      (∫ (x : E),
          TauCeti.Yamabe.bubble t b x ^ TauCeti.Yamabe.criticalExponent (Module.finrank ℝ E)) ^
        (2 / TauCeti.Yamabe.criticalExponent (Module.finrank ℝ E))
Uses 1

TheoremThe area of the unit round sphere#

|Sⁿ| = 2 π ^ ((n + 1) / 2) / Γ((n + 1) / 2).

TauCeti.roundSphereVolume_eq_two_mul_pi_rpow_div_Gamma (n : ℕ) :
  TauCeti.roundSphereVolume n = 2 * Real.pi ^ ((↑n + 1) / 2) / Real.Gamma ((↑n + 1) / 2)
Used by 1

DefinitionTauCeti.Yamabe.conformalLaplacian#

The conformal Laplacian L_g u = aₙ Δ⁺_g u + Scal_g u of a function u on a smooth Riemannian manifold of dimension n, where aₙ = 4 (n - 1) / (n - 2), Δ⁺_g = -Δ_g is the nonnegative Laplacian and Scal_g is the scalar curvature.

TauCeti.Yamabe.conformalLaplacian.{u_1, u_2, u_3} {E : Type u_1} [NormedAddCommGroup E]
  [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2} [TopologicalSpace H]
  (I : ModelWithCorners ℝ E H) {M : Type u_3} [TopologicalSpace M] [ChartedSpace H M] [T2Space M]
  [IsManifold I (↑⊤) M] [Bundle.RiemannianBundle fun x => TangentSpace I x]
  [IsContMDiffRiemannianBundle I (↑⊤) E fun x => TangentSpace I x] (u : M → ℝ) (x : M) : ℝ
Uses 1 · Used by 3

DefinitionBundle.ContMDiffRiemannianMetric.IsLocallyConformallyFlat#

A smooth Riemannian metric g is locally conformally flat if every point x has a neighbourhood on which a conformal rescaling f g, by a smooth positive function f, is flat: the curvature tensor of the Levi-Civita connection of f g vanishes at every point near x.

Bundle.ContMDiffRiemannianMetric.IsLocallyConformallyFlat.{u_1, u_2, u_3} {E : Type u_1}
  [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2}
  [TopologicalSpace H] {I : ModelWithCorners ℝ E H} {M : Type u_3} [TopologicalSpace M]
  [ChartedSpace H M] [T2Space M] [IsManifold I (↑⊤) M]
  (g : Bundle.ContMDiffRiemannianMetric I (↑⊤) E fun x => TangentSpace I x) : Prop
Uses 1 · Used by 2

DefinitionCovariantDerivative.cottonTensor#

The Cotton tensor C(u, v, w) = (∇_w P)(u, v) - (∇_v P)(u, w) of a smooth connection, where P is the Schouten tensor and ∇P is CovariantDerivative.derivSchoutenTensor. The usual formula on vector fields holds for a smooth metric (see CovariantDerivative.cottonTensor_apply_of_mdifferentiableAt); otherwise ∇P, and hence the Cotton tensor, may be 0.

CovariantDerivative.cottonTensor.{u_1, u_2, u_3} {E : Type u_1} [NormedAddCommGroup E]
  [NormedSpace ℝ E] [FiniteDimensional ℝ E] {H : Type u_2} [TopologicalSpace H]
  {I : ModelWithCorners ℝ E H} {M : Type u_3} [TopologicalSpace M] [ChartedSpace H M] [T2Space M]
  [IsManifold I (↑⊤) M] [Bundle.RiemannianBundle (TangentSpace I)]
  (cov : CovariantDerivative I E (TangentSpace I)) [cov.ContMDiffCovariantDerivative ↑⊤] (x : M) :
  TangentSpace I x →ₗ[ℝ] TangentSpace I x →ₗ[ℝ] TangentSpace I x →ₗ[ℝ] ℝ
Used by 1

TheoremWeyl–Schouten in dimension three, on a model space#

An arbitrary smooth Riemannian metric is locally conformally flat if and only if its Cotton tensor vanishes.

Bundle.ContMDiffRiemannianMetric.isLocallyConformallyFlat_iff_cottonTensor_eq_zero_model_space.{u_1}
  {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E]
  (g :
    Bundle.ContMDiffRiemannianMetric (modelWithCornersSelf ℝ E) (↑⊤) E fun x =>
      TangentSpace (modelWithCornersSelf ℝ E) x)
  (hn : Module.finrank ℝ E = 3) :
  g.IsLocallyConformallyFlat ↔
    ∀ (x : E),
      (CovariantDerivative.leviCivitaConnection (modelWithCornersSelf ℝ E) E).cottonTensor x = 0
Lean

Formalized in Tau Ceti as Bundle.ContMDiffRiemannianMetric.isLocallyConformallyFlat_iff_cottonTensor_eq_zero_model_space.

Other forms

Weyl–Schouten in dimension at least four, on a model space. Formalized in Tau Ceti as Bundle.ContMDiffRiemannianMetric.isLocallyConformallyFlat_iff_weylTensor_eq_zero_model_space.

Bundle.ContMDiffRiemannianMetric.isLocallyConformallyFlat_iff_weylTensor_eq_zero_model_space.{u_1}
  {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E]
  (g :
    Bundle.ContMDiffRiemannianMetric (modelWithCornersSelf ℝ E) (↑⊤) E fun x =>
      TangentSpace (modelWithCornersSelf ℝ E) x)
  (hn : 4 ≤ Module.finrank ℝ E) :
  g.IsLocallyConformallyFlat ↔
    ∀ (x : E),
      (CovariantDerivative.leviCivitaConnection (modelWithCornersSelf ℝ E) E).weylTensor x = 0
Sources
Uses 9

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