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Measurability of Bernstein numbers and of pushforwards

Lax606786.MeasurableOperatorFamilies · concepts/Lax606786/MeasurableOperatorFamilies.lean · lax-606786

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    Natural Language Statement

    Theorem

    Let XX be a separable real normed space with a σ\sigma-algebra containing the open sets, and let ω↦Aω\omega \mapsto A_\omega be a strongly measurable family of bounded operators on XX (each ω↦Aωx\omega \mapsto A_\omega x measurable). Then for k≥1k \ge 1:

    • ω↦ρk(Aω)\omega \mapsto \rho_k(A_\omega) is measurable, and so is ω↦∥Aω∥\omega \mapsto \|A_\omega\|;
    • ω↦Aωv(ω)\omega \mapsto A_\omega v(\omega) is measurable for every measurable v:Ω→Xv : \Omega \to X (if the σ\sigma-algebra of XX is Borel);
    • ω↦g(Aω,V)\omega \mapsto g(A_\omega, V) is measurable for every subspace V≠0V \ne 0, and ω↦g(Aω,E(ω))\omega \mapsto g(A_\omega, E(\omega)) is measurable for every measurable E:Ω→GkXE : \Omega \to \mathcal{G}_k X (if the σ\sigma-algebra of XX is Borel);
    • if moreover XX is complete and E:Ω→GkXE : \Omega \to \mathcal{G}_k X is measurable, then any F:Ω→GkXF : \Omega \to \mathcal{G}_k X with F(ω)=AωE(ω)F(\omega) = A_\omega E(\omega) for every ω\omega is measurable.

    For the norm topology on the operators, T↦ρk(T)T \mapsto \rho_k(T) is itself Borel measurable.

    Concept map
    3 concepts
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    Proven claimDefinitionThis conceptRelated conceptA → B: B builds on A
    Evidence

    In the paper

    • page 3 of this submission's paper

    Lean source view on GitLab

    1import Lax606786.OperatorStatistics
    2import Mathlib.MeasureTheory.Constructions.BorelSpace.Basic
    3
    4import Mathlib.MeasureTheory.Constructions.BorelSpace.ContinuousLinearMap
    5/-!
    6---
    7title: Measurability of Bernstein numbers and of pushforwards
    8type: theorem
    9---
    10Let XX be a separable real normed space with a σ\sigma-algebra containing the open sets, and let
    11ω↦Aω\omega \mapsto A_\omega be a strongly measurable family of bounded operators on XX (each
    12ω↦Aωx\omega \mapsto A_\omega x measurable). Then for k≥1k \ge 1:
    13
    14- ω↦ρk(Aω)\omega \mapsto \rho_k(A_\omega) is measurable, and so is ω↦∥Aω∥\omega \mapsto \|A_\omega\|;
    15- ω↦Aωv(ω)\omega \mapsto A_\omega v(\omega) is measurable for every measurable v:Ω→Xv : \Omega \to X (if the
    16 σ\sigma-algebra of XX is Borel);
    17- ω↦g(Aω,V)\omega \mapsto g(A_\omega, V) is measurable for every subspace V≠0V \ne 0, and
    18 ω↦g(Aω,E(ω))\omega \mapsto g(A_\omega, E(\omega)) is measurable for every measurable
    19 E:Ω→GkXE : \Omega \to \mathcal{G}_k X (if the σ\sigma-algebra of XX is Borel);
    20- if moreover XX is complete and E:Ω→GkXE : \Omega \to \mathcal{G}_k X is measurable, then any
    21 F:Ω→GkXF : \Omega \to \mathcal{G}_k X with F(ω)=AωE(ω)F(\omega) = A_\omega E(\omega) for every ω\omega is
    22 measurable.
    23
    24For the norm topology on the operators, T↦ρk(T)T \mapsto \rho_k(T) is itself Borel measurable.
    25-/
    26
    27namespace Lax606786.MeasurableOperatorFamilies
    28
    29open TopologicalSpace
    30open Lax606786.BanachGrassmannian Lax606786.OperatorStatistics
    31
    32/-- `ω ↦ ‖T_ω‖` is measurable for a strongly measurable family `T` on a separable space. -/
    33axiom measurable_opNorm {Ω X Y : Type*} [MeasurableSpace Ω] [NormedAddCommGroup X]
    34 [NormedSpace ℝ X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] [MeasurableSpace Y] [BorelSpace Y]
    35 [SeparableSpace X] {T : Ω → X →L[ℝ] Y} (hT : ∀ x, Measurable fun ω => T ω x) :
    36 Measurable fun ω => ‖T ω‖
    37
    38variable {X : Type*} [NormedAddCommGroup X] [NormedSpace ℝ X]
    39
    40/-- `ω ↦ ρ_k(A_ω)` is measurable for strongly measurable `A`. -/
    41axiom measurable_bernsteinNumber {Ω : Type*} [MeasurableSpace Ω] [MeasurableSpace X]
    42 [OpensMeasurableSpace X] [SeparableSpace X] (k : ℕ) [NeZero k] {A : Ω → X →L[ℝ] X}
    43 (hA : ∀ x : X, Measurable fun ω => A ω x) :
    44 Measurable fun ω => bernsteinNumber (A ω) k
    45
    46/-- A strongly measurable family applied to a measurable vector field is measurable. -/
    47axiom measurable_apply {Ω : Type*} [MeasurableSpace Ω] [MeasurableSpace X]
    48 [BorelSpace X] [SeparableSpace X] {A : Ω → X →L[ℝ] X}
    49 (hA : ∀ x : X, Measurable fun ω => A ω x) {v : Ω → X} (hv : Measurable v) :
    50 Measurable fun ω => A ω (v ω)
    51
    52/-- `ω ↦ g(A_ω, V)` is measurable for a fixed subspace `V ≠ 0`. -/
    53axiom measurable_growth {Ω : Type*} [MeasurableSpace Ω] [MeasurableSpace X]
    54 [OpensMeasurableSpace X] [SeparableSpace X] {A : Ω → X →L[ℝ] X}
    55 (hA : ∀ x : X, Measurable fun ω => A ω x) (V : Submodule ℝ X)
    56 (hV : (Metric.sphere (0 : X) 1 ∩ (V : Set X)).Nonempty) :
    57 Measurable fun ω => growth (A ω) V
    58
    59/-- `ω ↦ g(A_ω, E(ω))` is measurable for a measurable family `E` in `𝒢_k X`. -/
    60axiom measurable_growth_moving {Ω : Type*} [MeasurableSpace Ω] [MeasurableSpace X]
    61 [BorelSpace X] [SeparableSpace X] {m : ℕ} [NeZero m]
    62 {A : Ω → X →L[ℝ] X} (hA : ∀ x : X, Measurable fun ω => A ω x)
    63 {E : Ω → GrassmannianFin X m} (hE : Measurable E) :
    64 Measurable fun ω => growth (A ω) ((E ω).1 : Submodule ℝ X)
    65
    66/-- `ω ↦ A_ω E(ω)` is measurable in `𝒢_k X`. -/
    67axiom measurable_pushforward {Ω : Type*} [MeasurableSpace Ω] [MeasurableSpace X]
    68 [OpensMeasurableSpace X] [SeparableSpace X] [CompleteSpace X] (k : ℕ)
    69 [NeZero k] (E : Ω → GrassmannianFin X k) (hE : Measurable E)
    70 (A : Ω → X →L[ℝ] X) (hA : ∀ x : X, Measurable fun ω => A ω x)
    71 (F : Ω → GrassmannianFin X k)
    72 (hF : ∀ ω, (F ω).1 = Submodule.map ((A ω : X →ₗ[ℝ] X)) (E ω).1) :
    73 Measurable F
    74
    75/-- For the norm topology on the operators, `T ↦ ρ_k(T)` is measurable. -/
    76axiom measurable_bernsteinNumber_norm [SeparableSpace X] (k : ℕ) [NeZero k]
    77 [Nonempty (GrassmannianFin X k)] :
    78 Measurable (fun T : X →L[ℝ] X => bernsteinNumber T k)
    79
    80end Lax606786.MeasurableOperatorFamilies
    81
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