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Derivative bounds for partial maps along a compact parameter

Proved
Nemytskii.iteratedFDeriv_bounds_of_comp_compact

by Mazecto · Nov 12, 2026 · Mathlib 0df444a (Lean v4.33.1)

functional-analysisnonlinear-analysis

Let P,EP, EP,E be proper real normed spaces, FFF a real normed space, Ψ:P×E→F\Psi : P \times E \to FΨ:P×E→F of class C∞C^\inftyC∞, and p:X→Pp : X \to Pp:X→P continuous with values in a compact set KKK. Then for Φ(x,a)=Ψ(p(x),a)\Phi(x, a) = \Psi(p(x), a)Φ(x,a)=Ψ(p(x),a): each Φ(x,⋅)\Phi(x, \cdot)Φ(x,⋅) is C∞C^\inftyC∞, and for every kkk the derivative (x,a)↦DkΦ(x,⋅)(a)(x, a) \mapsto D^k\Phi(x, \cdot)(a)(x,a)↦DkΦ(x,⋅)(a) is jointly continuous and bounded on X×BˉR(0)X \times \bar B_R(0)X×BˉR​(0) for every RRR.

Role. Verifies the hypotheses of Nemytskii.exists_contDiff_superposition for integrands that depend on xxx through a compact parameter.

Preamble
import Mathlib

open Metric Set Filter Topology BoundedContinuousFunction
open scoped ContDiff
Formal statement
namespace Nemytskii

/-- If `Ψ : P × E → F` is `C^∞` (with `P`, `E` proper) and `p : X → P` is continuous with values
in a compact set, then the partial maps `a ↦ Ψ (p x, a)` are `C^∞`, and each of their derivatives
is jointly continuous in `(x, a)` and bounded on `X × closedBall 0 R` for every `R`. -/
theorem iteratedFDeriv_bounds_of_comp_compact {X : Type*} [TopologicalSpace X]
    {P E F : Type*} [NormedAddCommGroup P] [NormedSpace ℝ P] [ProperSpace P]
    [NormedAddCommGroup E] [NormedSpace ℝ E] [ProperSpace E]
    [NormedAddCommGroup F] [NormedSpace ℝ F]
    (Ψ : P × E → F) (hΨ : ContDiff ℝ ∞ Ψ) (p : X → P) (hp : Continuous p) (K : Set P)
    (hK : IsCompact K) (hpK : ∀ x, p x ∈ K) :
    (∀ x, ContDiff ℝ ∞ ((fun x a => Ψ (p x, a)) x)) ∧
      (∀ k : ℕ, Continuous (fun y : X × E => iteratedFDeriv ℝ k ((fun x a => Ψ (p x, a)) y.1) y.2)) ∧
      ∀ (k : ℕ) (R : ℝ), ∃ C : ℝ, ∀ x, ∀ a ∈ closedBall (0 : E) R,
        ‖iteratedFDeriv ℝ k ((fun x a => Ψ (p x, a)) x) a‖ ≤ C := by
  sorry

end Nemytskii
Source
Superposition (Nemytskii) operators, cf. J. Appell and P. P. Zabrejko, Nonlinear Superposition Operators, Cambridge Univ. Press 1990; used toward U. Hryniewicz, Fast finite-energy planes in symplectizations and applications, Trans. AMS 364 (2012), arXiv:0812.4076, Lemmas 6.10 and 6.20

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