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Bound on an intermediate derivative of a smooth map on a half-plane by its neighbouring derivatives

Proved
FiniteEnergyPlane.norm_iteratedFDeriv_succ_le_of_halfPlane

by Mazecto · Oct 29, 2026 · Mathlib 0df444a (Lean v4.33.1)

asymptotic-analysisinequalitiesreal-analysis

Let FFF be a real normed space, s0∈Rs_0\in\mathbb{R}s0​∈R, and let f:R×R→Ff:\mathbb{R}\times\mathbb{R}\to Ff:R×R→F be C∞C^\inftyC∞ on the open half-plane (s0,∞)×R(s_0,\infty)\times\mathbb{R}(s0​,∞)×R. Let j∈Nj\in\mathbb{N}j∈N, let p=(p1,p2)p=(p_1,p_2)p=(p1​,p2​) with p1>s0p_1>s_0p1​>s0​, and let A,B∈RA,B\in\mathbb{R}A,B∈R, h>0h>0h>0. Suppose that for every qqq with q1≥p1q_1\ge p_1q1​≥p1​ we have ∥Djf(q)∥≤A\|D^jf(q)\|\le A∥Djf(q)∥≤A and ∥Dj+2f(q)∥≤B\|D^{j+2}f(q)\|\le B∥Dj+2f(q)∥≤B. Then

∥Dj+1f(p)∥≤2(2Ah+Bh).\|D^{j+1}f(p)\|\le 2\Big(\frac{2A}{h}+Bh\Big).∥Dj+1f(p)∥≤2(h2A​+Bh).

This is a pointwise interpolation inequality for consecutive derivative orders on a half-plane, using only the region to the right of the point. It is meant for estimating derivatives of maps defined on cylindrical ends.

Formalization Note DkfD^kfDkf is iteratedFDeriv ℝ k f, with the norm of continuous multilinear maps on ℝ × ℝ (sup norm). The half-plane is Set.Ioi s₀ ×ˢ Set.univ, and the bounds are assumed on {q | p.1 ≤ q.1}.

Preamble
import Mathlib
Formal statement
namespace FiniteEnergyPlane

open scoped ContDiff

/-- On a half-plane `{s > s₀}`, the norm of the `(j+1)`-st derivative of a smooth map at `p` is
bounded by `2 (2A/h + Bh)` for every `h > 0`, where `A` and `B` bound the `j`-th and `(j+2)`-nd
derivatives on `{s ≥ p.1}`. -/
theorem norm_iteratedFDeriv_succ_le_of_halfPlane {F : Type*} [NormedAddCommGroup F]
    [NormedSpace ℝ F] (f : ℝ × ℝ → F) (s₀ : ℝ)
    (hf : ContDiffOn ℝ ∞ f (Set.Ioi s₀ ×ˢ Set.univ)) (j : ℕ) (p : ℝ × ℝ) (hp : s₀ < p.1)
    (A B h : ℝ) (hh : 0 < h)
    (hA : ∀ q : ℝ × ℝ, p.1 ≤ q.1 → ‖iteratedFDeriv ℝ j f q‖ ≤ A)
    (hB : ∀ q : ℝ × ℝ, p.1 ≤ q.1 → ‖iteratedFDeriv ℝ (j + 2) f q‖ ≤ B) :
    ‖iteratedFDeriv ℝ (j + 1) f p‖ ≤ 2 * (2 * A / h + B * h) := by
  sorry

end FiniteEnergyPlane
Source
Elementary real analysis (Landau-Kolmogorov type inequality); used for U. Hryniewicz, J. Symplectic Geom. 12 (2014), Theorem 2.10

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