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The LpL^pLp estimate for ∂s+J0∂t\partial_s+J_0\partial_t∂s​+J0​∂t​ on compactly supported maps R2→R2n\mathbb R^2\to\mathbb R^{2n}R2→R2n

Proved
CauchyRiemannIndex.plane_cauchyRiemann_estimate

by Mazecto · Oct 15, 2026 · Mathlib 0df444a (Lean v4.33.1)

cauchy-riemannelliptic-estimatesharmonic-analysis

Let 1<p<∞1<p<\infty1<p<∞ and n∈Nn\in\mathbb Nn∈N, and let J0=(0−IdId0)J_0=\begin{pmatrix}0&-\mathrm{Id}\\ \mathrm{Id}&0\end{pmatrix}J0​=(0Id​−Id0​) be the standard complex structure on R2n\mathbb R^{2n}R2n. There is a constant AAA such that every continuously differentiable map v:R2→R2nv:\mathbb R^2\to\mathbb R^{2n}v:R2→R2n with compact support satisfies

∥∂sv∥Lp(R2)+∥∂tv∥Lp(R2)≤A ∥∂sv+J0 ∂tv∥Lp(R2),\|\partial_s v\|_{L^p(\mathbb R^2)}+\|\partial_t v\|_{L^p(\mathbb R^2)}\le A\,\|\partial_s v+J_0\,\partial_t v\|_{L^p(\mathbb R^2)},∥∂s​v∥Lp(R2)​+∥∂t​v∥Lp(R2)​≤A∥∂s​v+J0​∂t​v∥Lp(R2)​,

where (s,t)(s,t)(s,t) are the coordinates of R2\mathbb R^2R2 and the measure is Lebesgue measure.

This is the basic LpL^pLp elliptic estimate for the Cauchy–Riemann operator ∂s+J0∂t\partial_s+J_0\partial_t∂s​+J0​∂t​ in real notation, in the form used for linear Cauchy–Riemann type operators on Riemann surfaces and cylinders (Wendl, Lecture 4).

Formalization Note. R2n\mathbb R^{2n}R2n is EuclideanSpace ℝ (Fin n ⊕ Fin n), J0J_0J0​ is Matrix.J, acting through Matrix.toEuclideanLin. Points of the plane are pairs (s,t)∈R×R(s,t)\in\mathbb R\times\mathbb R(s,t)∈R×R; ∂sv\partial_s v∂s​v and ∂tv\partial_t v∂t​v are partialS v x = fderiv ℝ v x (1, 0) and partialT v x = fderiv ℝ v x (0, 1). C1C^1C1 means ContDiff ℝ 1. Norms are eLpNorm with values in [0,∞][0,\infty][0,∞], for an exponent p : ℝ≥0∞ with 1<p1<p1<p and p≠∞p\ne\inftyp=∞. The constant AAA is chosen before vvv; the measure on R×R\mathbb R\times\mathbb RR×R is volume.

Preamble
import Definitions.Def_CauchyRiemannIndex_Setting
import Mathlib.Analysis.Calculus.ContDiff.Basic
Formal statement
namespace CauchyRiemannIndex

open MeasureTheory Matrix
open scoped ENNReal NNReal

/-- For `1 < p < ∞` there is `A` such that every `C¹` map `v : ℝ × ℝ → ℝ²ⁿ` with compact support
satisfies `‖∂ₛv‖_p + ‖∂ₜv‖_p ≤ A ‖∂ₛv + J₀ ∂ₜv‖_p` on the plane. -/
theorem plane_cauchyRiemann_estimate (n : ℕ) (p : ℝ≥0∞) (hp : 1 < p) (hp' : p ≠ ⊤) :
    ∃ A : ℝ≥0, ∀ v : ℝ × ℝ → Vec n, ContDiff ℝ 1 v → HasCompactSupport v →
      eLpNorm (partialS v) p volume + eLpNorm (partialT v) p volume ≤
        A * eLpNorm (fun x => partialS v x + Matrix.toEuclideanLin (J₀ n) (partialT v x)) p volume := by
  sorry

end CauchyRiemannIndex
Source
E. M. Stein, Singular Integrals and Differentiability Properties of Functions, Princeton 1970, Chapter III, § 1.3, Proposition 4 (p. 60), written for maps into R^{2n} = C^n with the standard complex structure J_0

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