Kernel dimension bound for Cauchy–Riemann operators with a large antilinear zeroth-order term

Proved

CauchyRiemannIndex.large_antilinear_kernel_le

by Mazecto · Oct 17, 2026 · Mathlib 0df444a (Lean v4.33.1)

cauchy-riemannfredholm-indexsobolev-spacessymplectic-geometry

Let $n=1$ , let $S^\pm$ be continuous loops of symmetric $2\times2$ matrices with nondegenerate asymptotic operators, and let $S:\mathbb{R}\times S^1\to\mathbb{R}^{2\times2}$ be continuous with $S(s,\cdot)\to S^\pm$ uniformly as $s\to\pm\infty$ . Let $\beta:\mathbb{R}\times S^1\to\mathbb{C}$ be smooth with bounded first derivative, such that $|\beta|$ and $1/|\beta|$ are bounded for large $|s|$ , and such that its zeros form finite sets $Z^+$ (positive zeros) and $Z^-$ (negative zeros) with the following normal form: for some $r>0$ and every $\zeta\in Z^\pm$ , on the disc $|z|<r$ around $\zeta$ one has $S=0$ and

\beta(\zeta+z)=z\ \ (\zeta\in Z^+),\qquad\beta(\zeta+z)=\bar z\ \ (\zeta\in Z^-).

For $\sigma>0$ let $D_\sigma\eta=\partial_s\eta+J_0\partial_t\eta+S\eta+\sigma\beta\bar\eta$ , acting from $W^{1,2}$ to $L^2$ .

Then there is $\sigma_0$ such that for all $\sigma\ge\sigma_0$ the kernel of $D_\sigma$ is finite-dimensional and

\dim\ker D_\sigma\le\#Z^+.

This is the kernel half of Wendl's Lemma 5.22; together with the cokernel bound and the two matching lower bounds it gives the kernel and cokernel of $D_\sigma$ for large $\sigma$ .

Formalization Note $\mathbb{C}=\mathbb{R}^2$ via $(x,y)\mapsto x+iy$ , and $\eta\mapsto\beta\bar\eta$ is the matrix antilinearMat β, so $D_\sigma$ is $D_{S_\sigma}$ with $S_\sigma=S+\sigma\cdot$ antilinearMat ∘ β. Points of the cylinder are pairs $(s,t)$ and $\zeta+z$ means $(\zeta_s+\operatorname{Re}z,\zeta_t+\operatorname{Im}z)$ ; $\beta$ is $1$ -periodic in $t$ , the zeros are listed in $\mathbb{R}\times[0,1)$ , and every zero of $\beta$ is a $t$ -translate by an integer of a listed one. The hypotheses are those of the milestone CauchyRiemannIndex.large_antilinear_perturbation_index (Wendl, Proposition 5.27, with the assumptions of § 5.5); the chart near each zero is a translation of the coordinates $(s,t)$ , the trivialization is the global one, and the area form is $ds\,dt$ . The exponent is $p=2$ . The conclusion is stated with FiniteDimensional and finrank of crKernel 2 ….

Preamble

import Definitions.Def_CauchyRiemannIndex_Setting
import Mathlib.Analysis.Calculus.ContDiff.Basic

Formal statement

namespace CauchyRiemannIndex

open MeasureTheory Matrix Filter Topology
open scoped ENNReal NNReal ContDiff RealInnerProductSpace

/-- Wendl, Lemma 5.22 (p. 97), kernel part, with the assumptions of § 5.5 (p. 94), on the cylinder
with the trivial line bundle and `p = 2`: let `D_S` (`n = 1`) be asymptotic to nondegenerate loops `S±` of symmetric
matrices, and let `β : ℝ × S¹ → ℂ` be smooth with bounded derivative, with `|β|` and `1/|β|`
bounded for large `|s|`, whose zeros (mod `t ↦ t + 1`) are the finite sets `Z⁺` (positive zeros) and
`Z⁻` (negative zeros), such that on a disc of radius `r` around each zero `ζ` the operator is `∂̄`
(`S = 0`) and `β(ζ + z) = z` for `ζ ∈ Z⁺`, `β(ζ + z) = z̄` for `ζ ∈ Z⁻`. Put
`D_σ η = D_S η + σ β η̄`. Then for all large `σ`, the kernel of
`D_σ : W^{1,2} → L²` is finite-dimensional of dimension at most `#Z⁺`. -/
theorem large_antilinear_kernel_le (Splus Sminus : ℝ → Mat 1) (hSplus : IsSymLoop Splus)
    (hSminus : IsSymLoop Sminus) (hplus : IsNondegenerate Splus) (hminus : IsNondegenerate Sminus)
    (S : ℝ × ℝ → Mat 1) (hS : IsAsymptoticTo S Splus Sminus)
    (β : ℝ × ℝ → ℂ) (hβ : ContDiff ℝ ∞ β) (hβper : ∀ s t, β (s, t + 1) = β (s, t))
    (hβC1 : ∃ C : ℝ, ∀ x, ‖fderiv ℝ β x‖ ≤ C)
    (hβends : ∃ (R c : ℝ), 0 < c ∧ ∀ s t, R ≤ |s| → c ≤ ‖β (s, t)‖ ∧ ‖β (s, t)‖ ≤ 1 / c)
    (Zplus Zminus : Finset (ℝ × ℝ)) (hZ : ∀ ζ ∈ Zplus ∪ Zminus, 0 ≤ ζ.2 ∧ ζ.2 < 1)
    (hdisj : Disjoint Zplus Zminus)
    (hzero : ∀ x : ℝ × ℝ, β x = 0 ↔ ∃ ζ ∈ Zplus ∪ Zminus, ∃ m : ℤ, x = (ζ.1, ζ.2 + m))
    (r : ℝ) (hr : 0 < r)
    (hplusZ : ∀ ζ ∈ Zplus, ∀ z : ℂ, ‖z‖ < r →
      β (ζ.1 + z.re, ζ.2 + z.im) = z ∧ S (ζ.1 + z.re, ζ.2 + z.im) = 0)
    (hminusZ : ∀ ζ ∈ Zminus, ∀ z : ℂ, ‖z‖ < r →
      β (ζ.1 + z.re, ζ.2 + z.im) = (starRingEnd ℂ) z ∧ S (ζ.1 + z.re, ζ.2 + z.im) = 0) :
    ∃ σ₀ : ℝ, ∀ σ ≥ σ₀,
      FiniteDimensional ℝ (crKernel 2 (fun x => S x + σ • antilinearMat (β x))) ∧
      Module.finrank ℝ (crKernel 2 (fun x => S x + σ • antilinearMat (β x))) ≤ Zplus.card := by
  sorry

end CauchyRiemannIndex

Source

C. Wendl, Lectures on Symplectic Field Theory, arXiv:1612.01009, https://arxiv.org/abs/1612.01009, Lecture 5, Lemma 5.22, p. 97, with the assumptions of § 5.5, p. 94 (cylinder, trivial line bundle, p = 2)

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