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The push-off of the boundary of an embedded disk along its ξ\xiξ-projected conormal has linking number 000

Proved
HryniewiczCriterion.disk_conormal_pushOff_linking_zero

by Mazecto · Oct 7, 2026 · Mathlib 0df444a (Lean v4.33.1)

geometry-topologysymplectic-geometry

Let S=H−1(1)S=H^{-1}(1)S=H−1(1) be strictly star-shaped, P=(x,T)P=(x,T)P=(x,T) a prime periodic orbit, and eee a smooth embedding of the closed unit disk into SSS with e(S1)=x(R)e(S^1)=x(\mathbb{R})e(S1)=x(R), and u(s)u(s)u(s) a smooth 111-periodic lift with e(u(s))=x(Ts)e(u(s))=x(Ts)e(u(s))=x(Ts). Push PPP off along the ξ\xiξ-projection of the outward conormal n(s)=de(u(s)) u(s)n(s)=de(u(s))\,u(s)n(s)=de(u(s))u(s),

γε(s)=x(Ts)+ε πξn(s)∣x(Ts)+ε πξn(s)∣,πξn=n−λ0(n)λ0(XH)XH.\gamma_\varepsilon(s)=\frac{x(Ts)+\varepsilon\,\pi_\xi n(s)}{|x(Ts)+\varepsilon\,\pi_\xi n(s)|},\qquad \pi_\xi n=n-\frac{\lambda_0(n)}{\lambda_0(X_H)}X_H .γε​(s)=∣x(Ts)+επξ​n(s)∣x(Ts)+επξ​n(s)​,πξ​n=n−λ0​(XH​)λ0​(n)​XH​.

Then for all small ε>0\varepsilon>0ε>0 the loops s↦x(Ts)/∣x(Ts)∣s\mapsto x(Ts)/|x(Ts)|s↦x(Ts)/∣x(Ts)∣ and γε\gamma_\varepsilonγε​ in S3S^3S3 have linking number 000, in the sense of IsLinkingNumber (Gauss integral after stereographic projection).

Proof idea: πξn\pi_\xi nπξ​n and nnn differ by a multiple of XHX_HXH​, which is tangent to PPP, so the two push-offs are homotopic in the complement of PPP. The push-off along nnn is close to e((1+ε)u(s))e((1+\varepsilon)u(s))e((1+ε)u(s)), which bounds the disk ρ↦e(ρ u)\rho\mapsto e(\rho\,u)ρ↦e(ρu), ρ≤1+ε\rho\le 1+\varepsilonρ≤1+ε, missing PPP after radial projection. So it is null-homotopic in S3∖PS^3\setminus PS3∖P, and the Gauss integral vanishes. This is the statement that the Seifert framing of a knot bounding a disk is 000.

Preamble
import Definitions.Def_HryniewiczCriterion_GlobalSection
import Definitions.Def_HryniewiczCriterion_SelfLinking

open HryniewiczCriterion
open scoped ContDiff
Formal statement
theorem HryniewiczCriterion.disk_conormal_pushOff_linking_zero (H : R4 → ℝ) (hS : IsStrictlyStarShapedLevel H) (P : PeriodicOrbit H) (hP : P.IsPrime)
    (e : Plane → R4) (he : IsSmoothDiskEmbedding e)
    (hDS : e '' closedUnitDisk ⊆ energySurface H) (hbd : e '' unitCircle = P.image)
    (u : ℝ → Plane) (hu : ContDiff ℝ ∞ u) (huper : ∀ s, u (s + 1) = u s)
    (hucirc : ∀ s, u s ∈ unitCircle) (heu : ∀ s, e (u s) = P.x (P.T * s)) :
    ∃ ε₀ : ℝ, 0 < ε₀ ∧ ∀ ε : ℝ, 0 < ε → ε < ε₀ →
      IsLinkingNumber (orbitLoop P)
        (fun s => radialNormalize (P.x (P.T * s) + ε • (fderiv ℝ e (u s) (u s) - (liouvilleForm (P.x (P.T * s)) (fderiv ℝ e (u s) (u s)) / liouvilleForm (P.x (P.T * s)) (hamiltonianVectorField H (P.x (P.T * s)))) • hamiltonianVectorField H (P.x (P.T * s))))) 0 := by sorry
Source
Seifert framing of the boundary of an embedded disk (Rolfsen, Knots and Links, 1976, Ch. 5D); the step link(P,P′)=0\mathrm{link}(P,P')=0link(P,P′)=0 for the disk push-off in U. Hryniewicz, Fast finite-energy planes in symplectizations and applications, Trans. Amer. Math. Soc. 364 (2012), 1859–1931, Proposition 2.1 (cited for necessity in Hryniewicz, Systems of global surfaces of section for dynamically convex Reeb flows on the 3-sphere, J. Symplectic Geom. 12 (2014), arXiv:1105.2077, outline after Theorem 1.8).

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