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Non-degenerate approximations of a star-shaped Hamiltonian, agreeing with it to first order along two given non-degenerate periodic orbits

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HryniewiczCriterion.exists_nondegenerate_approx_of_nondegenerate_along_orbits

by Mazecto · Oct 20, 2026 · Mathlib 0df444a (Lean v4.33.1)

dynamical-systemsgeometry-topologysymplectic-geometry

Let S=H−1(1)⊂R4S=H^{-1}(1)\subset\mathbb{R}^4S=H−1(1)⊂R4 be a strictly star-shaped level and let P=(xP,TP)P=(x_P,T_P)P=(xP​,TP​), Q=(xQ,TQ)Q=(x_Q,T_Q)Q=(xQ​,TQ​) be periodic orbits of XHX_HXH​ on SSS such that every periodic orbit of XHX_HXH​ on SSS whose image lies in xP(R)∪xQ(R)x_P(\mathbb{R})\cup x_Q(\mathbb{R})xP​(R)∪xQ​(R) (that is, PPP, QQQ and all their iterates) is non-degenerate. Then there is a sequence of Hamiltonians Hk:R4→RH_k:\mathbb{R}^4\to\mathbb{R}Hk​:R4→R such that

  1. every level Hk−1(1)H_k^{-1}(1)Hk−1​(1) is strictly star-shaped;
  2. every HkH_kHk​ is non-degenerate: every periodic orbit of XHkX_{H_k}XHk​​ on Hk−1(1)H_k^{-1}(1)Hk−1​(1), prime or multiply covered, is non-degenerate;
  3. Hk=HH_k=HHk​=H and dHk=dHdH_k=dHdHk​=dH at every point of xP(R)∪xQ(R)x_P(\mathbb{R})\cup x_Q(\mathbb{R})xP​(R)∪xQ​(R);
  4. Hk→HH_k\to HHk​→H in Cloc∞C^\infty_{\mathrm{loc}}Cloc∞​.

Hk→HH_k\to HHk​→H in Cloc∞C^\infty_{\mathrm{loc}}Cloc∞​ means: for every m∈Nm\in\mathbb{N}m∈N and every compact K⊂R4K\subset\mathbb{R}^4K⊂R4, DmHk→DmHD^mH_k\to D^mHDmHk​→DmH uniformly on KKK. A periodic orbit R=(x,T)R=(x,T)R=(x,T) on H−1(1)H^{-1}(1)H−1(1) (prime or not) is non-degenerate when, for its linearized flow YYY, the linearized return map on ξ=TS/RXH\xi=TS/\mathbb{R}X_{H}ξ=TS/RXH​, written in the global frame (Z1,Z2)(Z_1,Z_2)(Z1​,Z2​) as the matrix φ(1)\varphi(1)φ(1) of linearizedXiPath, does not have 111 as an eigenvalue. A Hamiltonian is non-degenerate when all its periodic orbits on the level 111 are.

Only the orbits PPP and QQQ are assumed non-degenerate; the other periodic orbits of XHX_HXH​ may be degenerate. In contact language this is the density, relative to a prescribed 111-jet along two non-degenerate closed orbits, of non-degenerate contact forms gλg\lambdagλ near λ\lambdaλ (Kupka–Smale for Reeb flows).

Preamble
import Definitions.Def_HryniewiczCriterion_ConleyZehnder
import Definitions.Def_HryniewiczCriterion_GlobalSection
import Definitions.Def_HryniewiczCriterion_SelfLinking

open HryniewiczCriterion
Formal statement
theorem HryniewiczCriterion.exists_nondegenerate_approx_of_nondegenerate_along_orbits (H : R4 → ℝ) (hS : IsStrictlyStarShapedLevel H) (P Q : PeriodicOrbit H)
    (hPQ : ∀ (R : PeriodicOrbit H), R.image ⊆ P.image ∪ Q.image →
      ∀ (Y : ℝ → (R4 →L[ℝ] R4)), IsLinearizedFlow H R.x Y →
      ∀ v : Fin 2 → ℝ, (linearizedXiPath H R Y 1).mulVec v = v → v = 0) :
    ∃ Hs : ℕ → R4 → ℝ, (∀ k, IsStrictlyStarShapedLevel (Hs k)) ∧
      (∀ k (R : PeriodicOrbit (Hs k)) (Y : ℝ → (R4 →L[ℝ] R4)), IsLinearizedFlow (Hs k) R.x Y →
      ∀ v : Fin 2 → ℝ, (linearizedXiPath (Hs k) R Y 1).mulVec v = v → v = 0) ∧
      (∀ k, ∀ y ∈ P.image ∪ Q.image, Hs k y = H y ∧ fderiv ℝ (Hs k) y = fderiv ℝ H y) ∧
      (∀ m : ℕ, ∀ K : Set R4, IsCompact K →
      TendstoUniformlyOn (fun k => iteratedFDeriv ℝ m (Hs k)) (iteratedFDeriv ℝ m H) Filter.atTop K) := by sorry
Source
U. Hryniewicz, Systems of global surfaces of section for dynamically convex Reeb flows on the 3-sphere, J. Symplectic Geom. 12 (2014) 791-862, arXiv:1105.2077, (28)-(29) on p. 20 (the set F\mathcal FF of non-degenerate forms is residual) and the proof of Lemma 3.12, p. 26.

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