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A smooth symmetric loop, possibly degenerate, has index α−+α+\alpha^- + \alpha^+α−+α+

Proved
FiniteEnergyPlane.exists_loopAlpha_czIndexOfPath_eq_of_symLoop

by Mazecto · Nov 6, 2026 · Mathlib 0df444a (Lean v4.33.1)

symplectic-geometry

Let S∞S_\inftyS∞​ be a smooth symmetric loop of 2×22 \times 22×2 matrices, not assumed nondegenerate. Then there are integers a,ba, ba,b with b∈{a,a+1}b \in \{a, a + 1\}b∈{a,a+1} such that aaa is the largest winding number of a negative eigenvalue of −J0∂t−S∞-J_0 \partial_t - S_\infty−J0​∂t​−S∞​, bbb is the smallest winding number of a nonnegative eigenvalue, and the fundamental solution Ψ\PsiΨ has czIndexOfPath equal to a+ba + ba+b.

Role. This extends the identity μCZ=α−+α+\mu_{CZ} = \alpha^- + \alpha^+μCZ​=α−+α+ to degenerate loops, with the lower semicontinuous index, so results on winding windows apply without a nondegeneracy assumption.

Formalization Note. The extremal properties are stated through HasLoopEigenWind.

Preamble
import Definitions.Def_FiniteEnergyPlane_Winding
Formal statement
namespace FiniteEnergyPlane

open scoped ContDiff Real

/-- For a smooth symmetric loop `S∞`, not necessarily nondegenerate, there are integers `a, b` with
`b ∈ {a, a + 1}` such that `a` is the largest winding number of a negative eigenvalue of
`-J₀ ∂ₜ - S∞`, `b` is the smallest winding number of a nonnegative eigenvalue, and the fundamental
solution has `czIndexOfPath` equal to `a + b`. -/
theorem exists_loopAlpha_czIndexOfPath_eq_of_symLoop (Sinf : ℝ → CauchyRiemannIndex.Mat 1)
    (hSinf : CauchyRiemannIndex.IsSymLoop Sinf)
    (hSinf' : ∀ i j, ContDiff ℝ ∞ (fun t => Sinf t i j)) :
    ∃ a b : ℤ,
      ((∃ μ < 0, HasLoopEigenWind Sinf μ a) ∧
        ∀ μ < 0, ∀ k : ℤ, HasLoopEigenWind Sinf μ k → k ≤ a) ∧
      ((∃ μ, 0 ≤ μ ∧ HasLoopEigenWind Sinf μ b) ∧
        ∀ μ, 0 ≤ μ → ∀ k : ℤ, HasLoopEigenWind Sinf μ k → b ≤ k) ∧
      (b = a ∨ b = a + 1) ∧
      ∀ Ψ : ℝ → CauchyRiemannIndex.Mat 1, CauchyRiemannIndex.IsFundamentalSolution Sinf Ψ →
        HryniewiczCriterion.czIndexOfPath
          (fun τ => Matrix.reindex finSumFinEquiv finSumFinEquiv (Ψ τ)) = a + b := by
  sorry

end FiniteEnergyPlane
Source
H. Hofer, K. Wysocki, E. Zehnder, Properties of pseudoholomorphic curves in symplectizations II (1995), Section 3; U. Hryniewicz, J. Symplectic Geom. 12 (2014), arXiv:1105.2077, Section 2.1.1

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