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A loop crossing a disk transversally crosses it at only finitely many points

Proved
HryniewiczCriterion.finite_transverse_disk_crossings

by Mazecto · Oct 8, 2026 · Mathlib 0df444a (Lean v4.33.1)

dynamical-systemsgeometry-topologysymplectic-geometry

Let E:R2→R4E:\mathbb{R}^2\to\mathbb{R}^4E:R2→R4 be C1C^1C1 on an open neighbourhood of the closed unit disk D‾\overline{\mathbb D}D and let γ:R→R4\gamma:\mathbb{R}\to\mathbb{R}^4γ:R→R4 be a C1C^1C1 loop of period 111. Suppose that at every crossing E(v)=γ(t)E(v)=\gamma(t)E(v)=γ(t) with v∈D‾v\in\overline{\mathbb D}v∈D,

det⁡(γ(t),γ′(t),∂1E(v),∂2E(v))≠0\det\big(\gamma(t),\gamma'(t),\partial_1E(v),\partial_2E(v)\big)\neq0det(γ(t),γ′(t),∂1​E(v),∂2​E(v))=0

(rows of a 4×44\times44×4 matrix). Then the set of crossing points {v∈D‾:∃t, E(v)=γ(t)}\{v\in\overline{\mathbb D}:\exists t,\ E(v)=\gamma(t)\}{v∈D:∃t, E(v)=γ(t)} is finite.

Proof. At a crossing the three vectors γ′(t),∂1E(v),∂2E(v)\gamma'(t),\partial_1E(v),\partial_2E(v)γ′(t),∂1​E(v),∂2​E(v) are linearly independent, so the map (t,v)↦γ(t)−E(v)(t,v)\mapsto\gamma(t)-E(v)(t,v)↦γ(t)−E(v) from R3\mathbb{R}^3R3 to R4\mathbb{R}^4R4 has injective differential there and is injective near (t,v)(t,v)(t,v) (inverse function theorem / local left inverse). Hence crossings are isolated in [0,1]×D‾[0,1]\times\overline{\mathbb D}[0,1]×D, a compact set, so there are finitely many pairs (t,v)(t,v)(t,v) with t∈[0,1]t\in[0,1]t∈[0,1], and by periodicity every crossing point vvv arises from such a pair.

Preamble
import Definitions.Def_HryniewiczCriterion_GlobalSection
import Definitions.Def_HryniewiczCriterion_SelfLinking

open HryniewiczCriterion
Formal statement
theorem HryniewiczCriterion.finite_transverse_disk_crossings (E : Plane → R4) (U : Set Plane) (hU : IsOpen U) (hDU : closedUnitDisk ⊆ U)
    (hE : ContDiffOn ℝ 1 E U)
    (γ : ℝ → R4) (hγ : ContDiff ℝ 1 γ) (hγper : ∀ t, γ (t + 1) = γ t)
    (htr : ∀ t, ∀ v ∈ closedUnitDisk, E v = γ t →
      Matrix.det (Matrix.of ![γ t, deriv γ t,
        fderiv ℝ E v (Pi.single 0 1), fderiv ℝ E v (Pi.single 1 1)]) ≠ 0) :
    {v : Plane | v ∈ closedUnitDisk ∧ ∃ t, E v = γ t}.Finite := by sorry
Source
Standard transversality and compactness argument, e.g. D. Rolfsen, Knots and Links, Publish or Perish 1976, Ch. 5D (linking number = algebraic intersection number with a Seifert surface)

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