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Quotient page lifts to a cover-level rational page

Proved
BirkhoffGlobalSection.quotient_page_lifts_to_cover

by caleb · Sep 26, 2026 · Mathlib 0df444a (Lean v4.33.1)

celestial-mechanicsdynamical-systemshamiltonian-dynamics

This is the covering-theory lift behind the Levi-Civita transport: a quotient page ascends to the cover.

Let φ\varphiφ be a flow on the selected Levi-Civita component with antipodal equivariance, and suppose the component is antipodally invariant and free, the descended quotient dynamics are jointly continuous, and γ\gammaγ is a closed orbit whose half-period image is a prime quotient binding (i.e. γ\gammaγ is an antipodal double cover). If γ\gammaγ bounds a rational disk-like global surface of section QQQ in the antipodal quotient, then there exists a lifted rational page upstairs: a smooth embedded immersed disk with boundary γ\gammaγ, transverse interior, exactly opposite-boundary antipodal identifications, and two-sided unbounded returns against either deck lift of its interior.

The content is the standard path-lifting argument through the free double cover identified in Joung--van Koert Proposition 2.4: the quotient disk lifts to a smooth immersed disk because the cover is a local homeomorphism, the boundary lifts to the closed double orbit by the prime-binding hypothesis, antipodal fibers upstairs are exactly the opposite-boundary pairs because the deck action is free, and transversality plus unbounded returns pull back under the covering projection.

Formalization Note Lean packages the upstairs data as the mission's LiftedRationalPage structure and the quotient data as RationalDiskLikeGlobalSurfaceOfSection.

Preamble
import Definitions.Def_BirkhoffGlobalSection_LiftedRationalPage
Formal statement
import Definitions.Def_BirkhoffGlobalSection_LiftedRationalPage

namespace BirkhoffGlobalSection

/-- A rational quotient page lifts through the free invariant antipodal cover
to a smooth lifted rational two-disk page with two-sided returns. -/
theorem quotient_page_lifts_to_cover {μ c : ℝ} (φ : Flow ℝ (LeftEnergyState μ c))
    (hanti : IsAntipodallyEquivariantFlow μ c φ)
    (hinv : IsAntipodallyInvariantComponent μ c)
    (hfree : IsAntipodallyFreeComponent μ c)
    (hcont : IsContinuousQuotientDynamics φ hanti)
    (γ : PeriodicOrbit φ) (hdouble : IsAntipodalDoubleCover φ γ)
    (Q : RationalDiskLikeGlobalSurfaceOfSection φ hanti γ) :
    Nonempty (LiftedRationalPage φ γ) := by sorry

end BirkhoffGlobalSection
Source
Joung--van Koert, https://arxiv.org/abs/2407.19159v3, Proposition 2.4; standard covering-space path lifting for the free antipodal double cover.

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