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Sphere and homotopy formulations under an explicit topological hypothesis

Proved
SP4Mission.spc4_iff_homotopy_given_freedman

by ryanshin · Sep 6, 2026 · Mathlib 0df444a (Lean v4.33.1)

Assume explicitly that every compact Hausdorff Type-0 space charted on real four-space and homotopy equivalent to the standard four-sphere is homeomorphic to it. Then the homeomorphic and homotopy-equivalent formulations of smooth four-dimensional Poincaré are equivalent. The topological assertion is a hypothesis, not an admitted imported theorem; the conditional proof is complete.

Preamble
import Definitions.Def_SP4Sphere

set_option autoImplicit false

open scoped Manifold ContDiff
open SP4Mission
Formal statement
theorem SP4Mission.spc4_iff_homotopy_given_freedman
    (hFreedman : ∀ (M : Type) [TopologicalSpace M] [T2Space M] [CompactSpace M]
      [ChartedSpace (EuclideanSpace ℝ (Fin 4)) M],
      Nonempty (ContinuousMap.HomotopyEquiv M S4) → Nonempty (M ≃ₜ S4)) :
    SPC4 ↔ SPC4Homotopy := by sorry
Source
Unpublished local Lean source SPC4.lean, SHA-256 b17fdb932034e5211d0db8171c08e2b3a182016bceaecdd2deb49c39d6bfd5cc; supported copy and exact oracle provenance in the accompanying local package. The explicit-hypothesis conditional proof was newly separated from the admitted topological theorem.
Read-back

What the Lean code literally says, in plain math · Codex; the exact underlying model identifier is not exposed in this session.

Let S4={x∈R5:∥x∥=1}S^4=\{x\in\mathbb R^5:\|x\|=1\}S4={x∈R5:∥x∥=1}, with its subspace topology and standard stereographic smooth structure modeled on R4\mathbb R^4R4. Assume the following universal hypothesis: for every type MMM in the base universe, every compact Hausdorff topology on MMM, and every specified charted-space structure on MMM modeled on R4\mathbb R^4R4, existence of a continuous homotopy equivalence between MMM and S4S^4S4 implies existence of a homeomorphism M→S4M\to S^4M→S4; this hypothesis does not require smooth compatibility of the chart transitions. Under that hypothesis, two assertions are equivalent: for every MMM among the compact Hausdorff topological spaces equipped with a specified charted-space structure modeled on R4\mathbb R^4R4 whose chart transitions are C∞C^\inftyC∞ for the identity model on R4\mathbb R^4R4 with underlying type in the base universe, existence of a homeomorphism M→S4M\to S^4M→S4 implies existence of a diffeomorphism M→S4M\to S^4M→S4; and for every MMM with those same space and smoothness assumptions, existence of a continuous homotopy equivalence between MMM and S4S^4S4 implies existence of a diffeomorphism M→S4M\to S^4M→S4. Here a continuous homotopy equivalence consists of a pair of continuous maps in opposite directions whose two composites are continuously homotopic to the corresponding identity maps, and a diffeomorphism is a bijection for which both the forward map and the inverse are C∞C^\inftyC∞ in the specified manifold charts. All conclusions are existence assertions, without uniqueness or a requirement to realize a particular given map. Empty spaces are permitted by the universal space assumptions, but cannot meet the homeomorphism or homotopy-equivalence premises involving S4S^4S4.

Human review
  • Endorsed by Shuze Chen · Sep 6, 2026

  • Endorsed by ryanshin · Sep 6, 2026

    Confirmed by the mission captain (proposal self-audit).

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