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Topological four-dimensional Poincaré theorem

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SP4Mission.freedman_poincare_top

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

Every compact Hausdorff Type-0 space charted on real four-space that is homotopy equivalent to the standard four-sphere is homeomorphic to it. A smooth-manifold hypothesis is not required by this exact statement. This is known in the literature, but its Lean proof remains open in this package.

Preamble
import Definitions.Def_SP4Sphere

set_option autoImplicit false

open scoped Manifold ContDiff
open SP4Mission
Formal statement
theorem SP4Mission.freedman_poincare_top
    (M : Type) [TopologicalSpace M] [T2Space M] [CompactSpace M]
    [ChartedSpace (EuclideanSpace ℝ (Fin 4)) M] :
    Nonempty (ContinuousMap.HomotopyEquiv M S4) → Nonempty (M ≃ₜ S4) := by sorry
Source
Unpublished local Lean source SPC4.lean, SHA-256 b17fdb932034e5211d0db8171c08e2b3a182016bceaecdd2deb49c39d6bfd5cc; supported copy and exact oracle provenance in the accompanying local package. Known mathematical source: Michael H. Freedman, The topology of four-dimensional manifolds, Journal of Differential Geometry 17 (1982), Theorem 1.6, printed p. 371. The literature attribution does not supply a Lean proof.
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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 the Euclidean subspace topology. For every type MMM in the base universe equipped with a topology making it compact and Hausdorff and with a specified charted-space structure modeled on R4\mathbb R^4R4, existence of a continuous homotopy equivalence between MMM and S4S^4S4—that is, a pair of continuous maps in opposite directions whose two composites are continuously homotopic to the corresponding identity maps—implies existence of a homeomorphism M→S4M\to S^4M→S4. There is no differentiability or smooth-compatibility assumption on the chart structure and no smoothness conclusion. The homeomorphism is required only to exist, without uniqueness or a prescribed homotopy class. Empty MMM is allowed by the quantified space assumptions, but its homotopy-equivalence premise cannot hold.

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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