diff --git a/LeanEval/Topology/Poincare4DTopological.lean b/LeanEval/Topology/Poincare4DTopological.lean new file mode 100644 index 0000000..3466ab2 --- /dev/null +++ b/LeanEval/Topology/Poincare4DTopological.lean @@ -0,0 +1,43 @@ +import Mathlib +import EvalTools.Markers + +namespace LeanEval +namespace Topology + +/-! +# 4D topological Poincaré conjecture (Freedman) + +A specialization of mathlib's generalized `proof_wanted +ContinuousMap.HomotopyEquiv.nonempty_homeomorph_sphere` in +`Mathlib/Geometry/Manifold/PoincareConjecture.lean`, restricted to +dimension 4: every Hausdorff 4-manifold homotopy-equivalent to the standard +4-sphere `𝕊⁴` is homeomorphic to `𝕊⁴`. + +This is Michael Freedman's 1982 theorem (Freedman, *The topology of +four-dimensional manifolds*, J. Differential Geom. 17), for which he was +awarded the Fields Medal in 1986. The proof uses Casson handles and the +"Bing topology" infinite-process construction; the corresponding **smooth** +4D Poincaré conjecture (every smooth homotopy 4-sphere is diffeomorphic +to `𝕊⁴`) remains famously **open** and is not included as an eval problem. + +mathlib has homotopy equivalences (`≃ₕ`), homeomorphisms (`≃ₜ`), +`ChartedSpace`, and the unit sphere as a topological space — but no Casson +handles, no Freedman's theorem, and no 4D Poincaré conjecture. +-/ + +open Metric (sphere) +open ContinuousMap + +/-- **4D topological Poincaré conjecture** (Freedman, 1982). Every Hausdorff +4-manifold that is homotopy-equivalent to the standard 4-sphere `𝕊⁴` is +homeomorphic to `𝕊⁴`. -/ +@[eval_problem] +theorem poincare_4d_topological + {M : Type*} [TopologicalSpace M] [T2Space M] + [ChartedSpace (EuclideanSpace ℝ (Fin 4)) M] + (_h : M ≃ₕ sphere (0 : EuclideanSpace ℝ (Fin 5)) 1) : + Nonempty (M ≃ₜ sphere (0 : EuclideanSpace ℝ (Fin 5)) 1) := by + sorry + +end Topology +end LeanEval diff --git a/manifests/problems/poincare_4d_topological.toml b/manifests/problems/poincare_4d_topological.toml new file mode 100644 index 0000000..f586f87 --- /dev/null +++ b/manifests/problems/poincare_4d_topological.toml @@ -0,0 +1,9 @@ +id = "poincare_4d_topological" +title = "4D topological Poincaré conjecture (Freedman)" +test = false +module = "LeanEval.Topology.Poincare4DTopological" +holes = ["poincare_4d_topological"] +submitter = "Kim Morrison" +notes = "Every Hausdorff 4-manifold homotopy-equivalent to 𝕊⁴ is homeomorphic to 𝕊⁴. Specialization to n = 4 of mathlib's generalized `proof_wanted ContinuousMap.HomotopyEquiv.nonempty_homeomorph_sphere` in `Mathlib/Geometry/Manifold/PoincareConjecture.lean`. Michael Freedman's 1982 theorem (J. Differential Geom. 17), Fields Medal 1986. Proof uses Casson handles and the Bing-topology infinite-process construction. The corresponding smooth 4D Poincaré conjecture (every smooth homotopy 4-sphere is diffeomorphic to S⁴) remains famously OPEN and is not included here. Mathlib has the homotopy-equiv / homeo / ChartedSpace scaffolding but no Casson handles or Freedman's theorem." +source = "M. H. Freedman, The topology of four-dimensional manifolds, J. Differential Geom. 17 (1982), 357-453. Specialization of `proof_wanted ContinuousMap.HomotopyEquiv.nonempty_homeomorph_sphere` in Mathlib/Geometry/Manifold/PoincareConjecture.lean." +informal_solution = "Freedman's main theorem: every topological 4-manifold homotopy equivalent to a topological model is homeomorphic to it, provided one can construct topological Casson handles. The construction of Casson handles is via a Bing-style limiting process (countably many handle additions, infinite tower of 2-handles) that produces a topological — but not smooth — 4-ball. Applying this to a homotopy 4-sphere M, one writes M as the union of two 2-handlebodies along their boundary, replaces handles with Casson handles, and assembles a homeomorphism to S⁴. The smooth analogue is open because Casson handles need not be smoothly standard (Donaldson's invariants distinguish smooth structures in dim 4)."