Finite Chains of Two-Complexes and Acyclic Covers
Whitehead's asphericity problem would have a negative answer if a non-aspherical two-complex began an infinite chain whose inclusions induce zero maps on second homotopy groups. This Lean formalization proves the article's main finite-length characterization: a connected two-complex K begins such chains of every finite length if and only if K has a connected acyclic regular cover. It also formalizes the finite acyclic enhancement.
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