A structure theorem for homology 4-manifolds with $g_2\leq 5$

Biplab Basak, Sourav Sarkar

Published 2023-02-05Version 1

Numerous structural findings of homology manifolds have been derived in various ways in relation to $g_2$-values. The homology $4$-manifolds with $g_2\leq 5$ are characterized combinatorially in this article. It is well-known that all homology $4$-manifolds for $g_2\leq 2$ are polytopal spheres. We demonstrate that homology $4$-manifolds with $g_2\leq 5$ are triangulated spheres and are derived from triangulated 4-spheres with $g_2\leq 2$ by a series of connected sum, bistellar 1- and 2-moves, edge contraction, edge expansion, and edge flipping operations. The inequality is also quite pronounced for these combinatorial classes.