A characterization of normal 3-pseudomanifolds with at most two singularities

TL;DR

This paper characterizes certain normal 3-pseudomanifolds with limited singularities based on the invariant g2, showing they can be constructed from boundary complexes of 4-simplices through specific operations, and establishes the sharpness of an upper bound.

Contribution

It provides a structural characterization of normal 3-pseudomanifolds with at most two singularities and bounds on g2, extending understanding of their combinatorial topology.

Findings

K is constructed from boundary complexes of 4-simplices via specific operations.

If K has one singularity, it is a handlebody with boundary coned off.

The upper bound on g2 is proven to be sharp for these pseudomanifolds.

Abstract

Characterizing face-number-related invariants of a given class of simplicial complexes has been a central topic in combinatorial topology. In this regard, one of the well-known invariants is $g_2$. Let $K$ be a normal $3$-pseudomanifold such that $g_2(K) \leq g_2(lk (v)) + 9$ for some vertex $v$ in $K$. Suppose either $K$ has only one singularity or $K$ has two singularities (at least) one of which is an $\mathbb{RP}^2$-singularity. We prove that $K$ is obtained from some boundary complexes of $4$-simplices by a sequence of operations of types connected sums, bistellar $1$-moves, edge contractions, edge expansions, vertex foldings, and edge foldings. In case $K$ has one singularity, $|K|$ is a handlebody with its boundary coned off. Further, we prove that the above upper bound is sharp for such normal $3$-pseudomanifolds.