Three-dimensional normal pseudomanifolds with relatively few edges

Abstract: Let $\Delta$ be a $d$-dimensional normal pseudomanifold, $d \ge 3.$ A relative lower bound for the number of edges in $\Delta$ is that $g_2$ of $\Delta$ is at least $g_2$ of the link of any vertex. When this inequality is sharp $\Delta$ has relatively minimal $g_2$. For example, whenever the one-skeleton of $\Delta$ equals the one-skeleton of the star of a vertex, then $\Delta$ has relatively minimal $g_2.$ Subdividing a facet in such an example also gives a complex with relatively minimal $g_2.$ We prove that in dimension three these are the only examples. As an application we determine the combinatorial and topological type of $3$-dimensional $\Delta$ with relatively minimal $g_2$ whenever $\Delta$ has two or fewer singularities. The topological type of any such complex is a pseudocompression body, a pseudomanifold version of a compression body. Complete combinatorial descriptions of $\Delta$ with $g_2(\Delta) \le 2$ are due to Kalai [12] $(g_2=0)$, Nevo and Novinsky [13] $(g_2=1)$ and Zheng [21] $(g_2=2).$ In all three cases $\Delta$ is the boundary of a simplicial polytope. Zheng observed that for all $d \ge 0$ there are triangulations of $S^d \ast \mathbb{RP}^2$ with $g_2=3.$ She asked if this is the only nonspherical topology possible for $g_2(\Delta)=3.$ As another application of relatively minimal $g_2$ we give an affirmative answer when $\Delta$ is $3$-dimensional.