Bounds for entries of $\gamma$-vectors of flag homology spheres

Abstract

We present some enumerative and structural results for flag homology spheres. For a flag homology sphere $\Delta$, we show that its $\gamma$-vector ${\gamma }^{\Delta }=(1,{\gamma }_1,{\gamma }_2,\dots )$ satisfies: \begin{align*} γ_j=0,\text{ for all } j>γ_1, \quad γ_2\leq\binom{γ_1}{2}, \quad γ_{γ_1}\in\{0,1\}, \quad \text{ and }γ_{γ_1-1}\in\{0,1,2,γ_1\}, \end{align*} supporting a conjecture of Nevo and Petersen. Further we characterize the possible structures for $\Delta$ in extremal cases. As an application, the techniques used produce infinitely many $f$-vectors of flag balanced simplicial complexes that are not $\gamma$-vectors of flag homology spheres (of any dimension); these are the first examples of this kind. In addition, we prove a flag analog of Perles' 1970 theorem on $k$-skeleta of polytopes with "few" vertices, specifically: the number of combinatorial types of $k$-skeleta of flag homology spheres with ${\gamma }_1\le b$, of any given dimension, is bounded independently of the dimension.

13 pages, to appear in SIAM Journal on Discrete Math