Bounds for entries of $γ$-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 $\gamma^\Delta=(1,\gamma_1,\gamma_2,\ldots)$ satisfies: \begin{align*} \gamma_j=0,\text{ for all } j>\gamma_1, \quad \gamma_2\leq\binom{\gamma_1}{2}, \quad \gamma_{\gamma_1}\in{0,1}, \quad \text{ and }\gamma_{\gamma_1-1}\in{0,1,2,\gamma_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 $\gamma_1\leq b$, of any given dimension, is bounded independently of the dimension.