On a Generalization of the Flag Complex Conjecture of Charney and Davis

Abstract: The Flag Complex Conjecture of Charney and Davis states that for a simplicial complex $S$ which triangulates a $(2n - 1)$-generalized homology sphere as a flag complex one has $(-1)^n \sum_{\sigma \in S} \left(\frac{-1}{2}\right)^{\dim\sigma + 1} \ge 0$, where the sum runs over all simplices $\sigma$ of $S$ (including the empty simplex). Interpreting the $1$-skeleta of $\sigma\in S$ as graphs of Coxeter groups, we present a stronger version of this conjecture, and prove the equivalence of the latter to the Flag Complex Conjecture.