Lyubeznik numbers of the face rings of simplicial posets

Abstract

For a simplicial poset $P$, Stanley assigned the face ring $A_P$, which is the quotient of the polynomial ring $S:=K[t_x \mid x \in P \setminus \{\widehat{0} \}]$ by the ideal $I_P$. This is a generalization of Stanley-Reisner rings, but the grading of $S$ is more involved. Building on our previous paper, we further develop and complete the study of the local cohomology modules $H_{I_P}^i(S)$. In our categorical framework, $\mathrm{Ext}_S(A_P, S)$ ``encodes" all the information of $H_{I_P}^i(S)$. We treat $H_{I_P}^i(S)$ in a reasonably explicit way. In particular, Lyubeznik numbers $λ_{i,j}(A_P)$ can be simply expressed in terms of $[\mathrm{Ext}_S^*(\mathrm{Ext}_S^*(A_P,S), S)]_\bf{0}$, and depend only on the homeomorphism class of the underlying space of the CW complex associated with $P$ and $\mathrm{char}(K)$.