Gorenstein Fano polytopes arising from order polytopes and chain polytopes

Abstract

Richard Stanley introduced the order polytope $\mathcal{O}(P)$ and the chain polytope $\mathcal{C}(P)$ arising from a finite partially ordered set $P$, and showed that the Ehrhart polynomial of $\mathcal{O}(P)$ is equal to that of $\mathcal{C}(P)$. In addition, the unimodular equivalence problem of $\mathcal{O}(P)$ and $\mathcal{C}(P)$ was studied by the first author and Nan Li. In the present paper, three integral convex polytopes $\Gamma(\mathcal{O}(P), -\mathcal{O}(Q))$, $\Gamma(\mathcal{O}(P), -\mathcal{C}(Q))$ and $\Gamma(\mathcal{C}(P), -\mathcal{C}(Q))$, where $P$ and $Q$ are partially ordered sets with $| P | = | Q |$, will be studied. First, it will be shown that the Ehrhart polynomial of $\Gamma(\mathcal{O}(P), -\mathcal{C}(Q))$ coincides with that of $\Gamma(\mathcal{C}(P), -\mathcal{C}(Q))$. Furthermore, when $P$ and $Q$ possess a common linear extension, it will be proved that these three convex polytopes have the same Ehrhart polynomial. Second, the problem of characterizing partially ordered sets $P$ and $Q$ for which $\Gamma(\mathcal{O}(P), -\mathcal{O}(Q))$ or $\Gamma(\mathcal{O}(P), -\mathcal{C}(Q))$ or $\Gamma(\mathcal{C}(P), -\mathcal{C}(Q))$ is a smooth Fano polytope will be solved. Finally, when these three polytopes are smooth Fano polytopes, the unimodular equivalence problem of these three polytopes will be discussed.