Semi-reflexive polytopes

Abstract

The Ehrhart function $L_P(t)$ of a polytope $P$ is usually defined only for integer dilation arguments $t$. By allowing arbitrary real numbers as arguments we may also detect integer points entering (or leaving) the polytope in fractional dilations of $P$, thus giving more information about the polytope. Nevertheless, there are some polytopes that only gain new integer points for integer values of $t$; that is, these polytopes satisfy $L_P(t)=L_P(\lfloor t\rfloor )$. We call those polytopes semi-reflexive. In this paper, we give a characterization of these polytopes in terms of their hyperplane description, and we use this characterization to show that a polytope is reflexive if and only if both it and its dual are semi-reflexive.