The Ehrhart $h^*$-polynomials of positroid polytopes

Yuhan Jiang

Published 2024-10-02, updated 2024-11-24Version 2

A positroid is a matroid realized by a matrix such that all maximal minors are non-negative. Positroid polytopes are matroid polytopes of positroids. In particular, they are lattice polytopes. The Ehrhart polynomial of a lattice polytope counts the number of integer points in the dilation of that polytope. The Ehrhart series is the generating function of the Ehrhart polynomial, a rational function with a numerator called the $h*$-polynomial. We give explicit formulas for the $h^*$-polynomials of an arbitrary positroid polytope regarding permutation descents. We also compute the $h^*$-polynomial of any positroid polytope with some facets removed and relate it to the descents of permutations. Our result generalizes that of Early, Kim, and Li for hypersimplices.