Lattice point enumeration of some arbor polytopes

Abstract

The $n$-dimensional lattice polytopes ${\mathcal{Q}}_{n,k}$ obtained by intersecting the $n$th dilate of the standard $n$-dimensional simplex in ${\mathbb{R}}^n$ with the half-spaces $x_i\le 1$ for $1\le i\le k$ form an interesting special case of Chapoton's arbor polytopes. They interpolate between the $n$th dilate of the standard $n$-dimensional simplex and the standard $n$-dimensional cube in ${\mathbb{R}}^n$. This paper provides an explicit combinatorial interpretation of the $h^{\ast }$-polynomial of ${\mathcal{Q}}_{n,k}$, as the ascent enumerator of certain words, and partly confirms some of Chapoton's conjectures on the lattice point enumeration of arbor polytopes in this special case. More specifically, the Ehrhart polynomial of ${\mathcal{Q}}_{n,k}$ is shown to be magic positive, by means of a new combinatorial parking model for cars, and the real-rootedness of its $h^{\ast }$-polynomial is deduced. The polynomial whose coefficients count the lattice points of ${\mathcal{Q}}_{n,k}$ by the number of their nonzero coordinates is shown to be gamma-positive and a combinatorial interpretation of the $h^{\ast }$-polynomial of any arbor polytope is conjectured.

14 pages