Acyclotopes and Tocyclotopes

TL;DR

This paper extends the combinatorial and geometric understanding of acyclotopes and introduces tocyclotopes, computing their Ehrhart polynomials and exploring their duality and face structures within the framework of root systems and signed graphs.

Contribution

It generalizes Ehrhart polynomial computations from graphical zonotopes to signed graphs and introduces tocyclotopes with their Ehrhart polynomials and duality properties.

Findings

Ehrhart polynomials of acyclotopes and tocyclotopes are explicitly computed.

A duality construction for acyclotopes and tocyclotopes is established.

The face structure of lattice Gale zonotopes is related to arithmetic matroids.

Abstract

There is a well-established dictionary between zonotopes, hyperplane arrangements, and their (oriented) matroids. Arguably one of the most famous examples is the class of graphical zonotopes, also called acyclotopes, which encode subzonotopes of the type-A root polytope, the permutahedron. Stanley (1991) gave a general interpretation of the coefficients of the Ehrhart polynomial (integer-point counting function for a polytope) of a zonotope via linearly independent subsets of its generators. Applying this to the graphical case shows that Ehrhart coefficients count induced forests of the graph of fixed sizes. Our first goal is to extend and popularize this story to other root systems, which on the combinatorial side is encoded by signed graphs analogously to the work by Greene and Zaslavsky (1983). We compute the Ehrhart polynomial of the acyclotope in the signed case, and we give a matroid-dual construction, giving rise to tocyclotopes, and compute their Ehrhart polynomials. Applying the same duality construction to a general integral matrix gives rise to a lattice Gale zonotope, whose face structure was studies by McMullen (1971) and whose duality nature is a special instance of D'Adderio--Moci's arithmetic matroids. We describe its Ehrhart polynomials in terms of the given matrix.