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arXiv · 2409.15227

Acyclotopes and Tocyclotopes

Eleon Bach·Matthias Beck·Sophie Rehberg
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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 Minkowski summands of the type-A 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 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. Our second goal is to translate matroid duality concepts not only to hyperplanes but also to zonotopes in such a way that the Ehrhart polynomial is uniquely defined. In the case of (signed) graphs we construct 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 studied by McMullen (1971) and whose duality concepts give a special instance of D'Adderio--Moci's arithmetic matroids (2013). We describe its Ehrhart polynomials in terms of the given matrix.

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Eleon Bach, Matthias Beck, Sophie Rehberg. 2024-09-23. Acyclotopes and Tocyclotopes. https://arxiv.org/abs/2409.15227

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