April 18, 2024 1 p.m. CEST Seminar room - 3th floor
Alessio D'Alì Politecnico di Milano

Symmetry counts: an introduction to equivariant Hilbert and Ehrhart series

Abstract The Ehrhart series of a lattice polytope $P$ is a combinatorial gadget that counts the number of lattice points of $P$ and of its dilations. The Hilbert series of a simplicial complex $S$ counts how many monomials supported on faces of $S$ exist in each possible degree. The aim of this talk is to introduce equivariant versions of such constructions, where we are not just interested in counting but we also want to record how the action of a finite group affects such collections of lattice points or monomials. Inspired by previous results by Betke-McMullen, Stembridge, Stapledon and Adams-Reiner, we will investigate which extra combinatorial features of the group action give rise to “nice” rational expressions of the equivariant Hilbert and Ehrhart series, and how the two are sometimes related.

This is joint work with Emanuele Delucchi (SUPSI and Pisa).

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