Counting faces in the extended Shi arrangement

Abstract

We define the level of a subset X of Euclidean space to be the dimension of the smallest subspace such that the distance between each element of X and the subspace is bounded. We prove that the number of faces in the n-dimensional extended Shi arrangement Aˆnr having codimension k and level m is given by m⋅(nk)⋅ΔrkΔm−1xn−1|x=rn−1, where Δ is the difference operator and Δr is the difference operator of step r, that is, Δrp(x)=p(x)−p(x−r). This generalizes a result of Athanasiadis which counts the number of faces of different dimensions in the extended Shi arrangement Aˆnr. The proof relies on a multi-variated Abel identity due to Françon.