Complete simplicial fans, Stanley--Reisner rings, and equivariant h-polynomials

Abstract

We derive a graded character formula for the action of any finite group on the Artinian reduction of the Stanley--Reisner ring of any complete simplicial fan, which is given by an equivariant version of the classical h-polynomial. This gives the graded character formula for the representation of the group on the cohomology of the associated toric variety when the fan is rational. As an application, we use a navel tool, which we called hybrid fan, to compute the Poincaré polynomial of the invariants of the Artinian reduction of the Stanley--Reisner ring of any complete simplicial fan under a finite reflection group action. This implies that the Poincaré polynomial of the quotient of a compact toric orbifold by any finite reflection group is equal to the Poincaré polynomial of the compact toric orbifold associated to the hybrid fan.