Invariant rings of the special orthogonal group have nonunimodal $h$-vectors

Abstract

For $K$ an infinite field of characteristic other than two, consider the action of the special orthogonal group ${\mathrm{SO}}_t(K)$ on a polynomial ring via copies of the regular representation. When $K$ has characteristic zero, Boutot's theorem implies that the invariant ring has rational singularities; when $K$ has positive characteristic, the invariant ring is $F$-regular, as proven by Hashimoto using good filtrations. We give a new proof of this, viewing the invariant ring for ${\mathrm{SO}}_t(K)$ as a cyclic cover of the invariant ring for the corresponding orthogonal group; this point of view has a number of useful consequences, for example it readily yields the $a$-invariant and information on the Hilbert series. Indeed, we use this to show that the $h$-vector of the invariant ring for ${\mathrm{SO}}_t(K)$ need not be unimodal.