Gelfand pairs for affine Weyl groups

Abstract: This paper is motivated by several combinatorial actions of the affine Weyl group of type $C_n$. Addressing a question of David Vogan, it was shown in an earlier paper that these permutation representations are essentialy multiplicity-free~\cite{arXiv:2009.13880}. However, the Gelfand trick, which was indispensable in~\cite{arXiv:2009.13880} to prove this property for types $C_n$ and $B_n$, is not applicable for other classical types. Here we present a unified approach to fully answer the analogous question for all irreducible affine Weyl groups. Given a finite Weyl group $W$ with maximal parabolic subgroup $P\leq W$, there corresponds to it a reflection subgroup $H$ of the affine Weyl group $\widetilde W$. It turns out that while the Gelfand property of $P\leq W$ does not imply that of $H\leq \widetilde{W}$, but $Q=N_W(P)\leq W$ has the Gelfand property if and only if $K=QH\leq \widetilde{W}$ has. Finally, for each irreducible type we describe when $(W,Q)$ is a Gelfand pair.