Flip actions and Gelfand pairs for affine Weyl groups

Abstract: Several combinatorial actions of the affine Weyl group of type $\widetilde{C}{n}$ on triangulations, trees, words and permutations are compared. Addressing a question of David Vogan, we show that, modulo a natural involution, these permutation representations are multiplicity-free. The proof uses a general construction of Gelfand subgroups in the affine Weyl groups of types $\widetilde{C}{n}$ and $\widetilde{B}_n$.