Length-generating functions for twisted involutions of Coxeter groups

For a Coxeter system $(W,S)$ of finite rank and an involutive automorphism $\ast:W\to W$ which preserves $S$, we prove that the length-generating function $\sum_{z\in\mathbf{I}_{W,\ast}}q^{\ell(z)}$ of the set of $\ast$-twisted involutions is rational. We derive recurrence relations analogous to the