In this note, we present an algorithm that allows to answer any individual instance of the following question. Let $G_{ \mathbb{ R}}$ be a semisimple real Lie group, and $V$ an irreducible representation of $G_{ \mathbb{ R}}$. How does the longest element $w_0$ of the restricted Weyl group $W$ act on the subspace $V^L$ of $V$ formed by vectors that are invariant by $L$, the centralizer of a maximal split torus of $G_{ \mathbb{ R}}$? This algorithm comprises two parts. First we describe a complete answer to this question in the particular case where $G_{ \mathbb{ R}} = \operatorname{ SO}(1,n)$ for any $n \geq 2$. Then, for an arbitrary $G_{ \mathbb{ R}}$, we show that it suffices to do the computation in a well-chosen subgroup $S_{ \mathbb{ R}} \subset G_{ \mathbb{ R}}$ which is (up to isogeny) the product of several groups that are either compact, abelian or isomorphic to $\operatorname{ SO}(1,n)$ for some $n \geq 2$.
Categories: math.RT math.GR