Cuspidal $\ell$-modular representations of $\mathrm{GL}_n(F)$ distinguished by a Galois involution

Abstract

Let $F/F_0$ be a quadratic extension of non-Archimedean locally compact fields of residual characteristic $p\neq2$ with Galois automorphism $\sigma$, and let $R$ be an algebraically closed field of characteristic $\ell\notin\{0,p\}$. We reduce the classification of $\mathrm{GL}_n(F_0)$-distinguished cuspidal $R$-representations of $\mathrm{GL}_n(F)$ to the level $0$ setting. Moreover, under a parity condition, we give necessary conditions for a $\sigma$-selfdual cuspidal $R$-representation to be distinguished. Finally, we classify the distinguished cuspidal $\overline{\mathbb{F}}_{\ell}$-representations of $\mathrm{GL}_n(F)$ having a distinguished cuspidal lift to $\overline{\mathbb{Q}}_\ell$.