Jacquet modules of Tate cohomology and base change lifting

Abstract: Let $G$ be a connected reductive algebraic group defined over a non-Archimedean local field $F$ of characteristic $p$. Let $\ell$ be a prime number with $\ell\neq p$. Let $\Gamma$ be a group of automorphisms of $G$ of order $\ell$. Let $\Pi$ be an $\ell$-modular irreducible smooth representation of $G(F)$ which extends as a representation of $G(F)\rtimes \Gamma$. We show that the Jacquet modules of the Tate cohomology groups $\widehat{H}^i(\Gamma, \Pi)$ are sub modules of certain Jacquet modules of $\Pi$. When $\Gamma$ is induced by a Galois automorphism, we use this result on Jacquet modules to show that $\widehat{H}^i(\Gamma, \Pi)$ is a finite length representation. We further use the Jacquet-modules to study some irreducibility of Tate cohomology in the cyclic base change lifting of cuspidal representations. The novelty in this article is that we do not assume that the base change lifting is cuspidal. Later in this article, we study these results for the finite fields case where we obtain a much precise description of the Tate cohomology groups. Finally, we end this article by studying the Tate cohomology of the mod-$\ell$ reduction of the unipotent cuspidal representation of ${\rm Sp}4(\mathbb{F}{q^\ell})$ for the action of ${\rm Gal}(\mathbb{F}_{q^\ell}/\mathbb{F}_q)$.