Tate cohomology of Whittaker lattices and base change of generic representations of ${\rm GL}_n$

Abstract

Let $p$ and $l$ be distinct odd primes and let $n\geq 2$ be a positive integer. Let $E$ be a finite Galois extension of degree $l$ of a $p$-adic field $F$. Let $q$ be the cardinality of the residue field of $F$. Let $\overline{\pi}_F$ be a generic mod-$l$ representation of ${\rm GL}_n(F)$ and let $\pi_F$ be an $l$-adic lift of $\overline{\pi}_F$. Let $\mathbb{W}^0(\pi_E, \psi_E)$ be the integral Whittaker model of $\pi_E$, i.e., the lattice of $\overline{\mathbb{Z}}_l$-valued functions in the Whittaker model of $\pi_E$. Assuming that $l$ does not divide $|{\rm GL}_{n-1}(\mathbb{F}_q)|$, we prove that the Frobenius twist of $\overline{\pi}_F$ is a $G_n(F)$ sub-quotient of the Tate cohomology group $\widehat{H}^0({\rm Gal}(E/F), \mathbb{W}^0(\pi_E, \psi_E))$.