Change of coefficients in the $p \neq \ell$ local Langlands correspondence for $GL_n$

Abstract

Let $\ell$ and $p$ be distinct primes, $n$ a positive integer, $F_\ell$ an $\ell$-adic local field of characteristic $0,$ and let $W(k)$ denote the ring of Witt vectors over an algebraically closed field of characteristic $p$. Work of Emerton-Helm, Helm and Helm-Moss defines and constructs a smooth $A[{GL}_n(F_\ell)]$-module $\tilde{\pi}(\rho_A)$ for a continuous Galois representation $\rho_A : G_{F_\ell} \to {GL}_n(A)$ over a $p$-torsionfree reduced complete local $W(k)$-algebra $A$ interpolating the local Langlands correspondence. However, since $\tilde{\pi}$ is not a functor, there is no clear way to speak about the local Langlands correspondence over non-reduced or finite characteristic $W(k)$-algebras. We describe two natural and reasonable variants of the local Langlands correspondence with arbitrary complete local $W(k)$-algebras as coefficients. They are isomorphic when evaluated on the universal framed deformation of a Galois representation $\overline{\rho}$ over $k$, and more generally we find a surjection in one direction. In many cases, including $n=2$ or $3,$ they both recover $\tilde{\pi}(\rho)$ when $\rho$ has coefficients in a finite extension of $W(k)[p^{-1}].$ On the Galois side, this requires finding minimal lifts between Galois deformations.