Computing the local $2$-component of a non-selfdual automorphic representation of $\mathrm{GL}_3$

Abstract

In this paper, we explicitly determine the local $2$-adic component of a non-selfdual automorphic representation $Π$ of $\mathrm{GL}_3$ constructed by van Geemen and Top. We prove that $Π_2$ is a parabolically induced representation of $\mathrm{GL}_3(\mathbb{Q}_2)$ given by $Π_2 = \mathrm{Ind}_P^{\mathrm{GL}_3(\mathbb{Q}_2)}(π\boxtimes χ)$, where $P$ is the standard parabolic subgroup of $\mathrm{GL}_3$ with Levi subgroup $\mathrm{GL}_2 \times \mathrm{GL}_1$, $χ$ is an unramified character of $\mathbb{Q}_2^\times$ satisfying $χ(2) = -2\sqrt{-1}$, and $π$ is a supercuspidal representation of $\mathrm{GL}_2(\mathbb{Q}_2)$. Furthermore, we describe $π$ explicitly as a compactly induced representation $π= \mathrm{c-Ind}_{J_α}^{\mathrm{GL}_2(\mathbb{Q}_2)} Λ$ and determine the representation $Λ$ explicitly. The proof relies on explicit computations of Hecke eigenvalues using computer calculations. The automorphic representation $Π$ is realized in the cuspidal cohomology of the congruence subgroup $Γ_0(128) \subset \mathrm{SL}_3(\mathbb{Z})$. By computing the Hecke eigenvalues of an associated Hecke eigenvector, we are able to uniquely identify the local structure of $Π_2$. As an application, we obtain an explicit description of the $2$-adic local component of the Galois representation $ρ_{\mathrm{vGT},\ell}$ associated with $Π$.