On Fields of rationality for automorphic representations

Abstract

This paper proves two results on the field of rationality \Q(\pi) for an automorphic representation $\pi$, which is the subfield of \C fixed under the subgroup of \Aut(\C) stabilizing the isomorphism class of the finite part of $\pi$. For general linear groups and classical groups, our first main result is the finiteness of the set of discrete automorphic representations $\pi$ such that $\pi$ is unramified away from a fixed finite set of places, $\pi_\infty$ has a fixed infinitesimal character, and [\Q(\pi):\Q] is bounded. The second main result is that for classical groups, [\Q(\pi):\Q] grows to infinity in a family of automorphic representations in level aspect whose infinite components are discrete series in a fixed $L$-packet under mild conditions