Published online by Cambridge University Press: 11 September 2014
This paper proves two results on the field of rationality $\mathbb{Q}({\it\pi})$ for an automorphic representation
${\it\pi}$, which is the subfield of
$\mathbb{C}$ fixed under the subgroup of
$\text{Aut}(\mathbb{C})$ stabilizing the isomorphism class of the finite part of
${\it\pi}$. For general linear groups and classical groups, our first main result is the finiteness of the set of discrete automorphic representations
${\it\pi}$ such that
${\it\pi}$ is unramified away from a fixed finite set of places,
${\it\pi}_{\infty }$ has a fixed infinitesimal character, and
$[\mathbb{Q}({\it\pi}):\mathbb{Q}]$ is bounded. The second main result is that for classical groups,
$[\mathbb{Q}({\it\pi}):\mathbb{Q}]$ 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.
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