Published online by Cambridge University Press: 26 April 2013
In this paper, we consider the  $\mathrm{SL} (2)$ analogue of two well-known theorems about period integrals of automorphic forms on
$\mathrm{SL} (2)$ analogue of two well-known theorems about period integrals of automorphic forms on  $\mathrm{GL} (2)$: one due to Harder–Langlands–Rapoport about non-vanishing of period integrals on
$\mathrm{GL} (2)$: one due to Harder–Langlands–Rapoport about non-vanishing of period integrals on  ${\mathrm{GL} }_{2} ({ \mathbb{A} }_{F} )$ of cuspidal automorphic representations on
${\mathrm{GL} }_{2} ({ \mathbb{A} }_{F} )$ of cuspidal automorphic representations on  ${\mathrm{GL} }_{2} ({ \mathbb{A} }_{E} )$ where
${\mathrm{GL} }_{2} ({ \mathbb{A} }_{E} )$ where  $E$ is a quadratic extension of a number field
$E$ is a quadratic extension of a number field  $F$, and the other due to Waldspurger involving toric periods of automorphic forms on
$F$, and the other due to Waldspurger involving toric periods of automorphic forms on  ${\mathrm{GL} }_{2} ({ \mathbb{A} }_{F} )$. In both these cases, now involving
${\mathrm{GL} }_{2} ({ \mathbb{A} }_{F} )$. In both these cases, now involving  $\mathrm{SL} (2)$, we analyze period integrals on global
$\mathrm{SL} (2)$, we analyze period integrals on global $L$-packets; we prove that under certain conditions, a global automorphic
$L$-packets; we prove that under certain conditions, a global automorphic  $L$-packet which at each place of a number field has a distinguished representation, contains globally distinguished representations, and further, an automorphic representation which is locally distinguished is globally distinguished.
$L$-packet which at each place of a number field has a distinguished representation, contains globally distinguished representations, and further, an automorphic representation which is locally distinguished is globally distinguished.
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