Invariant four-variable automorphic kernel functions

Abstract: Let $F$ be a number field, let $\mathbb{A}F$ be its ring of adeles, and let $g_1,g_2,h_1,h_2 \in \mathrm{GL}_2(\mathbb{A}_F)$. Previously the author provided an absolutely convergent geometric expression for the four variable kernel function $$ \sum{\pi} K_{\pi}(g_1,g_2)K_{\pi^{\vee}}(h_1,h_2)L(s,(\pi \times \pi^{\vee})^S), $$ where the sum is over isomorphism classes of cuspidal automorphic representations $\pi$ of $\mathrm{GL}2(\mathbb{A}_F)$. Here $K{\pi}$ is the typical kernel function representing the action of a test function on the space of the cuspidal automorphic representation $\pi$. In this paper we show how to use ideas from the circle method to provide an alternate expansion for the four variable kernel function that is visibly invariant under the natural action of $\mathrm{GL}_2(F) \times \mathrm{GL}_2(F)$.