Integrality of $\mathrm{GL}_2\times\mathrm{GL}_2$ Rankin-Selberg integrals for ramified representations

Abstract: Let $\pi_1,\pi_2$ be irreducible admissible generic tempered representations of $\mathrm{GL}2(F)$ for some $p$-adic field $F$ of odd residue characteristic. We introduce a natural notion of general $(\pi_1\times\pi_2)$-integral data $(\phi,g_1,g_2)\in \mathcal{S}(F^2)\times\mathrm{GL}_2(F)^2$ at which the Rankin-Selberg integral can be evaluated. This is inspired by work of Loeffler, and previous work of the author, on unramified zeta integrals. We then establish an integral variant of a result of Jacquet-Langlands for the local Rankin-Selberg zeta integral associated to $\pi_1\times\pi_2$; i.e. we show that for any such integral datum $(\phi,g_1,g_2)$, we have $$\frac{Z(\phi,g_1W{\pi_1}^\mathrm{new},g_2W_{\pi_2}^\mathrm{new};s)}{L(\pi_1\times\pi_2,s)}=\Phi(\phi,g_1W_{\pi_1}^\mathrm{new},g_2W_{\pi_2}^\mathrm{new};q^s)\in \mathbf{Z}[q^{-1},\Sigma^1][q^s,q^{-s}]$$for a finite set $\Sigma^1\subseteq\mathbf{C}^\times$ of roots of unity and unitary character values, depending only on $\pi_1,\pi_2$. This is compatible with the notion of integrality coming from newforms $f_1,f_2$ of even integral weights, satisfying a mild local dihedral condition at $2$. We show that if $\pi_1,\pi_2$ are local pieces of $f_1,f_2$ at any prime $p$, the coefficient algebra is $\mathcal{O}_{K}[p^{-1}]$ with $K$ a number field only depending on $f_1,f_2$. Our approach relies on a reinterpretation of the local Rankin-Selberg integral, and works of Assing and Saha on values of $p$-adic Whittaker new-vectors.