Jacobi forms, Saito-Kurokawa lifts, their Pullbacks and sup-norms on average

Abstract

We formulate a precise conjecture about the size of the $L^\infty$-mass of the space of Jacobi forms on $\mathbb H_n \times \mathbb C^{g \times n}$ of matrix index $S$ of size $g$. This $L^\infty$-mass is measured by the size of the Bergman kernel of the space. We prove the conjectured lower bound for all such $n,g,S$ and prove the upper bound in the $k$ aspect when $n=1$, $g \ge 1$. When $n=1$ and $g=1$, we make a more refined study of the sizes of the index-(old and) new spaces, the latter via the Waldspurger's formula. Towards this and with independent interest, we prove a power saving asymptotic formula for the averages of the twisted central $L$-values $L(1/2, f \otimes \chi_D)$ with $f$ varying over newforms of level a prime $p$ and even weight $k$ as $k,p \to \infty$ and $D$ being (explicitly) polynomially bounded by $k,p$. Here $\chi_D$ is a real quadratic Dirichlet character. We also prove that the size of the space of Saito-Kurokawa lifts (of even weight $k$) is $k^{5/2}$ by three different methods (with or without the use of central $L$-values), and show that the size of their pullbacks to the diagonally embedded $\mathbb H \times \mathbb H$ is $k^2$. In an appendix, the same question is answered for the pullbacks of the whole space $S^2_k$, the size here being $k^3$.