Improved subconvexity bounds for GL(2)xGL(3) and GL(3) L-functions by weighted stationary phase

Abstract

Let $f$ be a fixed self-contragradient Hecke-Maass form for $SL(3,\mathbb{Z})$, and $u$ an even Hecke-Maass form for $SL(2,\mathbb{Z})$ with Laplace eigenvalue $1/4+k^2$, $k>0$. A subconvexity bound $O\left(k^{4/3+\epsilon }\right)$ in the eigenvalue aspect is proved for the central value at $s=1/2$ of the Rankin-Selberg $L$-function $L(s,f\times u)$. Meanwhile, a subconvexity bound $O((1+|t|{)}^{2/3+\epsilon })$ in the $t$ aspect is proved for $L(1/2+it,f)$. These bounds improved corresponding subconvexity bounds proved by Xiaoqing Li (Annals of Mathematics, 2011). The main technique in the proof, other than those used by Li, is an $n$th-order asymptotic expansion of a weighted stationary phase integral, for arbitrary $n\ge 1$. This asymptotic expansion sharpened the classical result for $n=1$ by Huxley.

Published in Transactions of the American Mathematical Society online in December, 2016