The subconvexity problem for Rankin-Selberg $L$-functions

Let $f$ be a cusp form of prime level $p$ with central character $\chi$ (trivial or primitive)
and let $g$ be a fixed cusp form. We consider the subconvexity problem for the Rankin-Selberg
$L$-function $L(1/2, f \otimes g)$ as $p\to \infty$. This problem was studied by Michel, Harcos-Michel
and Michel-Venkatesh. Here we give another proof using the delta symbol method. The key input is an
estimate on bilinear sums of Kloosterman fractions, which does not rely on the spectral theory of automorphic
forms as previous methods did. This is based on joint work with Aggarwal, Kumar, Kwan, Leung and Young.