Level Reciprocity in the twisted second moment of Rankin-Selberg L-functions

We prove an exact formula for the second moment of Rankin-Selberg $L$-functions $L(1\mathrm{/}2,f\times g)$ twisted by ${\lambda }_f(p)$, where $g$ is a fixed holomorphic cusp form and $f$ is summed over automorphic forms of a given level $q$. The formula is a reciprocity relation that exchanges the twist parameter $p$ and the level $q$. The method involves the Bruggeman/Kuznetsov trace formula on both ends; finally the reciprocity relation is established by an identity of sums of Kloosterman sums.