Double square moments and bounds for resonance sums of cusp forms

Let $f$ and $g$ be holomorphic cusp forms for the modular group $SL_2(\mathbb Z)$ of weight $k_1$ and $k_2$ with Fourier coefficients $λ_f(n)$ and $λ_g(n)$, respectively. For real $α\neq0$ and $0<β\leq1$, consider a smooth resonance sum $S_X(f,g;α,β)$ of $λ_f(n)λ_g(n)$ against $e(αn^β)$ over $X\leq n\leq2X$. Double square moments of $S_X(f,g;α,β)$ over both $f$ and $g$ are nontrivially bounded when their weights $k_1$ and $k_2$ tend to infinity together. By allowing both $f$ and $g$ to move, these double moments are indeed square moments associated with automorphic forms for $GL(4)$. By taking out a small exceptional set of $f$ and $g$, bounds for individual $S_X(f,g;α,β)$ will then be proved. These individual bounds break the resonance barrier of $X^\frac58$ for $\frac16<β<1$ and achieve a square-root cancellation for $\frac13<β<1$ for almost all $f$ and $g$ as an evidence for Hypothesis S for cusp forms over integers. The methods used in this study include Petersson's formula, Poisson's summation formula, and stationary phase integrals.

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