Beyond the Weyl barrier for $\mathrm{GL}(2)$ exponential sums

Abstract

In this paper, we use the Bessel $\delta$-method, along with new variants of the van der Corput method in two dimensions, to prove non-trivial bounds for $\mathrm{GL}(2)$ exponential sums beyond the Weyl barrier. More explicitly, for sums of $\mathrm{GL}(2)$ Fourier coefficients twisted by $e(f(n))$, with length $N$ and phase $f(n)=N^{\beta} \log n / 2\pi$ or $a n^{\beta}$, non-trivial bounds are established for $ \beta < 1.63651... $, which is beyond the Weyl barrier at $\beta = 3/2$.