Divisor problems for restricted Fourier coefficients of modular forms

Abstract

Let $d(n)$ be the number of divisors of $n$. We investigate the average value of $d(a_f(p){)}^r$ for $r$ a positive integer and $a_f(p)$ the $p$-th Fourier coefficient of a cuspidal eigenform $f$ having integral Fourier coefficients, where $p$ is a prime subject to a constraint on the angle associated with the normalized Fourier coefficient.

27 pages, accepted by Science China Mathematics