Variants of Lehmer's speculation for newforms

TL;DR

This paper investigates which integers can appear as Fourier coefficients of newforms, providing nonexistence results for certain integers and primes, and establishing bounds on prime factors of these coefficients.

Contribution

It introduces a method to determine non-occurrence of specific integers as newform coefficients, extending Lehmer's conjecture and providing explicit bounds and nonvanishing results.

Findings

Certain primes between 3 and 37 are not coefficients of newforms with integer coefficients.

The Ramanujan tau-function avoids specific small integers and, under GRH, excludes certain larger primes.

Sharp lower bounds are established for the number of prime factors of newform coefficients.

Abstract

In the spirit of Lehmer's unresolved speculation on the nonvanishing of Ramanujan's tau-function, it is natural to ask whether a fixed integer is a value of $\tau(n)$ or is a Fourier coefficient $a_f(n)$ of any given newform $f(z)$. We offer a method, which applies to newforms with integer coefficients and trivial residual mod 2 Galois representation, that answers this question for odd integers. We determine infinitely many spaces for which the primes $3\leq \ell\leq 37$ are not absolute values of coefficients of newforms with integer coefficients. For $\tau(n)$ with $n>1$, we prove that $$\tau(n)\not \in \{\pm 1, \pm 3, \pm 5, \pm 7, \pm 13, \pm 17, -19, \pm 23, \pm 37, \pm 691\},$$ and assuming GRH we show for primes $\ell$ that $$\tau(n)\not \in \left \{ \pm \ell\ : \ 41\leq \ell\leq 97 \ {\textrm{with}}\ \left(\frac{\ell}{5}\right)=-1\right\} \cup \left \{ -11, -29, -31, -41, -59, -61, -71, -79, -89\right\}. $$ We also obtain sharp lower bounds for the number of prime factors of such newform coefficients. In the weight aspect, for powers of odd primes $\ell$, we prove that $\pm \ell^m$ is not a coefficient of any such newform $f$ with weight $2k>M^{\pm}(\ell,m)=O_{\ell}(m)$ and even level coprime to $\ell,$ where $M^{\pm}(\ell,m)$ is effectively computable.