The Explicit Sato-Tate Conjecture and Densities Pertaining to Lehmer-Type Questions

Abstract

Let $f(z)={\sum }_{n=1}^{\infty }a(n)q^n\in S_k^{\text{new}}({\Gamma }_0(N))$ be a newform with squarefree level $N$ that does not have complex multiplication. For a prime $p$, define ${\theta }_p\in [0,\pi ]$ to be the angle for which $a(p)=2p^{(k-1)/2}\cos {\theta }_p$. Let $I\subset [0,\pi ]$ be a closed subinterval, and let $d{\mu }_{ST}=\frac{2}{\pi }{\sin }^2\theta d\theta$ be the Sato-Tate measure of $I$. Assuming that the symmetric power $L$-functions of $f$ satisfy certain analytic properties (all of which follow from Langlands functoriality and the Generalized Riemann Hypothesis), we prove that if $x$ is sufficiently large, then \[ \left|\#\{p\leq x:θ_p\in I\} -μ_{ST}(I)\int_2^x\frac{dt}{\log t}\right|\ll\frac{x^{3/4}\log(N k x)}{\log x} \] with an implied constant of $3.34$. By letting $I$ be a short interval centered at $\frac{\pi }{2}$ and counting the primes using a smooth cutoff, we compute a lower bound for the density of positive integers $n$ for which $a(n)\ne 0$. In particular, if $\tau$ is the Ramanujan tau function, then under the aforementioned hypotheses, we prove that \[ \lim_{x\to\infty}\frac{\#\{n\leq x:τ(n)\neq0\}}{x}>1-1.54\times10^{-13}. \] We also discuss the connection between the density of positive integers $n$ for which $a(n)\ne 0$ and the number of representations of $n$ by certain positive-definite, integer-valued quadratic forms.

29 pages. Significant revisions, including improvements in Theorems 1.2, 1.3, and 1.5 and a more detailed account of the contour integration, are included. Acknowledgements are updated

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