Modular forms and an explicit Chebotarev variant of the Brun-Titchmarsh theorem

Abstract: We prove an explicit Chebotarev variant of the Brun--Titchmarsh theorem. This leads to explicit versions of the best-known unconditional upper bounds toward conjectures of Lang and Trotter for the coefficients of holomorphic cuspidal newforms. In particular, we prove that $$\lim_{x \to \infty} \frac{#{1 \leq n \leq x \mid \tau(n) \neq 0}}{x} > 1-1.15 \times 10^{-12},$$ where $\tau(n)$ is Ramanujan's tau-function. This is the first known positive unconditional lower bound for the proportion of positive integers $n$ such that $\tau(n) \neq 0$.