Prime values of Ramanujan's tau function

Abstract

We study the prime values of Ramanujan's tau function $\ddot{I}„(n)$. Lehmer found that $n=251^2=63001$ is the smallest $n$ such that $\ddot{I}„(n)$ is prime: $$\ddot{I}„(251^2)=-80561663527802406257321747.$$ We prove that in most arithmetic progressions (mod 23), the prime values $\ddot{I}„$ belonging to the progression form a thin set. As a consequence, there exists a set of primes of Dirichlet density $\frac{9}{11}$ which are not values of $\ddot{I}„$.

Accepted for publication in Proceedings of the American Mathematical Society