Odd values of the Ramanujan tau function

Abstract

We prove a number of results regarding odd values of the Ramanujan $$\tau $$ -function. For example, we prove the existence of an effectively computable positive constant $$\kappa $$ such that if $$\tau (n)$$ is odd and $$n \ge 25$$ then either $$\begin{aligned} P(\tau (n)) \; > \; \kappa \cdot \frac{\log \log \log {n}}{\log \log \log \log {n}} \end{aligned}$$ or there exists a prime $$p \mid n$$ with $$\tau (p)=0$$ . Here P(m) denotes the largest prime factor of m. We also solve the equation $$\tau (n)=\pm 3^{b_1} 5^{b_2} 7^{b_3} 11^{b_4}$$ and the equations $$\tau (n)=\pm q^b$$ where $$3\le q < 100$$ is prime and the exponents are arbitrary nonnegative integers. We make use of a variety of methods, including the Primitive Divisor Theorem of Bilu, Hanrot and Voutier, bounds for solutions to Thue–Mahler equations due to Bugeaud and Győry, and the modular approach via Galois representations of Frey–Hellegouarch elliptic curves.