On the number of prime divisors and radicals of non-zero Fourier coefficients of Hilbert cusp forms

Abstract

In this article, we derive lower bounds for the number of distinct prime divisors of families of non-zero Fourier coefficients of non-CM primitive cusp forms and more generally of non-CM primitive Hilbert cusp forms. In particular, for the Ramanujan $\Delta$-function, we show that for any $\epsilon > 0$, there exist infinitely many natural numbers $n$ such that $\tau(p^n)$ has at least $$ 2^{(1-\epsilon) \frac{\log n}{\log\log n}} $$ distinct prime factors for almost all primes $p$. This improves and refines the existing bounds. We also study lower bounds for absolute norms of radicals of non-zero Fourier coefficients of Modular forms alluded to above.