The strong Ramanujan bound for Drinfeld modular forms

Abstract

The Ramanujan bound states that the degree of the trace of the $\mathfrak{p}$-Hecke operator on a space of weight $k$ Drinfeld cusp forms is at most $d(k-2)/2$, where $\mathfrak{p}$ is a prime of degree $d$ in ${\mathbb{F}}_q[T]$. We prove the strong Ramanujan bound, which states that at level 1, this bound can be improved to $d(k-(q+1))/2$. In addition, we classify when the strong Ramanujan bound is sharp; surprisingly, this turns out to depend on $\mathfrak{p}$ only through the parity of its degree. We also prove results related to the smallest slope of the Atkin operator and mod $p$ distributions of Frobenius polynomials of Drinfeld modules over finite fields.

22 pages, comments welcome!