New applications of the Mellin transform to automorphic L-fuctions

Abstract

Let L(s) = L(s, \pi) be the standard L-function of a cuspidal representation \pi of GL(m,A) where A denotes the ad\`eles of the field of rationals. We consider the integral, on the real line Re(s)= 1/2, of the squared absolute value of L(s)/s. In an earlier paper, partly with P. Sarnak (arxiv:2203.12475) we obtained a universal lower bound on this integral, independently of m. In this paper, for m fixed, we first obtain a universal lower bound for the integral on an interval [-A logC, A log C] where C is the analytic conductor of \pi; this bound is of order c(log C)^{-1/2} ; A, c are absolute positive constants for m fixed. There is also an absolute lower bound on a shifted interval [X-T, X+T] where T is of the order of log X. In the second part of the paper, using the Mellin transform as in the previous paper, we estimate, for an irreducible, non trivial Galois representation \rho of Gal(E/F), E and F being number fields, the smallest norm of a prime ideal P of F at which \rho is unramified and \rho(Frob) is non-trivial, Frob being a Frobenius at P.