Second Moments and simultaneous non-vanishing of GL(2) automorphic L-series

Abstract: We obtain a second moment formula for the L-series of holomorphic cusp forms, averaged over twists by Dirichlet characters modulo a fixed conductor Q. The estimate obtained has no restrictions on Q, with an error term that has a close to optimal power savings in the exponent. However, one of the contributions to the main term is a special value of a shifted double Dirichlet series. We show that this special value is small on average, and obtain a corresponding estimate for a mean value of the second moment over Q. This mean value is non-zero even when applied to a product of two distinct L-series, leading to a simultaneous non-vanishing result. Our approach uses the theory of shifted multiple Dirichlet series to obtain some refined estimates for double shifted sums. These estimates are the key ingredient in the second moment estimate.