Non‐vanishing of Poincaré series on average

Abstract We study when Poincaré series for congruence subgroups do not vanish identically. We show that almost all Poincaré series with suitable parameters do not vanish when either the weight or the index varies in a dyadic interval. Crucially, analyzing the problem ‘on average’ over these weights or indices allows us to prove non‐vanishing in ranges where the index is significantly larger than —a range in which proving non‐vanishing for individual Poincaré series remains out of reach of current methods.