On Poincar\'e series of half-integral weight

Abstract

We use Poincar\'e series of $ K $-finite matrix coefficients of genuine integrable representations of the metaplectic cover of $ \mathrm{SL}_2(\mathbb R) $ to construct a spanning set for the space of cusp forms $ S_m(\Gamma,\chi) $, where $ \Gamma $ is a discrete subgroup of finite covolume in the metaplectic cover of $ \mathrm{SL}_2(\mathbb R) $, $ \chi $ is a character of $ \Gamma $ of finite order, and $ m\in\frac52+\mathbb Z_{\geq0} $. We give a result on the non-vanishing of the constructed cusp forms and compute their Petersson inner product with any $ f\in S_m(\Gamma,\chi) $. Using this last result, we construct a Poincar\'e series $ \Delta_{\Gamma,k,m,\xi,\chi}\in S_m(\Gamma,\chi) $ that corresponds, in the sense of the Riesz representation theorem, to the linear functional $ f\mapsto f^{(k)}(\xi) $ on $ S_m(\Gamma,\chi) $, where $ \xi\in\mathbb C_{\Im(z)>0} $ and $ k\in\mathbb Z_{\geq0} $. Under some additional conditions on $ \Gamma $ and $ \chi $, we provide the Fourier expansion of cusp forms $ \Delta_{\Gamma,k,m,\xi,\chi} $ and their expansion in a series of classical Poincar\'e series.