A modular framework of functions of Knopp and indefinite binary quadratic forms

Abstract: In this paper, we investigate functions introduced by Knopp and complete them to non-holomorphic bimodular forms of positive integral weight related to indefinite binary quadratic forms. We study further properties of our completions, which in turn motivates certain local cusp forms. We then define modular analogues of negative weight of our local cusp forms, which are locally harmonic Maass forms with continuously removable singularities. They admit local splittings in terms of Eichler integrals, and a realization as outputs of a certain theta lift.