Chiral differential operators on the upper half plane and modular forms

In this paper we study the vertex operator algebra D-ch(H,Gamma) constructed from the fixed points of the chiral differential operators on the upper half plane which is holomorphic at all the cusps, under the action of a congruence subgroup Gamma. To this end, we introduce an SL(2, R)-invariant filtration labeled by partition pairs and study its successive quotient. We show that the successive quotient under the cuspidal condition is isomorphic to the space of modular forms. We also give a description of the structure of D-ch(H, Gamma) and compute its character. (c) 2021 Elsevier Inc. All rights reserved.