Poincare series for modular graph forms at depth two. Part II. Iterated integrals of cusp forms

We continue the analysis of modular invariant functions, subject to inhomogeneous Laplace eigenvalue equations, that were determined in terms of Poincare series in a companion paper. The source term of the Laplace equation is a product of (derivatives of) two non-holomorphic Eisenstein series whence the modular invariants are assigned depth two. These modular invariant functions can sometimes be expressed in terms of single-valued iterated integrals of holomorphic Eisenstein series as they appear in generating series of modular graph forms. We show that the set of iterated integrals of Eisenstein series has to be extended to include also iterated integrals of holomorphic cusp forms to find expressions for all modular invariant functions of depth two. The coefficients of these cusp forms are identified as ratios of their L-values inside and outside the critical strip.

Automated summary

In this paper, we continue the analysis of modular invariant functions subject to inhomogeneous Laplace eigenvalue equations. We find that these functions can be expressed in terms of iterated integrals of Eisenstein series and that the set of iterated integrals needs to be extended to include iterated integrals of holomorphic cusp forms in order to find expressions for all modular invariant functions of depth two.