Overpartition M2M2-rank differences, class number relations, and vector-valued mock Eisenstein series

We prove that the generating function of overpartition $M2$-rank differences is, up to coefficient signs, a component of the vector-valued mock Eisenstein series attached to a certain quadratic form. We use this to compute analogs of the class number relations for $M2$-rank differences. As applications we split the Kronecker-Hurwitz relation into its "even" and "odd" parts and calculate sums over Hurwitz class numbers of the form $\sum_{r \in \mathbb{Z}} H(n - 2r^2)$.Comment: 13 pages. v2 includes a notation and background section and changes some notatio

Poincar\'e square series of small weight

We extend the author's earlier computation and give coefficient formulas for the (quasimodular) Poincar\'e square series of weight 3/2 and weight 2 for the dual Weil representation for an even lattice.Comment: 17 page