Graph Schemes, Graph Series, and Modularity

Abstract

To a simple graph we associate a so-called graph series, which can be viewed as the Hilbert--Poincar\'e series of a certain infinite jet scheme. We study new $q$-representations and examine modular properties of several examples including Dynkin diagrams of finite and affine type. Notably, we obtain new formulas for graph series of type $A_7$ and $A_8$ in terms of "sum of tails" series, and of type $D_4$ and $D_5$ in the form of indefinite theta functions of signature $(1,1)$. We also study examples related to sums of powers of divisors corresponding to $5$-cycles. For several examples of graphs, we prove that graph series are so-called mixed quantum modular forms.