Sheaves on P2 and generalized Appell functions

A closed expression is given for the generating function of (virtual) Poincar\'e polynomials of moduli spaces of semi-stable sheaves on the projective plane $\mathbb{P}^2$ with arbitrary rank $r$ and Chern classes. This generating function is known to equal the partition function of topologically twisted gauge theory with $\mathcal{N}=4$ supersymmetry and gauge group $U(r)$, which localizes on the Hermitian Yang-Mills solutions of the gauge field. To classify and study the novel generating functions, the notion of Appell functions with signature $(n_+,n_-)$ is introduced. For $n_-=1$, these novel functions reduce to the known class of Appell functions with multiple variables or higher level.