The Donaldson–Thomas partition function of the banana manifold \n (with an appendix coauthored with Stephen Pietromonaco)

A banana manifold is a compact Calabi-Yau threefold, fibered by Abelian surfaces, whose singular fibers have a singular locus given by a "banana configuration of curves". A basic example is given by $X_{ban}$, the blowup along the diagonal of the fibered product of a generic rational elliptic surface $S\to \mathbb{P}^{1}$ with itself. In this paper we give a closed formula for the Donaldson-Thomas partition function of the banana manifold $X_{ban }$ restricted to the 3-dimensional lattice $\Gamma$ of curve classes supported in the fibers of $X_{ban}\to \mathbb{P}^{1}$. It is given by \[ Z_{\Gamma}(X_{ban}) = \prod_{d_{1},d_{2},d_{3}\geq 0} \prod_{k} \left(1-p^{k}Q_{1}^{d_{1}}Q_{2}^{d_{2}}Q_{3}^{d_{3}}\right)^{-12c(||\mathbf{d} ||,k)} \] where $||\mathbf{d} || = 2d_{1}d_{2}+ 2d_{2}d_{3}+ 2d_{3}d_{1}-d_{1}^{2}-d_{2}^{2}-d_{3}^{2}$, and the coefficients $c(a,k)$ have a generating function given by an explicit ratio of theta functions. This formula has interesting properties and is closely realated to the equivariant elliptic genera of $\operatorname{Hilb} (\mathbb{C}^{2})$. In an appendix with S. Pietromonaco, it is shown that the corresponding genus $g$ Gromov-Witten potential $F_{g}$ is a genus 2 Siegel modular form of weight $2g-2$ for $g\geq 2$; namely it is the Skoruppa-Maass lift of a multiple of an Eisenstein series: $\frac{6|B_{2g}|}{g(2g-2)!} E_{2g}(\tau )$.