New formula for the prepotentials associated with Hurwitz-Frobenius manifolds and generalized WDVV equations

Abstract

We consider the Hurwitz spaces of ramified coverings of ${\mathbb{P}}^1$ with prescribed ramification profile over the point at infinity. By means of a particular symmetric bidifferential on a compact Riemann surface, we introduce quasi-homogeneous differentials. By following Dubrovin, we construct on Hurwitz spaces a family of Frobenius manifold structures associated with the quasi-homogeneous differentials. We explicitly derive new generating formulas for the corresponding prepotentials. This produces quasi-homogeneous solutions to the following generalized WDVV associativity equations: $F_i{\eta }^{-1}{F}_j=F_j{\eta }^{-1}{F}_i$, where the invertible constant matrix $\eta$ is a linear combination of the matrices $F_j$. In particular, our approach provides another look at Dubrovin's construction of semi-simple Hurwitz-Frobenius manifolds and establishes an alternative practical method to calculate their primary free energy functions. As applications, we use our formalism to obtain various explicit quasi-homogeneous solutions to the WDVV equations in genus zero and one and give a new proof of Ramanujan's differential equations for Eisenstein series.