Hurwitz numbers of a fixed partition (m, 1^{n-m}) via enumeration of unrooted hypermaps

Abstract

This manuscript studies a special case of the Hurwitz enumeration problem: for branched covers from genus g compact Riemann surface to the Riemann sphere, with three branch points, and require the branching data at one of the branch points to be of the partition (m 1^{n-m}), obtain a formula of Hurwitz number. The Hurwitz enumeration problem can be transformed into enumeration of a class of unrooted hypermaps. We first provide a enumeration formula for rooted hypermaps, thereby obtaining the weighted Hurwitz numbers. Next give the quantitative relationship between the enumeration of unrooted hypermaps and that of rooted hypermaps in the general setting. Finally, combining these two results, we obtain a formula for the unrooted hypermaps or the Hurwitz numbers in the special setting.