Algebraic Methods and Monotone Hurwitz Numbers
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We develop algebraic methods to solve join-cut equations, which are partial differential equations that arise in the study of permutation factorizations. Using these techniques, we give a detailed study of the recently introduced monotone Hurwitz numbers, which count factorizations of a given permutation into a fixed number of transpositions, subject to some technical conditions known as transitivity and monotonicity. Part of the interest in monotone Hurwitz numbers comes from the fact that they have been identified as the coefficients in a certain asymptotic expansion related to the Harish-Chandra-Itzykson-Zuber integral, which comes from the theory of random matrices and has applications in mathematical physics. The connection between random matrices and permutation factorizations goes through representation theory, with symmetric functions in the Jucys-Murphy elements playing a key role. As the name implies, monotone Hurwitz numbers are related to the more classical Hurwitz numbers, which count permutation factorizations regardless of monotonicity, and for which there is a significant body of work. Our results for monotone Hurwitz numbers are inspired by similar results for Hurwitz numbers; we obtain a genus expansion for the related generating functions, which yields explicit formulas and a polynomiality result for monotone Hurwitz numbers. A significant difference between the two cases is that our methods are purely algebraic, whereas the theory of Hurwitz numbers relies on some fairly deep results in algebraic geometry. Despite our methods being algebraic, it seems that there should be a connection between monotone Hurwitz numbers and geometry, although this is currently missing. We give some evidence for this connection by identifying some of the coefficients in the monotone Hurwitz genus expansion with coefficients in the classical Hurwitz genus expansion known to be Hodge integrals over the moduli space of curves.