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Asymptotics of quantum weighted Hurwitz numbers

This work concerns both the semiclassical and zero temperature asymptotics of quantum weighted double Hurwitz numbers. The partition function for quantum weighted double Hurwitz numbers can be interpreted in terms of the energy distri- bution of a quantum Bose gas with vanishing fugacity. We compute the leading semi- classical term of the partition function for three versions of the quantum weighted Hurwitz numbers, as well as lower order semiclassical corrections. The classical limit $\hbar \ra 0$ is shown to reproduce the simple single and double Hurwitz numbers studied by Pandharipande and Okounkov [20,22]. The KP-Toda $τ$-function that serves as generating function for the quantum Hurwitz numbers is shown to have the $τ$-function of [20,22] as its leading term in the classical limit, and, with suitable scaling, the same holds for the partition function, the weights and expectations of Hurwitz numbers. We also compute the zero temperature limit $T \ra 0$ of the partition function and quantum weighted Hurwitz numbers. The KP or Toda $τ$-function serving as generating function for the quantum Hurwitz numbers are shown to give the one for Belyi curves in the zero temperature limit and, with suitable scaling, the same holds true for the partition function, the weights and the expectations of Hurwitz numbers.