Integration on complex Grassmannians, deformed monotone Hurwitz numbers, and interlacing phenomena

TL;DR

This paper introduces deformed monotone Hurwitz numbers derived from integrals over complex Grassmannians, revealing their recursive structures and conjecturing their real-rootedness and interlacing properties, linking matrix models, combinatorics, and topological recursion.

Contribution

It develops a new family of polynomials from Grassmannian integrals, connecting them to monotone Hurwitz numbers, topological recursion, and interlacing phenomena, with extensive empirical evidence.

Findings

Polynomials satisfy cut-and-join and topological recursion

Conjecture that these polynomials are real-rooted with interlacing roots

Weighted enumeration of dessins d'enfant also exhibits similar properties

Abstract

We introduce a family of polynomials, which arise in three distinct ways: in the large $N$ expansion of a matrix integral, as a weighted enumeration of factorisations of permutations, and via the topological recursion. More explicitly, we interpret the complex Grassmannian $\mathrm{Gr}(M,N)$ as the space of $N \times N$ idempotent Hermitian matrices of rank $M$ and develop a Weingarten calculus to integrate products of matrix elements over it. In the regime of large $N$ and fixed ratio $\frac{M}{N}$, such integrals have expansions whose coefficients count factorisations of permutations into monotone sequences of transpositions, with each sequence weighted by a monomial in $t = 1 - \frac{N}{M}$. This gives rise to the desired polynomials, which specialise to the monotone Hurwitz numbers when $t = 1$. These so-called deformed monotone Hurwitz numbers satisfy a cut-and-join recursion, a one-point recursion, and the topological recursion. Furthermore, we conjecture on the basis of overwhelming empirical evidence that the deformed monotone Hurwitz numbers are real-rooted polynomials whose roots satisfy remarkable interlacing phenomena. An outcome of our work is the viewpoint that the topological recursion can be used to "topologise" sequences of polynomials, and we claim that the resulting families of polynomials may possess interesting properties. As a further case study, we consider a weighted enumeration of dessins d'enfant and conjecture that the resulting polynomials are also real-rooted and satisfy analogous interlacing properties.