# Generalisations of the Harer-Zagier recursion for 1-point functions

## TL;DR

This paper explores the existence of 1-point recursions in various enumerative problems with Schur function expansions, recovering known recursions and identifying limitations, and discusses their relation to topological recursion.

## Contribution

It proves the existence of 1-point recursions for a broad class of problems with Schur expansions and demonstrates their applicability and limitations.

## Key findings

- Recovered the Harer-Zagier recursion.
- Established 1-point recursions for dessins d'enfant, Bousquet-Mélou-Schaeffer numbers, and monotone Hurwitz numbers.
- Proved no 1-point recursion exists for simple Hurwitz numbers.

## Abstract

Harer and Zagier proved a recursion to enumerate gluings of a $2d$-gon that result in an orientable genus $g$ surface, in their work on Euler characteristics of moduli spaces of curves. Analogous results have been discovered for other enumerative problems, so it is natural to pose the following question: how large is the family of problems for which these so-called 1-point recursions exist? In this paper, we prove the existence of 1-point recursions for a class of enumerative problems that have Schur function expansions. In particular, we recover the Harer-Zagier recursion, but our methodology also applies to the enumeration of dessins d'enfant, to Bousquet-M\'{e}lou-Schaeffer numbers, to monotone Hurwitz numbers, and more. On the other hand, we prove that there is no 1-point recursion that governs simple Hurwitz numbers. Our results are effective in the sense that one can explicitly compute particular instances of 1-point recursions, and we provide several examples. We conclude the paper with a brief discussion and a conjecture relating 1-point recursions to the theory of topological recursion.