Bessenrodt--Ono inequalities for $\ell$-tuples of pairwise commuting permutations

TL;DR

This paper studies inequalities related to the number of commuting $ ext{S}_n$-tuples, proving stability of the sign of these inequalities across different parameters and providing explicit bounds.

Contribution

It establishes the stability of the sign of Bessenrodt--Ono inequalities for all $ ext{S}_n$-tuples with explicit bounds, extending previous results.

Findings

The sign of $\Delta_{a,b}^{\ell}$ stabilizes as $\ell$ varies.

Explicit bounds are provided for the stability of inequalities.

Connections to orbifold characteristics and Euler characteristics are discussed.

Abstract

Let $S_n$ denote the symmetric group. We consider \begin{equation*} N_{\ell}(n) := \frac{\left\vert Hom\left( \mathbb{Z}^{\ell},S_n\right) \right\vert}{n!} \end{equation*} which also counts the number of $\ell$-tuples $\pi=\left( \pi_1, \ldots, \pi_{\ell}\right) \in S_n^{\ell}$ with $\pi_i \pi_j = \pi_j \pi_i$ for $1 \leq i,j \leq \ell$ scaled by $n!$. A recursion formula, generating function, and Euler product have been discovered by Dey, Wohlfahrt, Bryman and Fulman, and White. Let $a,b, \ell \geq 2$. It is known by Bringman, Franke, and Heim, that the Bessenrodt--Ono inequality \begin{equation*} \Delta_{a,b}^{\ell}:= N_{\ell}(a) \, N_{\ell}(b) - N_{\ell}(a+b) >0 \end{equation*} is valid for $a,b \gg 1$ and by Bessenrodt and Ono that it is valid for $\ell =2$ and $a+b >9$. In this paper we prove that for each pair $(a,b)$ the sign of $\{\Delta_{a,b}^{\ell} \}_{\ell}$ is getting stable. In each case we provide an explicit bound. The numbers $N_{\ell}\left( n\right) $ had been identified by Bryan and Fulman as the $n$-th orbifold characteristics, generalizing work by Macdonald and Hirzebruch--H\"{o}fer concerning the ordinary and string-theoretic Euler characteristics of symmetric products, where $N_2(n)=p(n) $ represents the partition function.