Ballot Permutations and Odd Order Permutations

TL;DR

The paper explores a refined conjecture relating ballot permutations and odd order permutations, proving it for specific descent counts and deriving formulas involving Eulerian numbers.

Contribution

It refines Callan's conjecture by introducing a descent-based classification and proves the refined conjecture for select cases, providing explicit formulas.

Findings

Proved the refined conjecture for d=1, 2, 3, and floor((n-1)/2) cases.

Derived formulas for b(n,d) involving Eulerian and Eulerian-Catalan numbers.

Established a deeper connection between ballot permutations and permutations of odd order.

Abstract

A permutation $\pi$ is ballot if, for all $k$, the word $\pi_1\cdots \pi_k$ has at least as many ascents as it has descents. Let $b(n)$ denote the number of ballot permutations of order $n$, and let $p(n)$ denote the number of permutations which have odd order in the symmetric group $S_n$. Callan conjectured that $b(n)=p(n)$ for all $n$, which was proved by Bernardi, Duplantier, and Nadeau. We propose a refinement of Callan's original conjecture. Let $b(n,d)$ denote the number of ballot permutations with $d$ descents. Let $p(n,d)$ denote the number of odd order permutations with $M(\pi)=d$, where $M(\pi)$ is a certain statistic related to the cyclic descents of $\pi$. We conjecture that $b(n,d)=p(n,d)$ for all $n$ and $d$. We prove this stronger conjecture for the cases $d=1,\ 2,\ 3$, and $d=\lfloor(n-1)/2\rfloor$, and in each of these cases we establish formulas for $b(n,d)$ involving Eulerian numbers and Eulerian-Catalan numbers.