On a conjecture concerning the shuffle-compatible permutation statistics

TL;DR

This paper proves that the permutation statistics (udr, pk, des) are shuffle-compatible by constructing a preserving bijection, and further shows that (cpk, cdes) are cyclic shuffle-compatible, advancing understanding of permutation statistic properties.

Contribution

It establishes the shuffle-compatibility of (udr, pk, des) and the cyclic shuffle-compatibility of (cpk, cdes) through explicit bijections, confirming conjectures in the field.

Findings

(udr, pk, des) are shuffle-compatible

(cpk, cdes) are cyclic shuffle-compatible

Provides explicit bijections for these properties

Abstract

The notion of shuffle-compatible permutation statistics was implicit in Stanley's work on P-partitions and was first explicitly studied by Gessel and Zhuang. The aim of this paper is to prove that the triple ${\rm (udr, pk, des)}$ is shuffle-compatible as conjectured by Gessel and Zhuang, where ${\rm udr}$ denotes the number of up-down runs, ${\rm pk}$ denotes the peak number, and ${\rm des}$ denotes the descent number. This is accomplished by establishing an ${\rm (udr, pk, des)}$-preserving bijection in the spirit of Baker-Jarvis and Sagan's bijective proofs of shuffle-compatibility property of permutation statistics. As an application, our bijection also enables us to prove that the pair $({\rm cpk}, {\rm cdes})$ is cyclic shuffle-compatible, where ${\rm cpk}$ denotes the cyclic peak number and ${\rm cdes}$ denotes the cyclic descent number.