On Permutations with Bounded Drop Size

TL;DR

This paper establishes a bijective proof for the symmetry of polynomials related to permutations with bounded drop size and proves a conjecture on the unimodality of certain polynomials connected to signed permutations and type B descents.

Contribution

It provides a bijective proof of the symmetry property of specific polynomials and confirms a conjecture on their unimodality in the context of signed permutations.

Findings

Bijection $\

$ ext{establishes symmetry}$ of polynomials related to permutations with bounded drop size.

Proves a conjecture on the unimodality of polynomials associated with signed permutations and type B descents.

Abstract

The maximum drop size of a permutation $\pi$ of $[n]=\{1,2,\ldots, n\}$ is defined to be the maximum value of $i-\pi(i)$. Chung, Claesson, Dukes and Graham obtained polynomials $P_k(x)$ that can be used to determine the number of permutations of $[n]$ with $d$ descents and maximum drop size not larger than $k$. Furthermore, Chung and Graham gave combinatorial interpretations of the coefficients of $Q_k(x)=x^k P_k(x)$ and $R_{n,k}(x)=Q_k(x)(1+x+\cdots+x^k)^{n-k}$, and raised the question of finding a bijective proof of the symmetry property of $R_{n,k}(x)$. In this paper, we establish a bijection $\varphi$ on $A_{n,k}$, where $A_{n,k}$ is the set of permutations of $[n]$ and maximum drop size not larger than $k$. The map $\varphi$ remains to be a bijection between certain subsets of $A_{n,k}$. %related to the symmetry property. This provides an answer to the question of Chung and Graham. The second result of this paper is a proof of a conjecture of Hyatt concerning the unimodality of polynomials in connection with the number of signed permutations of $[n]$ with $d$ type $B$ descents and the type $B$ maximum drop size not greater than $k$.