Connecting descent and peak polynomials

TL;DR

This paper establishes a connection between descent and peak polynomials in permutations, providing a new combinatorial interpretation of peak polynomial coefficients and proving their positivity.

Contribution

It introduces a unitary expansion of descent polynomials in terms of peak polynomials and offers a combinatorial proof of the peak polynomial positivity conjecture.

Findings

Derived a unitary expansion linking descent and peak polynomials.

Provided a combinatorial interpretation of peak polynomial coefficients.

Proved the positivity conjecture for peak polynomials.

Abstract

A permutation $\sigma=\sigma_1 \sigma_2 \cdots \sigma_n$ has a descent at $i$ if $\sigma_i>\sigma_{i+1}$. A descent $i$ is called a peak if $i>1$ and $i-1$ is not a descent. The size of the set of all permutations of $n$ with a given descent set is a polynomials in $n$, called the polynomial. Similarly, the size of the set of all permutations of $n$ with a given peak set, adjusted by a power of $2$ gives a polynomial in $n$, called the peak polynomial. In this work we give a unitary expansion of descent polynomials in terms of peak polynomials. Then we use this expansion to give a combinatorial interpretation of the coefficients of the peak polynomial in a binomial basis, thus giving a new proof of the peak polynomial positivity conjecture.