A proof of the peak polynomial positivity conjecture

Abstract: We say that a permutation $\pi=\pi_1\pi_2\cdots \pi_n \in \mathfrak{S}n$ has a peak at index $i$ if $\pi{i-1} < \pi_i > \pi_{i+1}$. Let $\mathcal{P}(\pi)$ denote the set of indices where $\pi$ has a peak. Given a set $S$ of positive integers, we define $\mathcal{P}_S(n)={\pi\in\mathfrak{S}_n:\mathcal{P}(\pi)=S}$. In 2013 Billey, Burdzy, and Sagan showed that for subsets of positive integers $S$ and sufficiently large $n$, $| \mathcal{P}_S(n)|=p_S(n)2^{n-|S|-1}$ where $p_S(x)$ is a polynomial depending on $S$. They gave a recursive formula for $p_S(x)$ involving an alternating sum, and they conjectured that the coefficients of $p_S(x)$ expanded in a binomial coefficient basis centered at $\max(S)$ are all nonnegative. In this paper we introduce a new recursive formula for $|\mathcal{P}_S(n)|$ without alternating sums, and we use this recursion to prove that their conjecture is true.