  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.
