Some combinatorial results on smooth permutations

Abstract

We show that any smooth permutation $\sigma\in S_n$ is characterized by the set ${\mathbf{C}}(\sigma)$ of transpositions and $3$-cycles in the Bruhat interval $(S_n)_{\leq\sigma}$, and that $\sigma$ is the product (in a certain order) of the transpositions in ${\mathbf{C}}(\sigma)$. We also characterize the image of the map $\sigma\mapsto{\mathbf{C}}(\sigma)$. As an application, we show that $\sigma$ is smooth if and only if the intersection of $(S_n)_{\leq\sigma}$ with every conjugate of a parabolic subgroup of $S_n$ admits a maximum. This also gives another approach for enumerating smooth permutations and subclasses thereof. Finally, we relate covexillary permutations to smooth ones and rephrase the results in terms of the (co)essential set in the sense of Fulton.