A Refinement of the Eulerian Numbers, and the Joint Distribution of $π(1)$ and Des($π$) in $S_n$

Abstract

Given a permutation $π$ chosen uniformly from $S_n$, we explore the joint distribution of $π(1)$ and the number of descents in $π$. We obtain a formula for the number of permutations with $\des(π)=d$ and $π(1)=k$, and use it to show that if $\des(π)$ is fixed at $d$, then the expected value of $π(1)$ is $d+1$. We go on to derive generating functions for the joint distribution, show that it is unimodal if viewed correctly, and show that when $d$ is small the distribution of $π(1)$ among the permutations with $d$ descents is approximately geometric. Applications to Stein's method and the Neggers-Stanley problem are presented.