Elementary symmetric polynomials under the fixed point measure

Abstract

We identify a surprising inequality satisfied by elementary symmetric polynomials under the action of the fixed point measure of a random permutation. Concretely, for any collection of $n$ non-negative real numbers $a_1,\dots ,a_n\in {\mathbb{R}}_{\ge 0}$, we prove that \[ \frac{1}{n!} \sum_{π\in S_n} \left[\prod_{\{i:i=π(i)\}} a_i\right] \ge \frac{1}{\binom{n}{2}} \sum_{S \in\binom{[n]}{2}} \left[ \left(\prod_{\{i \in S\}} a_i \right)^{1/2}\right], \] and this bound is sharp. To prove this elementary inequality, we construct a collection of differential operators to set up a monotone flow that then allows us to establish the inequality.

14 pages, 0 figures