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 $a_1, \dots, a_n \in \mathbb{R}<em>{\geq 0}$, we prove that [ \frac{1}{n!} \sum{\pi \in S_n} \left[\prod_{{i:i=\pi(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.