An SOS counterexample to an inequality of symmetric functions

TL;DR

This paper presents a counterexample to a conjecture about the nonnegativity of differences of homogeneous symmetric functions, using semidefinite programming to explicitly demonstrate a negative case.

Contribution

It provides the first explicit counterexample to the conjecture that dominance order implies nonnegativity for homogeneous symmetric functions.

Findings

Counterexample polynomial $H_{44} - H_{521}$ is negative.

Sum of squares decomposition disproves the conjecture.

Partitions 44 and 521 are incomparable in dominance order.

Abstract

It is known that differences of symmetric functions corresponding to various bases are nonnegative on the nonnegative orthant exactly when the partitions defining them are comparable in dominance order. The only exception is the case of homogeneous symmetric functions where it is only known that dominance of the partitions implies nonnegativity of the corresponding difference of symmetric functions. It was conjectured by Cuttler, Greene, and Skandera in 2011 that the converse also holds, as in the cases of the monomial, elementary, power-sum, and Schur bases. In this paper we provide a counterexample, showing that homogeneous symmetric functions break the pattern. We use semidefinite programming to find an explicit sums of squares decomposition of the polynomial $H_{44} - H_{521}$ as a sum of 41 squares. This rational certificate of nonnegativity disproves the conjecture, since a polynomial which is a sum of squares cannot be negative, and since the partitions 44 and 521 are incomparable in dominance order.