Symmetric nonnegative forms and sums of squares

TL;DR

This paper explores the structure of symmetric nonnegative forms and sums of squares, characterizing their relationship using representation theory, and investigates their asymptotic behavior as the number of variables increases.

Contribution

It provides a uniform characterization of symmetric nonnegative forms and sums of squares cones, and analyzes their asymptotic proximity for fixed degrees as variables grow.

Findings

Symmetric nonnegative forms and sums of squares form closed, convex cones.

The difference between these cones does not grow arbitrarily large for fixed degree.

In degree 4, the cones asymptotically approach each other as variables increase.

Abstract

We study symmetric nonnegative forms and their relationship with symmetric sums of squares. For a fixed number of variables $n$ and degree $2d$, symmetric nonnegative forms and symmetric sums of squares form closed, convex cones in the vector space of $n$-variate symmetric forms of degree $2d$. Using representation theory of the symmetric group we characterize both cones in a uniform way. Further, we investigate the asymptotic behavior when the degree $2d$ is fixed and the number of variables $n$ grows. Here, we show that, in sharp contrast to the general case, the difference between symmetric nonnegative forms and sums of squares does not grow arbitrarily large for any fixed degree $2d$. We consider the case of symmetric quartic forms in more detail and give a complete characterization of quartic symmetric sums of squares. Furthermore, we show that in degree $4$ the cones of nonnegative symmetric forms and symmetric sums of squares approach the same limit, thus these two cones asymptotically become closer as the number of variables grows. We conjecture that this is true in arbitrary degree $2d$.