Polynomials non-negative on strips and half-strips

TL;DR

This paper extends Marshall's 2008 result on polynomials non-negative on the strip [0,1] x R to more general strips and half-strips, providing new characterizations of non-negative polynomials on certain non-compact semialgebraic sets in R^2.

Contribution

It generalizes Marshall's sum of squares representation to various strips and half-strips, broadening the understanding of non-negative polynomials on non-compact sets.

Findings

Characterization of non-negative polynomials on generalized strips and half-strips.

Representation of polynomials non-negative on U x R using sums of squares and generators.

New examples of non-compact semialgebraic sets with explicit polynomial descriptions.

Abstract

In 2008, M. Marshall settled a long-standing open problem by showing that if f(x,y) is a polynomial that is non-negative on the strip [0,1] x R, then there exist sums of squares s(x,y) and t(x,y) such that f(x,y) = s(x,y) + (x - x^2) t(x,y). In this paper, we generalize Marshall's result to various strips and half-strips in the plane. Our results give many new examples of non-compact semialgebraic sets in R^2 for which one can characterize all polynomials which are non-negative on the set. For example, we show that if U is a compact set in the real line and {g_1, ..., g_k} a specific set of generators for U as a semialgebraic set, then whenever f(x,y) is non-negative on U x R, there are sums of squares s_0, ..., s_k such that f = s_0 + s_1 g_1 + ... + s_k g_k.