  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.
