On Stability in the Large for Periodic Solution of Differential Systems
George Seifert · Annals of Mathematics · 1958
( 2 ) X~~~ + f(x).+ = gx) + pAt) where p(t) is continuous and of period r in t, and f(x) and are such that a relative bound R for the system exists. In fact, we require that for x in a finite closed interval determined by R, f'(x) and g(x) exist, and f(x) > 0, g'(x) < 0.