Periodic solutions to superlinear indefinite planar systems: A topological degree approach

Abstract

We deal with a planar differential system of the form {u'=h(t,v),v'=Ya(t)g(u), where h is T-periodic in the first variable and strictly increasing in the second variable, Y>0, a is a sign-changing T-periodic weight function and g is superlinear. Based on the coincidence degree theory, in dependence of Y, we prove the existence of T-periodic solutions (u,v) such that u(t)>0 for all tER. Our results generalize and unify previous contributions about Butler's problem on positive periodic solutions for second-order differential equations (involving linear or O-Laplacian-type differential operators).