Existence of periodic solutions of planar systems

In this paper, the existence of periodic solutions for the following nonlinear asymmetric oscillator, is discussed, where a±, b± are positive constants satisfying x± = max{±x, 0}, y± = max{±y, 0}, m, n ε N, f(x,y,t), g(x,y,t) are bounded, locally Lipschitz functions of x,y for fixed t ε [0, 2π], 2π-periodic in t, and the following limits exist