An application of coincidence degree theory to cyclic feedback type systems associated with nonlinear differential operators

Using Mawhin's coincidence degree theory, we obtain some new continuation theorems which are designed to have as a natural application the study of the periodic problem for cyclic feedback type systems. We also discuss some examples of vector ordinary differential equations with a φ-Laplacian operator where our results can be applied. Our main contribution in this direction is to obtain a continuation theorem for the periodic problem associated with (φ(u'))' + λ k(t,u,u') = 0, under the only assumption that φ is a homeomorphism.