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 $\phi$-Laplacian operator where our results can be applied. 
Guglielmo Feltrin, Fabio Zanolin
math.CA