Feltrin, Guglielmo, and Fabio Zanolin. 2017. “An Application of Coincidence Degree Theory to Cyclic Feedback Type Systems Associated With Nonlinear Differential Operators”. Topological Methods in Nonlinear Analysis 50 (2):683-726. https://apcz.umk.pl/TMNA/article/view/TMNA.2017.038.