Makarenkov, Oleg. “Influence of a Small Perturbation on Poincaré-Andronov Operators With Not Well Defined Topological Degree”. Topological Methods in Nonlinear Analysis 32, no. 1 (September 1, 2008): 165–175. Accessed April 26, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2008.043.