Wilczyński, Paweł. “Planar Nonautonomous Polynomial Equations III. Zeros of the Vector Field”. Topological Methods in Nonlinear Analysis 40, no. 1 (April 23, 2012): 71–110. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2012.023.