Llibre, Jaume. “On the Centers of Cubic Polynomial Differential Systems With Four Invariant Straight Lines”. Topological Methods in Nonlinear Analysis 55, no. 2 (May 30, 2020): 387–402. Accessed April 26, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2020.004.