Llibre, J., and M. F. da Silva. “Global Phase Portraits of Kukles Differential Systems With Homogenous Polynomial Nonlinearities of Degree 5 Having a Center”. Topological Methods in Nonlinear Analysis, vol. 48, no. 1, Aug. 2016, pp. 257-82, https://apcz.umk.pl/TMNA/article/view/TMNA.2016.049.