LLIBRE, J.; SALHI, T. On a class of polynomial differential systems of degree 4: phase portraits and limit cycles. Topological Methods in Nonlinear Analysis, [S. l.], v. 57, n. 2, p. 441–463, 2021. DOI: 10.12775/TMNA.2020.042. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2020.042. Acesso em: 22 jul. 2024.