BUICĂ, A.; GINÉ, J.; LLIBRE, J. Periodic solutions for nonlinear differential systems: the second order bifurcation function. Topological Methods in Nonlinear Analysis, [S. l.], v. 43, n. 2, p. 403–419, 2016. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2014.024. Acesso em: 28 mar. 2023.