WANG, F.; CHU, J.; SIEGMUND, S. Periodic solutions for second order singular differential systems with parameters. Topological Methods in Nonlinear Analysis, [S. l.], v. 46, n. 2, p. 549–562, 2015. DOI: 10.12775/TMNA.2015.058. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2015.058. Acesso em: 26 apr. 2024.