HATA, K.; LIU, J.; WANG, Z.-Q. Note on periodic solutions of relativistic pendulum type systems. Topological Methods in Nonlinear Analysis, [S. l.], v. 42, n. 2, p. 417–425, 2013. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2013.043. Acesso em: 29 mar. 2023.