HAKL, Robert; TORRES, Pedro J.; ZAMORA, Manuel. Periodic solutions to singular second order differential equations: the repulsive case. Topological Methods in Nonlinear Analysis, [S. l.], v. 39, n. 2, p. 199–220, 2012. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2012.012. Acesso em: 30 may. 2026.