UREÑA, A. Periodic solutions of singular equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 47, n. 1, p. 55–72, 2016. DOI: 10.12775/TMNA.2015.087. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2015.087. Acesso em: 22 jul. 2024.