AMSTER, P.; HADDAD, J. A Hartman-Nagumo type condition for a class of contractible domains. Topological Methods in Nonlinear Analysis, [S. l.], v. 41, n. 2, p. 287–304, 2013. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2013.013. Acesso em: 17 jan. 2022.