WALCZAK, S. Variational and boundary value problems with perturbations. Topological Methods in Nonlinear Analysis, [S. l.], v. 18, n. 1, p. 103–118, 2001. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2001.026. Acesso em: 25 apr. 2024.