DUC, D. M.; HUNG, T. V.; KHAI, N. T. Critical points of non-$C^{2}$ functionals. Topological Methods in Nonlinear Analysis, [S. l.], v. 29, n. 1, p. 35–68, 2007. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2007.003. Acesso em: 26 apr. 2024.