FELMER, P.; WANG, Z.-Q. Multiplicity for symmetric indefinite functionals: applications to Hamiltonian and elliptic systems. Topological Methods in Nonlinear Analysis, [S. l.], v. 12, n. 2, p. 207–226, 1998. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1998.038. Acesso em: 27 apr. 2024.