HABETS, P.; SERRA, E.; TARALLO, M. Multiple periodic solutions for problems at resonance with arbitrary eigenvalues. Topological Methods in Nonlinear Analysis, [S. l.], v. 12, n. 2, p. 293–307, 1998. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1998.043. Acesso em: 22 jul. 2024.