BERKOVITS, J.; LEINFELDER, H.; MUSTONEN, V. Existence and multiplicity results for wave equations with time-independent nonlinearity. Topological Methods in Nonlinear Analysis, [S. l.], v. 22, n. 2, p. 273–295, 2003. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2003.040. Acesso em: 23 jan. 2022.