BERKOVITS, Juha; LEINFELDER, Herbert; MUSTONEN, Vesa. 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: 8 jul. 2025.