BEN-NAOUM, A. K.; BERKOVITS, J. Nontrivial solutions for some semilinear problems and applications to wave equations on balls and spheres. Topological Methods in Nonlinear Analysis, [S. l.], v. 5, n. 1, p. 177–192, 1995. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1995.010. Acesso em: 24 apr. 2024.