Ben-Naoum, A. K., & Berkovits, J. (1995). Nontrivial solutions for some semilinear problems and applications to wave equations on balls and spheres. Topological Methods in Nonlinear Analysis, 5(1), 177–192. Pobrano z https://apcz.umk.pl/TMNA/article/view/TMNA.1995.010