RICCERI, Biagio. Three solutions for a Neumann problem. Topological Methods in Nonlinear Analysis, [S. l.], v. 20, n. 2, p. 275–281, 2002. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2002.037. Acesso em: 8 jul. 2025.