PERERA, K. Nontrivial critical groups in $p$-Laplacian problems via the Yang index. Topological Methods in Nonlinear Analysis, [S. l.], v. 21, n. 2, p. 301–309, 2003. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2003.018. Acesso em: 22 lip. 2024.