Perera, K. (2003) “Nontrivial critical groups in $p$-Laplacian problems via the Yang index”, Topological Methods in Nonlinear Analysis, 21(2), pp. 301–309. Available at: https://apcz.umk.pl/TMNA/article/view/TMNA.2003.018 (Accessed: 26 April 2024).