HEID, Michael; HEINZ, Hans-Peter. Nonlinear eigenvalue problems admitting eigenfunctions with known geometric properties. Topological Methods in Nonlinear Analysis, [S. l.], v. 13, n. 1, p. 17–51, 1999. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1999.002. Acesso em: 8 jul. 2025.