BARILLON, Cristelle; VOLPERT, Vitaly A. Topological degree for a class of elliptic operators in ${\mathbb R}^n$. Topological Methods in Nonlinear Analysis, [S. l.], v. 14, n. 2, p. 275–293, 1999. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1999.034. Acesso em: 9 mar. 2025.