Barillon, Cristelle, and Vitaly A. Volpert. “Topological Degree for a Class of Elliptic Operators in ${\mathbb R}^n$”. Topological Methods in Nonlinear Analysis 14, no. 2 (December 1, 1999): 275–293. Accessed April 20, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.1999.034.