Pejsachowicz, Jacobo. 2001. “Index Bundle, Leray-Schauder Reduction and Bifurcation of Solutions of Nonlinear Elliptic Boundary Value Problems”. Topological Methods in Nonlinear Analysis 18 (2):243-67. https://apcz.umk.pl/TMNA/article/view/TMNA.2001.033.