Fitzpatrick, P. M., Pejsachowicz, J., & Rabier, P. J. (1994). The degree of proper $C^2$ Fredholm mappings: Convariant theory. Topological Methods in Nonlinear Analysis, 3(2), 325–367. Retrieved from https://apcz.umk.pl/TMNA/article/view/TMNA.1994.017