Fitzpatrick, P. M., J. Pejsachowicz, i P. J. Rabier. „The Degree of Proper $C^2$ Fredholm Mappings: Convariant Theory”. Topological Methods in Nonlinear Analysis, t. 3, nr 2, czerwiec 1994, s. 325-67, https://apcz.umk.pl/TMNA/article/view/TMNA.1994.017.