FITZPATRICK, Patrick M., PEJSACHOWICZ, Jacobo and RABIER, Patrick J. The degree of proper $C^2$ Fredholm mappings: Convariant theory. Topological Methods in Nonlinear Analysis. Online. 1 June 1994. Vol. 3, no. 2, pp. 325 - 367. [Accessed 22 July 2024].