Ize, J., and A. Vignoli. “Equivariant Degree for Abelian Actions. Part III: Orthogonal Maps”. Topological Methods in Nonlinear Analysis, vol. 13, no. 1, Mar. 1999, pp. 105-46, https://apcz.umk.pl/TMNA/article/view/TMNA.1999.006.