Ize, J., and A. Vignoli. “Equivariant Degree for Abelian Actions. Part I: Equivariant Homotopy Groups”. Topological Methods in Nonlinear Analysis, vol. 2, no. 2, Dec. 1993, pp. 367-13, https://apcz.umk.pl/TMNA/article/view/TMNA.1993.048.