Hirano, N., and S. Rybicki. “Some Remarks on Degree Theory for ${\text{\rm SO}(2)}$-Equivariant Transversal Maps”. Topological Methods in Nonlinear Analysis, vol. 22, no. 2, Dec. 2003, pp. 253-72, https://apcz.umk.pl/TMNA/article/view/TMNA.2003.039.