Pötzsche, Christian, and Stefan Siegmund. “$C^{m}$-Smoothness of Invariant Fiber Bundles”. Topological Methods in Nonlinear Analysis 24, no. 1 (September 1, 2004): 107–145. Accessed April 16, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2004.021.