Mawhin, Jean. “Parameter Dependent Pull-Back of Closed Differential Forms and Invariant Integrals”. Topological Methods in Nonlinear Analysis 26, no. 1 (September 1, 2005): 17–33. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2005.023.