Colonius, F., R. Fabbri, R. Johnson, and M. Spadini. “Bifurcation Phenomena in Control Flows”. Topological Methods in Nonlinear Analysis, vol. 30, no. 1, Sept. 2007, pp. 87-111, https://apcz.umk.pl/TMNA/article/view/TMNA.2007.020.