Furi, M., and M. P. Pera. “Carathéodory Periodic Perturbations of the Zero Vector Field on Manifolds”. Topological Methods in Nonlinear Analysis, vol. 10, no. 1, Sept. 1997, pp. 79-92, https://apcz.umk.pl/TMNA/article/view/TMNA.1997.022.