Hakl, R., Torres, P. J. and Zamora, M. (2012) “Periodic solutions to singular second order differential equations: the repulsive case”, Topological Methods in Nonlinear Analysis, 39(2), pp. 199–220. Available at: https://apcz.umk.pl/TMNA/article/view/TMNA.2012.012 (Accessed: 20 January 2022).