Hakl, R., P. J. Torres, and M. Zamora. “Periodic Solutions to Singular Second Order Differential Equations: The Repulsive Case”. Topological Methods in Nonlinear Analysis, vol. 39, no. 2, Apr. 2012, pp. 199-20, https://apcz.umk.pl/TMNA/article/view/TMNA.2012.012.