Motreanu, D., V. V. Motreanu, and N. S. Papageorgiou. “Periodic Solutions for Nonautonomous Systems With Nonsmooth Quadratic or Superquadratic Potential”. Topological Methods in Nonlinear Analysis, vol. 24, no. 2, Dec. 2004, pp. 269-96, https://apcz.umk.pl/TMNA/article/view/TMNA.2004.028.