Hu, S., and N. S. Papageorgiou. “Nontrivial Solutions for Superquadratic Nonautonomous Periodic Systems”. Topological Methods in Nonlinear Analysis, vol. 34, no. 2, Dec. 2009, pp. 327-38, https://apcz.umk.pl/TMNA/article/view/TMNA.2009.045.