Motreanu, D., Motreanu, V. V., & Papageorgiou, N. S. (2004). Periodic solutions for nonautonomous systems with nonsmooth quadratic or superquadratic potential. Topological Methods in Nonlinear Analysis, 24(2), 269–296. Retrieved from https://apcz.umk.pl/TMNA/article/view/TMNA.2004.028