1.
MOTREANU, Dumitru, MOTREANU, Viorica V. and PAPAGEORGIOU, Nikolaos S. Periodic solutions for nonautonomous systems with nonsmooth quadratic or superquadratic potential. Topological Methods in Nonlinear Analysis. Online. 1 December 2004. Vol. 24, no. 2, pp. 269 - 296. [Accessed 22 July 2024].