Aizicovici, S., N. S. Papageorgiou, and V. Staicu. “Nonlinear Periodic System With Unilateral Constraints”. Topological Methods in Nonlinear Analysis, vol. 54, no. 2B, Nov. 2019, pp. 871-85, https://apcz.umk.pl/TMNA/article/view/TMNA.2019.062.