Aizicovici, S., N. S. Papageorgiou, and V. Staicu. “Resonant Nonlinear Periodic Problems With the Scalar $p$-Laplacian and a Nonsmooth Potential”. Topological Methods in Nonlinear Analysis, vol. 27, no. 2, June 2006, pp. 269-88, https://apcz.umk.pl/TMNA/article/view/TMNA.2006.011.