Aizicovici, Sergiu, Nikolaos S. Papageorgiou, and Vasile Staicu. 2006. “Resonant Nonlinear Periodic Problems With the Scalar $p$-Laplacian and a Nonsmooth Potential”. Topological Methods in Nonlinear Analysis 27 (2):269-88. https://apcz.umk.pl/TMNA/article/view/TMNA.2006.011.