Aizicovici, S., N. S. Papageorgiou, and V. Staicu. “On a $p$-Superlinear Neumann $p$-Laplacian Equation”. Topological Methods in Nonlinear Analysis, vol. 36, no. 1, Sept. 2009, pp. 111-30, https://apcz.umk.pl/TMNA/article/view/TMNA.2009.032.