Aizicovici, S., N. S. Papageorgiou, and V. Staicu. “Existence and Multiplicity of Solutions for Resonant Nonlinear Neumann Problems”. Topological Methods in Nonlinear Analysis, vol. 35, no. 2, Apr. 2010, pp. 235-52, https://apcz.umk.pl/TMNA/article/view/TMNA.2010.014.