Aizicovici, S., N. S. Papageorgiou, and V. Staicu. “Positive Solutions for Parametric Dirichlet Problems With Indefinite Potential and Superdiffusive Reaction”. Topological Methods in Nonlinear Analysis, vol. 47, no. 2, June 2016, pp. 423-38, doi:10.12775/TMNA.2016.014.