Gámez, J. L., and J. F. Riuz. “Bifurcation of Solutions of Elliptic Problems: Local and Global Behaviour”. Topological Methods in Nonlinear Analysis, vol. 23, no. 2, June 2004, pp. 203-12, https://apcz.umk.pl/TMNA/article/view/TMNA.2004.009.