Karakostas, George L., and Panagiotis Ch. Tsamatos. “Existence of Multiple Positive Solutions for a Nonlocal Boundary Value Problem”. Topological Methods in Nonlinear Analysis 19, no. 1 (March 1, 2002): 109–121. Accessed January 17, 2022. https://apcz.umk.pl/TMNA/article/view/TMNA.2002.007.