Capietto, Anna, et al. “On the Existence of Two Solutions With a Prescribed Number of Zeros for a Superlinear Two-Point Boundary Value Problem”. Topological Methods in Nonlinear Analysis, vol. 6, no. 1, Sept. 1995, pp. 175-88, https://apcz.umk.pl/TMNA/article/view/TMNA.1995.039.