Capietto, A., M. Henrard, J. Mawhin, and F. Zanolin. “A Continuation Approach to Some Forced Superlinear Sturm-Liouville Boundary Value Problems”. Topological Methods in Nonlinear Analysis, vol. 3, no. 1, Mar. 1994, pp. 81-100, https://apcz.umk.pl/TMNA/article/view/TMNA.1994.005.