Ge, B., V. D. Rădulescu, and C. J. Zhang. “Infinitely Many Positive Solutions of Fractional Boundary Value Problems”. Topological Methods in Nonlinear Analysis, vol. 49, no. 2, Feb. 2017, pp. 647-64, https://apcz.umk.pl/TMNA/article/view/TMNA.2017.001.