Zhang, Xingqiu, et al. “Triple Positive Solutions for Semipositone Fractional Differential Equations M-Point Boundary Value Problems With Singularities and p–q-Order Derivatives”. Nonlinear Analysis: Modelling and Control, vol. 23, no. 6, Dec. 2018, pp. 889-03, https://doi.org/10.15388/NA.2018.6.5.