Hai-yan Zhang & Yao-hong Li. (2019). Multiple Positive Solutions to Singular Fractional Differential System with Riemann-Stieltjes Integral Boundary Condition.
            <i>Communications in Mathematical Research </i>. <i>35</i> (3).
            208-218.
         doi:10.13447/j.1674-5647.2019.03.02