“…In recent years, there are several contributions focusing on fractional differential equations, mainly on the existence, uniqueness, and stability of solutions. For more details, the readers are referred to the previous studies [4,5,6,7,9,15,17,18,19,20,21,22,23,24,35,36,41,43,49,50,51,52,53,54,55] and the references therein. The main techniques used in these studies are fixed-point techniques, Leray-Schauder theory, coincidence theory, or monotone iterative technique combined with the method of upper and lower solutions.…”