“…This is because of both the intensive development of the theory of fractional calculus itself and the wide range of applications of such kind of equations in various scientific fields such as physics, mechanics, chemistry, economics, engineering and biological sciences, etc., see [18,20,21,28]. In recent years, the study of positive solutions for fractional differential equation boundary value problems has attracted considerable attention, and many results have been achieved, and here we refer the reader to [5,6,12,14,15,19,25,29,30,31,32,35,36,37] and the references therein for details.…”