“…In the past decades, the theory of fractional differential equations has become an important area of investigation because of its wide applicability in many branches of physics, economics and technical sciences, see [18,20,[30][31][32][33][34][35]38] and references therein. The BVPs of fractional differential equations have also attracted more and more attention (for example [2,10,12,17]). The classical approaches to study such problems mainly include fixed point theorems, degree theory, the method of upper and lower solutions and so on.…”