“…Fractional differential equations have attracted much attention and have been the focus of many studies due mainly to their varied applications in many fields of science and engineering. In other words, fractional differential equations are widely used to describe many important phenomena in various fields such as physics, biophysics, chemistry, biology, control theory, economy and so on; see [14,19,23,29,33]. For an extensive literature in the study of fractional differential equations, we refer the reader to [2,11,15,16,18,20,21,24,26,30,32].…”