“…Since then, the new theory turned out to be very attractive to c 2011 Diogenes Co., Sofia pp. 523-537 , DOI: 10.2478/s13540-011-0032-6 mathematicians and many different forms of fractional operators were introduced: the Grunwald-Letnikov, Riemann-Liouville, Riesz, and Caputo fractional derivatives (see, e.g., [25,35,40]), and the more recent notions of [26], [15], and [23,24]. Besides in mathematics, fractional derivatives and integrals appear in physics, mechanics, engineering, elasticity, dynamics, control theory, economics, biology, chemistry, etc.…”