“…The interested reader may be referred to several recent papers on the subject [10,16,17,27,30,31]. The special case when τ 1, τ 2 = 1 in (1.7) give the familiar representations of Appell's hypergeometric function [4,5,14,15,17,18,20,22,23]. However, recently there has been an increasing interest in and widely extended use of differential equations and systems of fractional order (that is, of arbitrary order), as better models of phenomena of various physics, engineering, automatization, biology and biomedicine, chemistry, earth science, economics, nature and so on.…”