Remarks on some families of fractional-order differential equations

The main purpose of this note is to draw the reader's attention towards some errors and omissions in a recent work involving solutions of some families of fractional-order differential equations, which was published in this Journal (see, for details, [Tomovski Ž, Hilfer R, Srivastava HM. Fractional and operational calculus with generalized fractional derivative operators and Mittag–Leffler type functions. Integral Transforms Spec Funct. 2010;21:797–814]). Several relevant remarks and observations on some other related recent developments on this subject are also presented.