Can we split fractional derivative while analyzing fractional differential equations?

Fractional derivatives are generalization to classical integer-order derivatives. The rules which are true for classical derivative need not hold for the fractional derivatives. For example, it is proved in the literature that we cannot simply add the fractional orders alpha and beta in (DD beta)-D-alpha to produce the fractional derivative D alpha+beta of order alpha +beta, in general. In this article we discuss the details of such compositions and propose the conditions to split a linear fractional differential equation into systems involving lower order derivatives. We provide some examples, which show that the conditions of the related results in the literature are sufficient but not necessary. Further, we point out that the fractional differential equations formed using the derivatives which satisfy the composition rule (DD beta)-D-alpha = (DD alpha)-D-beta = D alpha+beta produce only a trivial solution. (C) 2019 Elsevier B.V. All rights reserved.