  After reviewing the definition of two differential operators which have been
recently introduced by Caputo and Fabrizio and, separately, by Atangana and
Baleanu, we present an argument for which these two integro-differential
operators can be understood as simple realizations of a much broader class of
fractional operators, i.e. the theory of Prabhakar fractional integrals.
Furthermore, we also provide a series expansion of the Prabhakar integral in
terms of Riemann-Liouville integrals of variable order. Then, by using this
last result we finally argue that the operator introduced by Caputo and
Fabrizio cannot be regarded as fractional. Besides, we also observe that the
one suggested by Atangana and Baleanu is indeed fractional, but it is
ultimately related to the ordinary Riemann-Liouville and Caputo fractional
operators. All these statements are then further supported by a precise
analysis of differential equations involving the aforementioned operators. To
further strengthen our narrative, we also show that these new operators do not
add any new insight to the linear theory of viscoelasticity when employed in
the constitutive equation of the Scott-Blair model.
