  The goal of this communication is to propose a generalized notion of the
"traditional derivative". This generalization includes the fractional
derivatives such as the Riemann-Liouville, Gruenwald-Letnikov, Weyl, Riesz,
Caputo, Marchaud derivatives and other variants as special cases. The approach
is useful to describe mechanical problems in material systems with
microstructures without characteristic scale such as self-similar and fractal
materials.
