  Let $D(s)$ be a fractional derivation of order $s$. For a real $p\ne 0$, we
construct an integral operator $A(p)$ in an appropriate functional space such
that $A(p) D(s) A(p)^{-1}=D(p s)$ for all $s$. The kernel of the operator
$A(p)$ is expressed in terms of a function similar to the stable densities.
