Given $f$ a function measurable in $[0,\infty]$ defined as $$\frac{d\mu}{d\nu}$$
such that, for every measurable set $A$ we have:
$$\mu(A)=\int_A{}fd\nu$$ the function $f$ is called $Radon - Nikodym\ derivative$ of $\mu$ respect to $\nu$
My question is: is it possible to define a $fractional\ Radon - Nikodym\ derivative$ of $\mu$ respect to $\nu$?
Thanks.