I have been looking for results or theorems which give me regularity conditions of the Radon-Nikodym derivative, but I have not found any :(
For instance, we know that if $\nu\ll\mu$ then there exists $f\in L^1$ s.t. $\nu = \int f d\mu$.
I wonder if, under extra conditions, we can say more about $f$, like $f\in \mathcal{C}$ or similar.
Are there results? Thank you very much for any help!