Coincidence of standard derivative and weak derivative Let $f:\mathbb{R}^n \to \mathbb{R}$ be in $W^{1,p}(\mathbb{R}^n)$ and differentiable (in the classical sense) almost everywhere. 
Is it true that the standard derivative and the weak derivative conicide?
When $p>n$ this is a corollary from theorem 4.9 ("LECTURES ON LIPSCHITZ ANALYSIS"- by Heinonen; In fact, in that case $f \in W^{1,p}$ implies that $f$ is differentiable almost everywhere).
What happens for other values of $p$?

This follows from the ACL characterization of Sobolev spaces, see https://en.wikipedia.org/wiki/Sobolev_space#Absolutely_continuous_on_lines_.28ACL.29_characterization_of_Sobolev_functions.
