# Derivative is Zero on a dense G_delta set

I have the following question: I have a function $f: \mathbb R \to \mathbb R$ which is differentiable everywhere. I also have a set $G\subset\mathbb R$ which is dense in $\mathbb R$ and a $G_\delta$-set. I know that $f'(x)=0 \forall x\in G$.

Can I conclude that $f$ is constant?

The answer is yes if $f'$ is continuous, but unfortunately I don't know that.

Many thanks!

Since the zero set is always a $G_\delta$, the question is whether it being dense implies that the function is constant. This is false, and a counterexample is known as a Pompeiu derivative.
• The WP article is sloppy; it says the derivative at some points "$=+\infty$" and papers over what that means. Maybe there are senses in which it makes sense to call this everywhere-differentiable anyway, but I would call it ae-differentiable. Nov 29, 2016 at 16:35
• @R if you read more carefully, $g(x)$ is the inverse of a function $f$ with finite derivative at every point, and $f'(x)=0$ iff $g'(f(x))=+\infty$. Of course "derivative" here has the usual meaning of limit of the incremental ratio in the extended real line. Quite a nice and well-written article, indeed. Nov 29, 2016 at 22:22