I'm having a little problem in understanding this question...
Let $f \colon D \rightarrow \mathbb{R}$ with $D \subset \mathbb{R}$ and $a \in D'$. If $V$ is a neighbourhood of $a$ and $E= D \cap V$, show that $a \in E'$.
The exercise also asks to show that if $f|_E$ has a limit at the point $a$ then $f$ also has a limit at the point $a$ .
More specifically, I don't understand how to analyze the restricted function...
Thanks the attention!
(This is an exercise of my real analysis list of exercises. I spent a good time trying to do it, but I'm stuck... If anyone could give me a light... I'm studying for my final exam...)