Call **a computable function** a total function $\mathbb{R} \to \mathbb{R}$, for which there exists a Turing machine outputting arbitrary close approximation to $f(x)$ given arbitrary close approximation to $x$.

- Obviously not every computable function is differentiable (for example, absolute value). For arbitrary continuous functions, the set of points of differentiability is $\Pi_{3}^0$. Can this be improved for computable functions?
- Suppose $f$ is computable and continously differentiable everywhere. Must $f'$ be computable?