Let $C^{m, \omega} ( \mathbb{R}^n)$ be the space of functions on $\mathbb{R}^n$ whose $m^{\sf th}$ derivatives have modulus of continuity $\omega$. For $E \subset \mathbb{R}^n$, let $C^{m , \omega} (E)$ be the space of all restrictions to $E$ of functions in $C^{m , \omega} ( \mathbb{R}^n)$. We show that there exists a bounded linear operator $T: C^{m , \omega} ( E ) \rightarrow C^{m , \omega } ( \mathbb{R}^n)$ such that, for any $f \in C^{m , \omega} ( E )$, we have $T f = f$ on $E$.

We exhibit efficient algorithms to perform the following task: Given a function $f$ defined on a finite subset $E \subset \mathbb R^n$, compute a $C^m$ function $F$ on $\mathbb R^n$, with a controlled $C^m$ norm, that approximates $f$ on the subset $E$.

We give algorithms to compute a function $F$ on $\mathbb R^n$, having prescribed Taylor polynomials (or taking prescribed values) at $N$ given points, with the $C^m$-norm of $F$ close to least possible.