Suppose that, for each point $x$ in a given subset $E \subset
\mathbb{R}^n$, we are given an $m$-jet $f(x)$ and a convex,
symmetric set $\sigma(x)$ of $m$-jets at $x$. We ask whether
there exist a function $F \in C^{m , \omega} ( \mathbb{R}^n )$ and a
finite constant $M$, such that the $m$-jet of $F$ at $x$ belongs to
$f ( x ) + M \sigma ( x )$ for all $x \in E$. We give a necessary
and sufficient condition for the existence of such $F , M$, provided
each $\sigma(x)$ satisfies a condition that we call ``Whitney
$\omega$-convexity''.