Stability of compact actions and a result on divided differences

Abstract

We study smooth locally free actions of ${\mathbb{R}}^n$ on manifolds $M$ of dimension $n+1$. We are interested in compact orbits and in compact actions: actions with all orbits compact. Given a compact orbit in a neighborhood of compact orbits, we give necessary and sufficient conditions for the existence of a $C^k$ perturbation with noncompact orbits in the given neighborhood. We prove that if such a perturbation exists it can be assumed to differ from the original action only in a smaller neighborhood of the initial orbit. As an application, for each $k$, we give examples of compact actions which admit $C^{k-1}$-perturbations with noncompact orbits but such that all $C^k$-perturbations are compact. The main result generalizes for $k>1$ a previous result for the case $C^1$. A critical auxiliary result is an estimate on divided differences.

23 pages, 1 figure; minor clarifications added in this version