Stability of non-proper functions

Abstract

The purpose of this paper is to give a sufficient condition for (strong) stability of non-proper smooth functions (with respect to the Whitney $C^{\infty }$-topology). We show that a Morse function is stable if it is end-trivial at any point in its discriminant, where end-triviality (which is also called local triviality at infinity) is a property concerning behavior of functions around the ends of the source manifolds. We further show that a Morse function $f:N\to \mathbb{R}$ is strongly stable (i.e. there exists a continuous mapping $g\mapsto ({\Phi }_g,{\varphi }_g)\in \mathrm{Diff}(N)\times \mathrm{Diff}(\mathbb{R})$ such that ${\varphi }_g\circ g\circ {\Phi }_g=f$ for any $g$ close to $f$) if (and only if) $f$ is quasi-proper. This result yields existence of a strongly stable but not infinitesimally stable function. Applying our result on stability, we give a reasonable sufficient condition for stability of Nash functions, and show that any Nash function becomes stable after a generic linear perturbation.

29 pages, no figures. V2: details of the proof of the main theorem are added and typos are fixed