Generalized Hestenes’ Lemma and extension of functions

Abstract:

Suppose we have an $m$-jet field on $V\subset \mathbf {R}^{n}$ which is a Whitney field on the nonsingular part $M$ of $V$. We show that, under certain hypotheses about the relationship between geodesic and euclidean distance on $V$, if the field is flat enough at the singular part $S$, then it is a Whitney field on $V$ (the order of flatness required depends on the coefficients in the hypotheses). These hypotheses are satisfied when $V$ is subanalytic. In Section II, we show that a $C^{2}$ function $f$ on $M$ can be extended to one on $V$ if the differential $df$ goes to $0$ faster than the order of divergence of the principal curvatures of $M$ and if the first covariant derivative of $df$ is sufficiently flat. For the general case of $C^{m}$ functions with $m >2$, we give a similar result for $\operatorname {codim} M=1$ in Section III.