$C^{1,\omega }$ extension formulas for $1$-jets on Hilbert spaces

Abstract

We provide necessary and sufficient conditions for a $1$-jet $(f,G):E\to \mathbb{R}\times X$ to admit an extension $(F,\nabla F)$ for some $F\in C^{1,\omega }(X)$. Here $E$ stands for an arbitrary subset of a Hilbert space $X$ and $\omega$ is a modulus of continuity. As a corollary, in the particular case $X={\mathbb{R}}^n$, we obtain an extension (nonlinear) operator whose norm does not depend on the dimension $n$. Furthermore, we construct extensions $(F,\nabla F)$ in such a way that: (1) the (nonlinear) operator $(f,G)\mapsto (F,\nabla F)$ is bounded with respect to a natural seminorm arising from the constants in the given condition for extension (and the bounds we obtain are almost sharp); (2) $F$ is given by an explicit formula; (3) $(F,\nabla F)$ depend continuously on the given data $(f,G)$; (4) if $f$ is bounded (resp. if $G$ is bounded) then so is $F$ (resp. $F$ is Lipschitz). We also provide similar results on superreflexive Banach spaces.

We have added some comments in the Introduction and also corrected some misprints