English

Convex $C^1$ extensions of $1$-jets from compact subsets of Hilbert spaces

Functional Analysis 2020-04-03 v3

Abstract

Let XX denote a Hilbert space. Given a compact subset KK of XX and two continuous functions f:KRf:K\to\mathbb{R}, G:KXG:K\to X, we show that a necessary and sufficient condition for the existence of a convex function FC1(X)F\in C^1(X) such that F=fF=f on KK and F=G\nabla F=G on KK is that the 11-jet (f,G)(f, G) satisfies (1) f(x)f(y)+G(y),xyf(x)\geq f(y)+ \langle G(y), x-y\rangle for all x,yKx, y\in K, and (2) if x,yKx, y\in K and f(x)=f(y)+G(y),xyf(x)= f(y)+ \langle G(y), x-y\rangle then G(x)=G(y)G(x)=G(y). We also solve a similar problem for KK replaced with an arbitrary bounded subset of XX, and for C1(X)C^1(X) replaced with the class Cb1,u(X)C^{1,u}_{b}(X) of differentiable functions with uniformly continuous derivatives on bounded subsets of XX.

Keywords

Cite

@article{arxiv.1911.04166,
  title  = {Convex $C^1$ extensions of $1$-jets from compact subsets of Hilbert spaces},
  author = {Daniel Azagra and Carlos Mudarra},
  journal= {arXiv preprint arXiv:1911.04166},
  year   = {2020}
}

Comments

Final version (with an improvement in the proof suggested by a referee)