Isometries on certain non-complete vector-valued function spaces

TL;DR

This paper extends the characterization of surjective isometries between vector-valued function spaces to non-complete and non-strictly convex cases, providing new proofs and generalizations for various function spaces.

Contribution

It describes surjective isometries between certain normed subspaces of vector-valued continuous functions without assuming strict convexity, broadening previous results.

Findings

Provides a short proof for isometries without strict convexity assumption.

Generalizes results to spaces of Lipschitz and differentiable functions.

Applies to non-complete and more general vector-valued function spaces.

Abstract

In the recent paper \cite{Hos}, surjective isometries, not necessarily linear, $T: {\rm AC}(X,E) \longrightarrow {\rm AC}(Y,F)$ between vector-valued absolutely continuous functions on compact subsets $X$ and $Y$ of the real line, has been described. The target spaces $E$ and $F$ are strictly convex normed spaces. In this paper, we assume that $X$ and $Y$ are compact Hausdorff spaces and $E$ and $F$ are normed spaces, which are not assumed to be strictly convex. We describe (with a short proof) surjective isometries $T: (A,\|\cdot\|_A) \longrightarrow (B,\|\cdot\|_B)$ between certain normed subspaces $A$ and $B$ of $C(X,E)$ and $C(Y,F)$, respectively. We consider three cases for $F$ with some mild conditions. The first case, in particular, provides a short proof for the above result, without assuming that the target spaces are strictly convex. The other cases give some generalizations in this topic. As a consequence, the results can be applied, for isometries (not necessarily linear) between spaces of absolutely continuous vector-valued functions, (little) Lipschitz functions and also continuously differentiable functions.