Metric invariants in Banach and Jordan--Banach algebras

TL;DR

This paper surveys and extends recent research on metric invariants in complex Banach and Jordan--Banach algebras, focusing on natural subsets where isometries imply algebraic isomorphisms, with new results and simplified proofs.

Contribution

It introduces new results and simplified proofs regarding metric invariants and isometries in specific subsets of Banach and Jordan--Banach algebras, highlighting conditions for algebraic isomorphisms.

Findings

Surjective isometries on invertible elements extend to algebra isometries.

Isometries on positive invertible elements relate to algebraic structure preservation.

Unitary element sets admit isometries that imply algebraic isomorphisms.

Abstract

In this note we collect some significant contributions on metric invariants for complex Banach algebras and Jordan--Banach algebras established during the last fifteen years. This note is mainly expository, but it also contains complete proofs and arguments, which in many cases are new or have been simplified. We have also included several new results. The common goal in the results is to seek for "natural" subsets, $\mathfrak{S}_{A},$ associated with each complex Banach or Jordan--Banach algebra $A$, sets which when equipped with a certain metric, $d_{A}$, enjoys the property that each surjective isometry from $(\mathfrak{S}_{A},d_A)$ to a similar set, $(\mathfrak{S}_{B},d_B),$ associated with another Banach or Jordan--Banach algebra $B$, extends to a surjective real-linear isometry from $A$ onto $B$. In case of a positive answer to this question, the problem of discussing whether in such a case the algebras $A$ and $B$ are in fact isomorphic or Jordan isomorphic is the subsequent question. The main results presented here will cover the cases in which the sets $(\mathfrak{S}_{A},d_A)$ and $(\mathfrak{S}_{B},d_B)$ are in one of the following situations: $(\checkmark)$ Subsets of the set of invertible elements in a unital complex Banach algebra or in a unital complex Jordan--Banach algebra with the metric induced by the norm. Specially in the cases of unital C$^*$- and JB$^*$-algebras. $(\checkmark)$ The sets of positive invertible elements in unital C$^*$- or JB$^*$-algebras with respect to the metric induced by the norm and with respect to the Thompson's metric. $(\checkmark)$ Subsets of the set of unitary elements in unital C$^*$- and JB$^*$-algebras.