Approximately invertible elements in non-unital normed algebras

TL;DR

This paper introduces a new concept of approximate invertibility in non-unital normed algebras, extending classical invertibility notions and exploring its implications in various algebraic structures and their representations.

Contribution

It defines approximate invertibility in non-unital algebras, relating it to topological divisors of zero and ideals, and examines its properties across different algebra classes.

Findings

Approximate invertibility coincides with non-vanishing in Wiener algebras with approximate identities.

It connects approximate invertibility with Gelfand theory and representation theory in non-unital Banach and C*-algebras.

Provides examples in group algebras, Wiener algebras, and operator ideals.

Abstract

We introduce a concept of approximately invertible elements in non-unital normed algebras which is, on one side, a natural generalization of invertibility when having approximate identities at hand, and, on the other side, it is a direct extension of topological invertibility to non-unital algebras. Basic observations relate approximate invertibility with concepts of topological divisors of zero and density of (modular) ideals. We exemplify approximate invertibility in the group algebra, Wiener algebras, and operator ideals. For Wiener algebras with approximate identities (in particular, for the Fourier image of the convolution algebra), the approximate invertibility of an algebra element is equivalent to the property that it does not vanish. We also study approximate invertibility and its deeper connection with the Gelfand and representation theory in non-unital abelian Banach algebras as well as abelian and non-abelian C*-algebras.