Fredholm operators on $C^*$-algebras

TL;DR

This paper generalizes the concept of Fredholm operators to arbitrary $C^*$-algebras by defining finite type elements and establishing an index theorem, unifying various classical and modern Fredholm theories.

Contribution

It introduces a unified axiomatic framework for Fredholm operators on $C^*$-algebras, encompassing classical, von Neumann, and Hilbert $C^*$-module cases.

Findings

Established an index theorem for generalized Fredholm elements.

Unified different notions of Fredholm operators under a common framework.

Showed that classical and modern Fredholm operators are special cases.

Abstract

The aim of this note is to generalize the notion of Fredholm operator to an arbitrary $C^*$-algebra. Namely, we define "finite type" elements in an axiomatic way, and also we define Fredholm type element $a$ as such element of a given $C^*$-algebra for which there are finite type elements $p$ and $q$ such that $(1-q)a(1-p)$ is "invertible". We derive index theorem for such operators. In applications we show that classical Fredholm operators on a Hilbert space, Fredholm operators in the sense of Breuer, Atiyah and Singer on a properly infinite von Neumann algebra, and Fredholm operators on Hilbert $C^*$-modules over an unital $C^*$-algebra in the sense of Mishchenko and Fomenko are special cases of our theory.