Some Character Generating Functions on Banach Algebras

Abstract

We consider a multiplicative variation on the classical Kowalski-S\l{}odkowski Theorem which identifies the characters among the collection of all functionals on a Banach algebra $A$. In particular we show that, if $A$ is a $C^*$-algebra, and if $\phi:A\mapsto\mathbb C$ is a continuous function satisfying $\phi(\mathbf 1)=1$ and $\phi(x)\phi(y) \in \sigma(xy)$ for all $x,y\in A$ (where $\sigma$ denotes the spectrum), then $\phi$ generates a corresponding character $\psi_\phi$ on $A$ which coincides with $\phi$ on the principal component of the invertible group of $A$. We also show that, if $A$ is any Banach algebra whose elements have totally disconnected spectra, then, under the aforementioned conditions, $\phi$ is always a character.