Commutativity via Spectral Equivalences of the Jordan Product in Banach Algebras

Abstract

Spectral characterizations of algebraic structure have a long history in the theory of Banach algebras. It is known that weak spectral information may force strong algebraic consequences, such as commutativity throughout the algebra or centrality of a single element. In this paper we study when spectral invariance, applied to the Jordan product $x\circ y=(xy+yx)/2$, forces commutativity or centrality in a semisimple Banach algebra $A$. We first investigate permutations of three elements, proving that equality of the spectral radii $\rho (xyz)=\rho (xzy)$ for all $x,y,z\in A$ implies $A$ is commutative, thereby complementing earlier cardinality and diameter results of Braatvedt et al. We next consider Jordan products and show that if the spectrum (or spectral radius) cannot distinguish the Jordan product from the ordinary product then $A$ must be commutative. By using representation-theoretic methods, we also obtain local spectral characterizations of central elements, showing that boundedness or omission properties of the spectrum (or spectral argument) of the Jordan product $x\circ (a{x}^{-1})$, as $x$ runs through the exponential group of $A$, imply $a$ belongs to the center of $A$. These results show that coarse spectral data associated with the Jordan product can determine commutativity and centrality.

There is an error in the proof of Theorem 2.2 whereby the assumption/hypothesis to the theorem is not enough to proceed in the first part of the proof. Specifically, assuming the spectral radius of xyz is the same as that of xzy does not produce the required invertibility necessary in line 6 of the proof