On automorphisms of Banach algebras

We prove an extension of the Jacobson density theorem for Banach algebras with automorphisms, and discuss its applications. In particular, automorphisms a \alpha of Banach algebras such that the map a- 1 \alpha - 1 is spectrally bounded are characterized.     

On character amenability of Banach algebras

We continue our work [E. Kaniuth, A.T. Lau, J. Pym, On φ-amenability of Banach algebras, Math. Proc. Cambridge Philos. Soc. 144 (2008) 85-96] in the study of amenability of a Banach algebra A defined with respect to a character φ of A. Various necessary and sufficient conditions of a global and a pointwise nature are found for a Banach algebra to possess a φ-mean of norm 1. We also completely determine the size of the set of φ-means for a separable weakly sequentially complete Banach algebra A with no φ-mean in A itself. A number of illustrative examples are discussed.     

On spectral and central states on Banach algebras

Acta Mathematica Hungarica -     

On the simplicial cohomology of Banach operator algebras

We investigate the simplicial cohomology of certain Banach operator algebras. The two main examples considered are the Banach algebra of all bounded operators on a Banach space and its ideal of approximable operators. Sufficient conditions will be given forcing Banach algebras of this kind to be simplicially trivial.     

On the symbolic calculus in homogeneous Banach algebras

We construct a strongly homogeneous Banach algebraB so that for an appropriate positive integern, (1)A(T) ⊊B ⊊C(T); (2)n-times continuously differentiable functions operate onB. This work was supported in part by NSF Grant MCS75-08552-A02.     

On identifying the maximal ideals in Banach algebras

We consider the problem of identifying the maximal ideals of a Banach algebra S that is contained in a larger Banach algebra B. If S is dense in B in an appropriate sense and if the spectral radii in S and B are the same, then S and B have the same maximal ideals. The result is illustrated by two examples of Banach algebras S in which the density and spectral radius conditions are easily shown to be valid with respect to a larger algebra B whose maximal ideals are known. The first example is a convolution algebra on a group where the functions in the algebra have specified rate of decay. The second example is a generalized version of an algebra introduced by I. Hirschman. Two applications of the generalized Hirschman algebra are presented: these relate to filtering and prediction of stationary random sequences and concern the asymptotic behavior of the errors incurred by the finite memory predictor and by the finite lag filter.     

On the semi-continuity of generalized inverses in Banach algebras

The main purpose of this paper is to study the continuity of several kinds of generalized inverses of elements in a Banach algebra with identity. We first obtain a sufficient and necessary condition for the lower semi-continuity of reflexive generalized inverses as set-valued mappings. Based on this result, we characterize the continuity of the Moore-Penrose inverse in a C^∗-algebra and therefore, derive some new and well-known criteria in operator theory.     

Cones in Banach algebras

We develop spectral theory for elements in an ordered Banach algebra.     

On commutativity and the numerical range in Banach algebras

If x and y are noncommuting elements in a complex Banach algebra with unit then the generalized numerical ranges W₀(e^λvxe^−λv) cover the complex plane.     

Commutatively ordered Banach algebras

Abstract

Spectral theory in ordered Banach algebras (OBAs) has been investigated and several authors have made contributions. However, the results are not applicable to non-commutative C^*-algebras, since a non-commutative C^*-algebra is not an OBA. In this paper we introduce a more general structure, called a commutatively ordered Banach algebra (COBA), which includes the class of OBAs. Every C^* - algebra is a COBA. We will give the basic properties of COBAs and show how known results in OBAs can be generalized to the COBA setting. We will then discuss two spectral problems regarding COBA elements. The results obtained, of course, hold true in an OBA as well. These results extend the theory of COBAs and OBAs.     

Arens semi-regular Banach algebras

For some important Banach algebras, the first and the second Arens product on their biduals are different, i. e. these algebras are not (Arens) regular. Arens semi-regularity is a property strictly weaker than regularity; it characterizes those non-regular algebras (having a bounded two-sided approximate identity) for which the Arens products, though different, still behave in a reasonable way. The definition of semi-regularity is based on the relation of two natural embeddings of the space of double multipliers into the bidual of the Banach algebra. It is shown that each commutative Banach algebra is semi-regular and that semi-regularity is equivalent to the equality of the Arens products on certain subspaces of the bidual. Among others, group algebras and algebras of compact operators are treated as examples.     

On normed Jordan algebras which are Banach dual spaces

Alfsen, Shultz, and Størmer have defined a class of normed Jordan algebras called JB-algebras, which are closely related to Jordan algebras of self-adjoint operators. We show that the enveloping algebra of a JB-algebra can be identified with its bidual. This is used to show that a JB-algebra is a dual space iff it is monotone complete and admits a separating set of normal states; in this case the predual is unique and consists of all normal linear functionals. Such JB-algebras (“JBW-algebras”) admit a unique decomposition into special and purely exceptional summands. The special part is isomorphic to a weakly closed Jordan algebra of self-adjoint operators. The purely exceptional part is isomorphic to C(X, M₃⁸) (the continuous functions from X into M₃⁸).