Commutators with finite spectrum II

The purpose of this paper is to study derivations d, d′ defined on a Banach algebra A such that the spectrum a（[dx, d′x]） is finite for all x ∈ A. In particular we show that if the algebra is semisimple, then there exists an element a in the socle of A such that [d, d′] is the inner derivation implemented bv a.     

An abstract Nyquist criterion containing old and new results

We prove an abstract Nyquist criterion in a general set up. As applications, we recover various versions of the Nyquist criterion, some of which are new.     

The first divisible sum

We consider the distribution of the first sum of a sequence of positive integer valued iid random variables which is divisible byd. This is known to converge, when divided byd, to a geometric distribution asd. We obtain results on the rate of convergence using two contrasting approaches. In the first, Stein's method is adapted to geometric limit distributions. The second method is based on the theory of Banach algebras. Each method is shown to have its merits.     

Divisibility in the K0‐Group of the Algebra of Operators on a Banach Space

Let Figiel's reflexive Banach space which is not isomorphic to its Cartesian square. We show that the K ₀group of the algebra of continuous, linear operators on contain a subgroup isomorphic to the group c ₀₀( ) of sequences rational numbers with z _n=0 eventually.     

A Periodicity Theorem in K-Theory for Finite Covariant Systems

Let A be a complex Banach algebra. In this paper, we prove a periodicity theorem that K(S ² A, G, )K(A, G,), where (A,G,) is a finite covariant system and K(A,G,) is an Abelian group associated with (A,G,).     

Noninvertibility preservers on Banach algebras

It is proved that a linear surjection Ф: A → B, which preserves noninvertibility between semisimple, unital, complex Banach algebras, is automatically injective.     

Homotopical stable ranks for Banach algebras

The connected stable rank and the general stable rank are homotopy invariants for Banach algebras, whereas the Bass stable rank and the topological stable rank should be thought of as dimensional invariants. This paper studies the two homotopical stable ranks, viz. their general properties as well as specific examples and computations. The picture that emerges is that of a strong affinity between the homotopical stable ranks, and a marked contrast with the dimensional ones.     

Polynomially Riesz perturbations

In this paper we investigate perturbation of left (right) Fredholm, Weyl and Browder operators by polynomially Riesz operators. We show how Baklouti’s idea of “communication” enhances the perturbation properties of polynomially Riesz operators.