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Finite rank elements in semisimple Banach algebras

Matej Brešar, Peter Šemrl (1998)

Studia Mathematica

Let A be a semisimple Banach algebra. We define the rank of a nonzero element a in the socle of A to be the minimum of the number of minimal left ideals whose sum contains a. Several characterizations of rank are proved.

Finite spectra and quasinilpotent equivalence in Banach algebras

Rudi M. Brits, Heinrich Raubenheimer (2012)

Czechoslovak Mathematical Journal

This paper further investigates the implications of quasinilpotent equivalence for (pairs of) elements belonging to the socle of a semisimple Banach algebra. Specifically, not only does quasinilpotent equivalence of two socle elements imply spectral equality, but also the trace, determinant and spectral multiplicities of the elements must agree. It is hence shown that quasinilpotent equivalence is established by a weaker formula (than that of the spectral semidistance). More generally, in the second...

Functionals on Banach Algebras with Scattered Spectra

H. S. Mustafayev (2004)

Bulletin of the Polish Academy of Sciences. Mathematics

Let A be a complex, commutative Banach algebra and let M A be the structure space of A. Assume that there exists a continuous homomorphism h:L¹(G) → A with dense range, where L¹(G) is a group algebra of the locally compact abelian group G. The main results of this note can be summarized as follows: (a) If every weakly almost periodic functional on A with compact spectra is almost periodic, then the space M A is scattered (i.e., M A has no nonempty perfect subset). (b) Weakly almost periodic functionals...

Géométrie du spectre dans une algèbre de Banach et domaine numérique

Mohamed Chraibi Kaadoud (2004)

Studia Mathematica

Dans une algèbre de Banach et dans deux cas particuliers, nous montrons la continuité du centre du plus petit disque contenant le spectre. Pour a ∈ , on donne une condition nécessaire et suffisante pour avoir R K = d ( a ) où d(a) est la distance de a aux scalaires et R K le rayon du plus petit disque contenant K qui représente le spectre ou le domaine numérique algébrique de a. Dans un espace de Hilbert complexe, K peut représenter certains types de spectres ou de domaines numériques de a.

Geometry of the spectral semidistance in Banach algebras

Gareth Braatvedt, Rudi Brits (2014)

Czechoslovak Mathematical Journal

Let A be a unital Banach algebra over , and suppose that the nonzero spectral values of a and b A are discrete sets which cluster at 0 , if anywhere. We develop a plane geometric formula for the spectral semidistance of a and b which depends on the two spectra, and the orthogonality relationships between the corresponding sets of Riesz projections associated with the nonzero spectral values. Extending a result of Brits and Raubenheimer, we further show that a and b are quasinilpotent equivalent if...

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