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- W1505196222 abstract "There are a number of spectra studied in literature which do not fit into the axiomatic theory of Żelazko. This paper is an attempt to give an axiomatic theory for these spectra which, apart from the usual types of spectra, like one-sided, approximate point or essential spectra, include also the local spectra, the Browder spectrum and various versions of the Apostol spectrum (studied under various names, e.g. regular, semi-regular or essentially semi-regular). I. Basic properties of regularities The axiomatic theory of spectrum was introduced by W. Żelazko [21], see also S lodkowski and Żelazko [17]. He gave a classification of various types of spectra defined for commuting n-tuples of elements of a Banach algebra. The most important notion is that of subspectrum. All algebras in this paper are complex and unital. Denote by Inv(A) the set of all invertible elements in a Banach algebra A and by σ(a) = {λ ∈ C, a− λ / ∈ Inv(A)} the ordinary spectrum of an element a ∈ A. The spectral radius of a ∈ A will be denoted by r(a). Definition 1.1. Let A be a Banach algebra. A subspectrum σ in A is a mapping which assigns to every n-tuple (a1, . . . , an) of mutually commuting elements of A a non-empty compact subset σ(a1, . . . , an) ⊂ C such that (1) σ(a1, . . . , an) ⊂ σ(a1)× · · · × σ(an), (2) σ(p(a1, . . . , an)) = p(σ(a1, . . . , an)) for every commuting a1, . . . , an ∈ A and every polynomial mapping p = (p1, . . . , pm) : C → C. This notion has proved to be quite useful since it includes for example the left (right) spectrum, the left (right) approximate point spectrum, the Harte (= the union of the left and right) spectrum, the Taylor spectrum and various essential spectra. However, there are also many examples of spectrum, usually defined only for single elements of A, which are not covered by the axiomatic theory of Żelazko. The aim of this paper is to give an axiomatic description of such spectra. Instead of describing a spectrum, it is possible to describe equivalently the set of regular elements. Definition 1.2. Let A be a Banach algebra. A non-empty subset R of A is called a regularity if (1) if a ∈ A and n ∈ N then a ∈ A ⇔ a ∈ A, (2) if a, b, c, d are mutually commuting elements of A and ac + bd = 1A, then ab ∈ R ⇔ a ∈ R and b ∈ R." @default.
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- W1505196222 date "1996-01-01" @default.
- W1505196222 modified "2023-10-14" @default.
- W1505196222 title "On the axiomatic theory of spectrum II" @default.
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- W1505196222 doi "https://doi.org/10.4064/sm-119-2-129-147" @default.
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