On the universal convergence sets

Summary In this paper we study various universal convergence sets, in connection with some problem arising from the Korovkin approximation theory. This analysis is made in the context of topological vector lattices, of commutative Banach algebras, of Banach algebras with an involution and of C ^*-algebras. Some applications and examples are given in specific function spaces and function algebras.Work performed out under the auspices of the G.N.A.F.A. (C.N.R.) and of Ministero della Pubblica Istruzione (60%) for the year 1982.     

Metrically and topologically projective ideals of Banach algebras

In the present paper, necessary conditions for the metric and topological projectivity of closed ideals of Banach algebras are given. In the case of commutative Banach algebras, a criterion for the metric and topological projectivity of ideals admitting a bounded approximate identity is obtained. The main result of the paper is as follows: a closed ideal of an arbitrary C*-algebra is metrically or topologically projective if and only if it admits a self-adjoint right identity.     

Algebraic properties of isometries between groups of invertible elements in Banach algebras

We prove that an isometry T between open subgroups of the invertible groups of unital Banach algebras A and B is extended to a real-linear isometry up to translation between these Banach algebras. While a unital isometry between unital semisimple commutative Banach algebras need not be multiplicative, we prove in this paper that if A is commutative and A or B are semisimple, then (T(eA))⁻¹T is extended to an isometric real algebra isomorphism from A onto B. In particular, A−1 is isometric as a metric space to B−1 if and only if they are isometrically isomorphic to each other as metrizable groups if and only if A is isometrically isomorphic to B as a real Banach algebra; it is compared by the example of ?elazko concerning on non-isomorphic Banach algebras with the homeomorphically isomorphic invertible groups. Isometries between open subgroups of the invertible groups of unital closed standard operator algebras on Banach spaces are investigated and their general forms are given.     

On existence of finite universal Korovkin sets in Segal algebras

We prove that there exists a finite universal Korovkin set w.r.t positive operators for the centre of a Segal algebra on a compact groupG if and only ifG is metrizable. As a consequence it follows that a Segal algebra on a compact abelian group admits a finite universal Korovkin set w.r.t. positive operators iff the group is metrizable.     

Weak spectral synthesis in commutative Banach algebras. II

Let A be a semisimple and regular commutative Banach algebra with structure space Δ(A). Continuing our investigation in [E. Kaniuth, Weak spectral synthesis in commutative Banach algebras, J. Funct. Anal. 254 (2008) 987-1002], we establish various results on intersections and unions of weak spectral sets and weak Ditkin sets in Δ(A). As an important example, the algebra of n-times continuously differentiable functions is studied in detail. In addition, we prove a theorem on spectral synthesis for projective tensor products of commutative Banach algebras which applies to Fourier algebras of locally compact groups.     

Universal equivalence of some countably generated partially commutative structures

We study universal theories of partially commutative Lie algebras, partially commutative metabelian Lie algebras, and partially commutative metabelian groups such that their defining graphs are trees with countably many vertices. Also we find universal equivalence criteria for each of these classes of Lie algebras and groups.     

Quadratic double centralizers and quadratic multipliers

In this paper, we establish stability of quadratic double centralizers and quadratic multipliers on Banach algebras. We also prove the super stability of quadratic double centralizers on Banach algebras which are weakly commutative and weakly without order, and of quadratic multipliers on Banach algebras which are weakly without order.     

Approximate regularity of commutative Beurling algebras and Korovkin approximation

We study generalizations of the classical Korovkin theorem to the context of commutative Beurling algebras associated to a weight on a locally compact abelian groupG. On the way, we prove that a commutative Beurling algebra is approximately regular iff has rate of growth identically one, and we calculate diverse stable ranks of commutative Beurling algebras in terms of the dimension of the dual group.     

Subalgebras of finite codimension

We consider a commutative algebra A over a field K whose algebraic closure is a finite extension of K, and describe the subalgebras of A that are of finite codimension. We solve a similar problem for the closed subalgebras of Banach algebras.Translated from Matematicheskie Zametki,Vol. 6, No. 3, pp. 321–328, September, 1969.     

SOME SPECTRAL PROPERTIES OF MULTIPLIERS ON SEMI-PRIME BANACH ALGEBRAS

Abstract

We extend to arbitrary semi-prime Banach algebras some results of spectral theory and Fredholm theory obtained in [1] and [2] for multipliers defined in commutative semi-simple Banach algebras.     

Endomorphism monoids in varieties of commutative semigroups

A concrete category is almost universal if its class of non-constant morphisms contains an isomorphic copy of every category of algebras as a full subcategory. This paper characterizes almost universal varieties of commutative semigroups. As a consequence we obtain that for every infinite cardinal κ there exists a commutative semigroup of cardinality κ such that it has exactly two endomorphisms, the identity endomorphism and a single constant endomorphism.     

Uniformities and Complemented Group Algebra Ideals

We continue an investigation of complementation in closed idealsof commutative group algebras. The main result here is a sufficientcondition for a closed ideal of a commutative group algebrato have a Banach space complement. This involves the comparisonof uniformities on the hull of the ideal.     

P-commutativity of the Banach algebra L1**(G)

Let G be a compact abelian group. It is known that the second conjugate space L₁**(G) of the group algebra L₁(G) is a noneommutative, nonsemisimple, Banach algebra. It is not known if L₁**(G) admits an involution. If it does, we show that it is P-coimnutative, and hence enjoys many of the properties of commutative Banach algebras.     

On Universal Equivalence of Partially Commutative Metabelian Lie Algebras

In this paper, we consider partially commutative metabelian Lie algebras whose defining graphs are cycles. We show that such algebras are universally equivalent iff the corresponding cycles have the same length. Moreover, we give an example showing that the class of partially commutative metabelian Lie algebras such that their defining graphs are trees is not separable by universal theory in the class of all partially commutative metabelian Lie algebras.     

Remarks on Segal algebras

Let B be an abstract Segal algebra in some Banach algebra A. There was some belief that in the commutative case A should be semi-simple, if B is, but this is not so (Section I). It is well known that a (proper) abstract Segal algebra does not have bounded right approximate units. It may however have a left unit. Pseudosymmetric Segal algebras in the sense of Reiter do not have bounded left approximate units (Section II). A nonfactorization proof is given for a class of algebras which contains most of the known examples of Segal algebras on abelian groups (Section III).     

具有连续对合运算的实Banach*代数的Jordan结构

本文讨论了实Banach*代数的Jordan结构．主要结果:第一部分指出映射到 *-半单实Banach*代数上的Jordan*同态是连续的,且其核空间是闭*理想;由映射到交换实Banach*代数上的Jordan*同态诱导的因子代数也是交换的．第二部分介绍了两个不同的锥,并讨论了他们间的关系．另外,我们得到了关于实Banach*代数*- 根基的一个新的刻画．本文是Satish Shirali的工作的实化．     

SVEP for multipliers on a faithful commutative Banach algebra

We give some sufficient conditions that each multiplier on a faithful commutative Banach algebra has SVEP. On the other hand, we show that there exist a faithful commutative Banach algebra and a multiplier on it without SVEP. Such examples of multipliers can actually be found within the class of multiplication operators on unital commutative Banach algebras. This answers in negative a question that is stated as Open problem 6.2.1 by Laursen and Neumann, 2000.

     

On universal Korovkin sets w.r.t. positive spectral contractions

We prove that there exists a finite universal Korovkin set with respect to positive spectral contraction operators for a Segal algebra on a locally compact abelian groupG if and only ifĜ is finite dimensional separable metrizable space.