18Th Workshop: Noncommutative Probability, Operator Algebras, Random Matrices and Related Topics, with Applications
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Noncommutative Uniform Algebras
Noncommutative Uniform Algebras Mati Abel and Krzysztof Jarosz Abstract. We show that a real Banach algebra A such that a2 = a 2 , for a A is a subalgebra of the algebra C (X) of continuous quaternion valued functions on a compact setk kX. ∈ H ° ° ° ° 1. Introduction A well known Hirschfeld-Zelazkoú Theorem [5](seealso[7]) states that for a complex Banach algebra A the condition (1.1) a2 = a 2 , for a A k k ∈ implies that (i) A is commutative, and further° ° implies that (ii) A must be of a very speciÞc form, namely it ° ° must be isometrically isomorphic with a uniformly closed subalgebra of CC (X). We denote here by CC (X)the complex Banach algebra of all continuous functions on a compact set X. The obvious question about the validity of the same conclusion for real Banach algebras is immediately dismissed with an obvious counterexample: the non commutative algebra H of quaternions. However it turns out that the second part (ii) above is essentially also true in the real case. We show that the condition (1.1) implies, for a real Banach algebra A,thatA is isometrically isomorphic with a subalgebra of CH (X) - the algebra of continuous quaternion valued functions on acompactsetX. That result is in fact a consequence of a theorem by Aupetit and Zemanek [1]. We will also present several related results and corollaries valid for real and complex Banach and topological algebras. To simplify the notation we assume that the algebras under consideration have units, however, like in the case of the Hirschfeld-Zelazkoú Theorem, the analogous results can be stated for none unital algebras as one can formally add aunittosuchanalgebra. -
Topological Divisors of Zero and Shilov Boundary
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Topology and its Applications 153 (2006) 1152–1163 www.elsevier.com/locate/topol Topological divisors of zero and Shilov boundary Alain Escassut Laboratoire de Mathématiques UMR 6620, Université Blaise Pascal, Clermont-Ferrand, Les Cézeaux, 63177 Aubiere cedex, France Received 17 February 2005; accepted 1 March 2005 Abstract Let L be a field complete for a non-trivial ultrametric absolute value and let (A, ·) be a com- mutative normed L-algebra with unity whose spectral semi-norm is ·si.LetMult(A, ·) be the set of continuous multiplicative semi-norms of A,letS be the Shilov boundary for (A, ·si) and let ψ ∈ Mult(A, ·si).Thenψ belongs to S if and only if for every neighborhood U of ψ in Mult(A, ·), there exists θ ∈ U and g ∈ A satisfying gsi = θ(g) and γ(g)<gsi ∀γ ∈ S \ U. Suppose A is uniform, let f ∈ A and let Z(f ) ={φ ∈ Mult(A, ·) | φ(f)= 0}.Thenf is a topo- logical divisor of zero if and only if there exists ψ ∈ S such that ψ(f) = 0. Suppose now A is complete. If f is not a divisor of zero, then it is a topological divisor of zero if and only if the ideal fAis not closed in A. Suppose A is ultrametric, complete and Noetherian. All topological divisors of zero are divisors of zero. This applies to affinoid algebras. Let A be a Krasner algebra H(D)without non-trivial idempotents: an element f ∈ H(D)is a topological divisor of zero if and only if fH(D) is not a closed ideal; moreover, H(D) is a principal ideal ring if and only if it has no topological divisors of zero but 0 (this new condition adds to the well-known set of equivalent conditions found in 1969). -
Regularity Conditions for Banach Function Algebras
Regularity conditions for Banach function algebras Dr J. F. Feinstein University of Nottingham June 2009 1 1 Useful sources A very useful text for the material in this mini-course is the book Banach Algebras and Automatic Continuity by H. Garth Dales, London Mathematical Society Monographs, New Series, Volume 24, The Clarendon Press, Oxford, 2000. In particular, many of the examples and conditions discussed here may be found in Chapter 4 of that book. We shall refer to this book throughout as the book of Dales. Most of my e-prints are available from www.maths.nott.ac.uk/personal/jff/Papers Several of my research and teaching presentations are available from www.maths.nott.ac.uk/personal/jff/Beamer 2 2 Introduction to normed algebras and Banach algebras 2.1 Some problems to think about Those who have seen much of this introductory material before may wish to think about some of the following problems. We shall return to these problems at suitable points in this course. Problem 2.1.1 (Easy using standard theory!) It is standard that the set of all rational functions (quotients of polynomials) with complex coefficients is a field: this is a special case of the “field of fractions" of an integral domain. Question: Is there an algebra norm on this field (regarded as an algebra over C)? 3 Problem 2.1.2 (Very hard!) Does there exist a pair of sequences (λn), (an) of non-zero complex numbers such that (i) no two of the an are equal, P1 (ii) n=1 jλnj < 1, (iii) janj < 2 for all n 2 N, and yet, (iv) for all z 2 C, 1 X λn exp (anz) = 0? n=1 Gap to fill in 4 Problem 2.1.3 Denote by C[0; 1] the \trivial" uniform algebra of all continuous, complex-valued functions on [0; 1]. -
Book of Abstracts for the 3Rd Conference of Settat on Operator Algebras and Applications, November 1–5, 2011
Book of Abstracts for the 3rd Conference of Settat on Operator Algebras and Applications, November 1{5, 2011 1. Speaker: Ahmed Al-Rawashdeh (United Arab Emirates University, Al Ain) Title: On Certain Projections of C*-Matrix Algebras ∗ Abstract: H. Dye defined the projections Pi;j(a) of a C -matrix algebra by ∗ −1 ∗ −1 Pi;j(a) = (1 + aa ) ⊗ Ei;i + (1 + aa ) a ⊗ Ei;j ∗ ∗ −1 ∗ ∗ −1 + a (1 + aa ) ⊗ Ej;i + a (1 + aa ) a ⊗ Ej;j; and he used it to show that in the case of factors not of type I2n, the unitary group determines the algebraic type of that factor. We study these projections and we show that in M2(C), the set of such projections includes all the projections. For infinite C∗-algebra A, having a system of matrix units, including the Cuntz algebra On, we have A ' Mn(A). M. Leen proved that in a simple, ∗ purely infinite C -algebra A, the ∗-symmetries generate U0(A). We revise and modify Leen's proof to show that part of such ∗-isometry factors are of the form 1−2Pi;j(!);! 2 U(A). In simple, unital ∗ purely infinite C -algebras having trivial K1-group, we prove that all Pi;j(!) have trivial K0-class. In particular, if u 2 U(On), then u can be factorized as a product of ∗-symmetries, where eight of them are of the form 1 − 2Pi;j(!). 2. Speaker: Pere Ara (Universitat Aut`onomade Barcelona, Spain) Title:(m; n)-dynamical systems and their C*-algebras Abstract: Given positive integers n and m, we consider dynamical systems in which n copies of a topological space is homeomorphic to m copies of that same space. -
Geometry of the Shilov Boundary of a Bounded Symmetric Domain Jean-Louis Clerc
Geometry of the Shilov Boundary of a Bounded Symmetric Domain Jean-Louis Clerc To cite this version: Jean-Louis Clerc. Geometry of the Shilov Boundary of a Bounded Symmetric Domain. journal of geometry and symmetry in physics, 2009, Vol. 13, pp. 25-74. hal-00381665 HAL Id: hal-00381665 https://hal.archives-ouvertes.fr/hal-00381665 Submitted on 6 May 2009 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Geometry of the Shilov Boundary of a Bounded Symmetric Domain Jean-Louis Clerc today Abstract In the first part, the theory of bounded symmetric domains is pre- sented along two main approaches : as special cases of Riemannian symmetric spaces of the noncompact type on one hand, as unit balls in positive Hermitian Jordan triple systems on the other hand. In the second part, an invariant for triples in the Shilov boundary of such a domain is constructed. It generalizes an invariant constructed by E. Cartan for the unit sphere in C2 and also the triple Maslov index on the Lagrangian manifold. 1 Introduction The present paper is an outgrowth of the cycle of conferences delivred by the author at the Tenth International Conference on Geometry, Integrability and Quantization, held in Varna in June 2008. -
Extension of Characters and Generalized Shilov Boundaries
Revista de la 211 Union Matem~tica Argentina . Volumen 32, 1986. EXTENSION OF CHARACTERS AND GENERALIZED SHILOV BOUNDARIES Gustavo Corach and Alejandra M~estripieri ABSTRACT. We prove in a very simple way Shilov extension theorem (if A is a commutative Banach algebra, h a character in the Shilov boundary of A and B a commutative superalgebra of A then h admits an extension to B which can be chosen in the Shilov boundary of B) and use this proof t~ extend similar results to higher dimensional Shilov boundaries. 1. I NTRODUCT ION. Let A be a (complex commutative) Banach algebra with unity and X(A) the spectrum space of characters of A. The Shilov boundary ro(A) is a compact subset of X(A) which plays an important role in various problems, in particular in the search of existence of I-dimensional analytic structure on X(A). For higher dimensions ro(A) seems to be too small and Basener [2] and S;ibony [11} have defined a bigger subset of X(A), rn(A), which is an appropiate generalized Shilov boundary (see als0 [1,5,6,7,11] for more information on rnCA)). In this paper we are concerned w;ith the following property of r 0 (A), ca.lled Shilov extension theo'l'em: if A is a closed subalgebra of a Banach algebra B then every· chal'acter in r 0 (A) admits an extension to some character k of B. Moreover k can be chosen in ro(B), by a result of; Rickart [9,(3.3.25)]. -
Uniform Algebras As Banach Spaces
The Erwin Schrodinger International Boltzmanngasse ESI Institute for Mathematical Physics A Wien Austria Uniform Algebras as Banach Spaces TW Gamelin SV Kislyakov Vienna Preprint ESI June Supp orted by Federal Ministry of Science and Transp ort Austria Available via httpwwwesiacat UNIFORM ALGEBRAS AS BANACH SPACES TW Gamelin SV Kislyakov June Abstract Any Banach space can b e realized as a direct summand of a uniform algebra and one do es not exp ect an arbitrary uniform algebra to have an abundance of prop erties not common to all Banach spaces One general result concerning arbitrary uniform algebras is that no prop er uniform algebra is linearly homeomorphic to a C K space Nevertheless many sp ecic uniform algebras arising in complex analysis share or are susp ected to share certain Banach space prop erties of C K We discuss the family of tight algebras which includes algebras of analytic functions on strictly pseudo con vex domains and algebras asso ciated with rational approximation theory in the plane Tight algebras are in some sense close to C K spaces and along with C K spaces they have the Pelczynski and the DunfordPettis prop erties We also fo cus on certain prop erties of C K spaces that are inherited by the disk algebra This includes a dis p cussion of interp olation b etween H spaces and Bourgains extension of Grothendiecks theorem to the disk algebra We conclude with a brief description of linear deformations of uniform algebras and a brief survey of the known classication results Supp orted in part by the Russian -
Class Notes, Functional Analysis 7212
Class notes, Functional Analysis 7212 Ovidiu Costin Contents 1 Banach Algebras 2 1.1 The exponential map.....................................5 1.2 The index group of B = C(X) ...............................6 1.2.1 p1(X) .........................................7 1.3 Multiplicative functionals..................................7 1.3.1 Multiplicative functionals on C(X) .........................8 1.4 Spectrum of an element relative to a Banach algebra.................. 10 1.5 Examples............................................ 19 1.5.1 Trigonometric polynomials............................. 19 1.6 The Shilov boundary theorem................................ 21 1.7 Further examples....................................... 21 1.7.1 The convolution algebra `1(Z) ........................... 21 1.7.2 The return of Real Analysis: the case of L¥ ................... 23 2 Bounded operators on Hilbert spaces 24 2.1 Adjoints............................................ 24 2.2 Example: a space of “diagonal” operators......................... 30 2.3 The shift operator on `2(Z) ................................. 32 2.3.1 Example: the shift operators on H = `2(N) ................... 38 3 W∗-algebras and measurable functional calculus 41 3.1 The strong and weak topologies of operators....................... 42 4 Spectral theorems 46 4.1 Integration of normal operators............................... 51 4.2 Spectral projections...................................... 51 5 Bounded and unbounded operators 54 5.1 Operations.......................................... -
1. Introduction
FACTORIZATION IN COMMUTATIVE BANACH ALGEBRAS H. G. DALES, J. F. FEINSTEIN, AND H. L. PHAM Abstract. Let A be a (non-unital) commutative Banach algebra. We consider when A has a variety of factorization properties: we list the (ob- vious) implications between these properties, and then consider whether any of these implications can be reversed in various classes of commu- tative Banach algebras. We summarize the known counter-examples to these possible reverse implications, and add further counter-examples. Some results are used to show the existence of a large family of prime ideals in each non-zero, commutative, radical Banach algebra with a dense set of products. 1. Introduction Let A be a (non-unital) commutative Banach algebra. We wish to examine when A factors in a variety of senses. Our main results are counter-examples to a number of questions that have been raised. Indeed, we shall list seven such factorization properties, called (I)(VII), and note that each of these immediately implies the next one. We shall also, in x4, discuss two other `lo- cal' factorization properties, called (A) and (B) (where (A) ) (B)); these properties are relevant for Esterle's classication of commutative, radical Banach algebras that is given in [14]. We shall then discuss whether or not any of these implications can be reversed when we restrict attention to par- ticular classes of commutative Banach algebras. We shall show that several cannot be reversed, but we leave open other possible reverse implications. A summary in x6 describes our knowledge at the present time. We shall concentrate on two particular classes of commutative Banach algebras A: rst, on the case where A is semi-simple (so that A is a Banach function algebra), and in particular when A is a maximal ideal in a uniform algebra on a compact space, and, second, on the other extreme case where A 2010 Mathematics Subject Classication. -
Noncommutative Functional Analysis for Undergraduates"
N∞M∞T Lecture Course Canisius College Noncommutative functional analysis David P. Blecher October 22, 2004 2 Chapter 1 Preliminaries: Matrices = operators 1.1 Introduction Functional analysis is one of the big fields in mathematics. It was developed throughout the 20th century, and has several major strands. Some of the biggest are: “Normed vector spaces” “Operator theory” “Operator algebras” We’ll talk about these in more detail later, but let me give a micro-summary. Normed (vector) spaces were developed most notably by the mathematician Banach, who not very subtly called them (B)-spaces. They form a very general framework and tools to attack a wide range of problems: in fact all a normed (vector) space is, is a vector space X on which is defined a measure of the ‘length’ of each ‘vector’ (element of X). They have a huge theory. Operator theory and operator algebras grew partly out of the beginnings of the subject of quantum mechanics. In operator theory, you prove important things about ‘linear functions’ (also known as operators) T : X → X, where X is a normed space (indeed usually a Hilbert space (defined below). Such operators can be thought of as matrices, as we will explain soon. Operator algebras are certain collections of operators, and they can loosely be thought of as ‘noncommutative number fields’. They fall beautifully within the trend in mathematics towards the ‘noncommutative’, linked to discovery in quantum physics that we live in a ‘noncommutative world’. You can study a lot of ‘noncommutative mathematics’ in terms of operator algebras. The three topics above are functional analysis. -
Order Structure, Multipliers, and Gelfand Representation of Vector-Valued Function Algebras
ORDER STRUCTURE, MULTIPLIERS, AND GELFAND REPRESENTATION OF VECTOR-VALUED FUNCTION ALGEBRAS JORMA ARHIPPAINEN, JUKKA KAUPPI∗ AND JUSSI MATTAS Abstract. We continue the study begun by the third author of C∗-Segal algebra valued function algebras, with an emphasis on the order structure. Our main result is a characterization theorem for C∗-Segal algebra valued function algebras with an order unitization. As an intermediate step, we establish a function algebraic description of the multiplier module of arbitrary Segal algebra valued function algebras. We also consider Gelfand representation of these algebras in the commutative case. Introduction The concept of a Segal algebra originated in the work of Reiter on subalgebras of the L1-algebra of a locally compact group; cf. [19]. It was generalized to arbitrary Banach algebras by Burnham in [8]. A C∗-Segal algebra is a Banach algebra which is a dense (not necessarily self-adjoint) ideal in a C∗-algebra. Despite many important examples in analysis, such as the Schatten classes for instance, little has been known about the general structure and properties of C∗-Segal algebras. In particular, most results have relied on the additional assumption of an approximate identity. The multiplier algebra and the bidual of self-adjoint C∗-Segal algebras were described in [2, 14], and the form of the closed ideals of C∗-Segal algebras with an approximate identity was given in [6]. Recently, a new approach to Segal algebras was introduced in [5, 12], which allows a more detailed study of their structure and properties, particularly in the absence of an approximate identity ([5] the commutative case and [12] the noncommutative case). -
Function Representation of a Noncommutative Uniform Algebra
Function representation of a noncommutative uniform algebra Krzysztof Jarosz Abstract. We construct a Gelfand type representation of a real noncommu- tative Banach algebra A satisfying f 2 = f 2, for all f A: k k 2 1. Introduction A uniform algebra A is a Banach algebra such that (1.1) f 2 = f 2 , for all f A: k k 2 The commutative, complex uniform algebras constitute the most classical class of Banach algebras, one which has been intensely studied for decades. It is well known that any such algebra is isometrically isomorphic with a subalgebra of CC (X) - the algebra of all continuous complex valued functions de…ned on a compact set X and equipped with the sup-norm topology. Hirschfeld and Zelazko· [6] proved that the assumption that the algebra is commutative is super‡uous - commutativity al- ready follows from (1.1). Algebras of analytic functions serve as standard examples of complex uniform algebras. Commutative, real uniform algebras have also been studied for years [9]. Any such algebra A is isometrically isomorphic with a real subalgebra of CC (X) for some compact set X; furthermore X can be often divided into three parts X1;X2; and X3 such that A X1 is a complex uniform algebra, A X2 consists of complex conjugates j j of the functions from A X1 , and A X3 is equal to CR (X3). Any commutative, real uniform algebra can bej naturally identi…edj with a real subalgebra of df C (X; ) = f C (X): f ( (x)) = f (x) for x X C 2 C 2 n 2 o where : X X is a surjective homeomorphism with = idX .