Arens Regularity for Quotients Ap(E) of the Herz Algebra

If the subset T supports a synthesizable p-pseudofunction T,then the restriction A_p(E) to E of the Herz algebra on T isnot Arens regular. The proof is direct, by brute force, anddoes not show the existence of Day points for A_p(E). 2000 MathematicsSubject Classification 43A15, 43A10 (primary), 46L10 (secondary).     

Arens regularity of projective tensor products

For completely contractive Banach algebras A and B (respectively operator algebras A and B), the necessary and sufficient conditions for the operator space projective tensor product \({A\widehat{\otimes}B}\) (respectively the Haagerup tensor product \({A\otimes^{h}B}\)) to be Arens regular are obtained. Using the non-commutative Grothendieck inequality, we show that, for C*-algebras A and B, \({A\otimes^{\gamma} B}\) is Arens regular if \({A\widehat{\otimes}B}\) and \({A\widehat{\otimes}B^{op}}\) are Arens regular whereas \({A\widehat{\otimes}B}\) is Arens regular if and only if \({A\otimes^{h}B}\) and \({B\otimes^{h}A}\) are, where \({\otimes^h}\), \({\otimes^{\gamma}}\), and \({\widehat{\otimes}}\) are the Haagerup, the Banach space projective tensor norm, and the operator space projective tensor norm, respectively.     

Products of Commuting Boolean Algebras of Projections and Banach Space Geometry

New criteria and Banach spaces are presented (for example, GL-spacesand Banach spaces with property () that ensure that the Booleanalgebra generated by a pair of bounded, commuting Boolean algebrasof projections is itself bounded. The notion of R-boundednessplays a fundamental role. It is shown that the strong operatorclosure of any R-bounded Boolean algebra of projections is necessarilyBade complete. Also, for a Dedekind -complete Banach latticeE, the Boolean algebra consisting of all band projections inE is R-bounded if and only if E has finite cotype. In this situation,every bounded Boolean algebra of projections in E is R-boundedand has a Bade complete strong closure. 2000 Mathematics SubjectClassification 46B20, 47L10 (primary), 46B42, 47B40, 47B60 (secondary).     

Module Arens regularity for semigroup algebras

We extend the concept of Arens regularity of a Banach algebra to the case that there is an -module structure on , and show that when S is an inverse semigroup with totally ordered subsemigroup E of idempotents, then A=ℓ ¹(S) is module Arens regular if and only if an appropriate group homomorphic image of S is finite. When S is a discrete group, this is just Young’s theorem which asserts that ℓ ¹(S) is Arens regular if and only if S is finite. An erratum to this article can be found at     

The Second Transpose of a Derivation

Let A be a Banach algebra, and let D: A A* be a continuousderivation, where A* is the topological dual space of A. Thepaper discusses the situation when the second transpose D**:A** (A**)* is also a derivation in the case where A** has thefirst Arens product.     

Generating Countable Sets of Permutations

Let E be an infinite set. In answer to a question of Wagon,I show that every countable subset of the symmetric group Sym(E)is contained in a 2-generator subgroup of Sym(E). In answerto a question of Macpherson and Neumann, I show that, if Sym(E)is generated by A B where |B| ||E||, then Sym(E) is generatedby A {} for some permutation in Sym(E).     

Structural Projections on JBW*-Triples

A linear projection R on a Jordan*-triple A is said to be structuralprovided that, for all elements a, b and c in A, the equality{Rab Rc} = R{a Rbc} holds. A subtriple B of A is said to becomplemented if A = B + Ker(B), where Ker(B) = {aA: {B a B}= 0}. It is shown that a subtriple of a JBW*-triple is complementedif and only if it is the range of a structural projection. A weak* closed subspace B of the dual E* of a Banach space Eis said to be an N*-ideal if every weak* continuous linear functionalon B has a norm preserving extension to a weak* continuous linearfunctional on E* and the set of elements in E which attain theirnorm on the unit ball in B is a subspace of E. It is shown thata subtriple of a JBW*-triple A is complemented if and only ifit is an N*-ideal, from which it follows that complemented subtriplesof A are weak* closed, and structural projections on A are weak*continuous and norm non-increasing. It is also shown that everyN*-ideal in A possesses a triple product with respect to whichit is a JBW*-triple which is isomorphic to a complemented subtripleof A.     

Rees Algebras of Modules

We study Rees algebras of modules within a fairly general framework.We introduce an approach through the notion of Bourbaki idealsthat allows the use of deformation theory. One can talk aboutthe (essentially unique) generic Bourbaki ideal I(E) of a moduleE which, in many situations, allows one to reduce the natureof the Rees algebra of E to that of its Bourbaki ideal I(E).Properties such as Cohen–Macaulayness, normality and beingof linear type are viewed from this perspective. The known numericalinvariants, such as the analytic spread, the reduction numberand the analytic deviation, of an ideal and its associated algebrasare considered in the case of modules. Corresponding notionsof complete intersection, almost complete intersection and equimultiplemodules are examined in some detail. Special consideration isgiven to certain modules which are fairly ubiquitous becauseinteresting vector bundles appear in this way. For these modulesone is able to estimate the reduction number and other invariantsin terms of the Buchsbaum–Rim multiplicity. 2000 MathematicsSubject Classification 13A30 (primary), 13H10, 13B21 (secondary)     

Geometrical Spines of Lens Manifolds

We introduce the concept of ‘geometrical spine’for 3-manifolds with natural metrics, in particular, for lensmanifolds. We show that any spine of L_p,q that is close enoughto its geometrical spine contains at least E(p,q) – 3vertices, which is exactly the conjectured value for the complexityc(L_p,q). As a byproduct, we find the minimal rotation distance(in the Sleator–Tarjan–Thurston sense) between atriangulation of a regular p-gon and its image under rotation.     

Arens regularity ofK(X)

LetX be a reflexive Banach space andK(X) the operator algebra of compact linear operatorsu:XX. In this note we prove the following two results: a) Any decomposable bilinear operator fromK(X)×K(X) intoK(X) is Arens regular; b) ifX has an unconditional basis then any bounded bilinear operator fromK(X)×K(X) intoK(X) is Arens regular.     

Volumes of Projections of unit Cubes

Let Z and be the images of a regular unit n-cube in Eⁿ under orthogonal projection on toorthogonal complementary subspaces of dimensions d and n —d respectively. It is shown that the d-volume of Z and the (n— d)-volume of are equal. This generalizes to a connexion between the volumes of stronglyassociated zonotopes.     

The Direct Limits Of The Banach-mazur Compacta

Let 1 p . For each n-dimensional Banach space E = (E, || ·||), we define a norm || · ||_p on E x R as follows: [formula] It is shown that the correspondence (E, || · ||) (Ex R, || · ||_p) defines a topological embedding of oneBanach–Mazur compactum, BM(n), into another, BM(n 1),and hence we obtain a tower of Banach–Mazur compacta:BM(1) BM(2) BM(3) ···. Let BM_p be thedirect limit of this tower. We prove that BM_p is homeomorphicto Q = dir lim Qⁿ, where Q = [0, 1] is the Hilbert cube. 1991Mathematics Subject Classification 46B04, 46B20, 52A21, 57N20,54H15.     

Purely Infinite Cuntz-Krieger Algebras of Directed Graphs

For arbitrary infinite directed graphs E, the characterisationof the (not necessarily simple) Cuntz–Krieger algebrasC*(E) which are purely infinite in the sense of Kirchberg–Rørdamis given. It is also shown that C*(E) has real rank zero ifand only if the graph E satisfies Condition (K). 2000 MathematicsSubject Classification 46L05.     

Arens semi-regular Banach algebras

For some important Banach algebras, the first and the second Arens product on their biduals are different, i. e. these algebras are not (Arens) regular. Arens semi-regularity is a property strictly weaker than regularity; it characterizes those non-regular algebras (having a bounded two-sided approximate identity) for which the Arens products, though different, still behave in a reasonable way. The definition of semi-regularity is based on the relation of two natural embeddings of the space of double multipliers into the bidual of the Banach algebra. It is shown that each commutative Banach algebra is semi-regular and that semi-regularity is equivalent to the equality of the Arens products on certain subspaces of the bidual. Among others, group algebras and algebras of compact operators are treated as examples.     

POWERS OF OPERATORS DOMINATED BY STRICTLY SINGULAR OPERATORS

It is proved that every positive operator R on a Banach latticeE dominated by a strictly singular operator T:E E satisfiesthat the R⁴ is strictly singular. Moreover, if E is order continuousthen the R² is already strictly singular.     

Embeddings of Sz(32) in E8(5)

We show that the Suzuki group Sz(32) is a subgroup of E₈(5),and so is its automorphism group. Both are unique up to conjugacyin E₈(F) for any field F of characteristic 5, and the automorphismgroup Sz(32):5 is maximal in E₈(5). 1991 Mathematics SubjectClassification 20E28.     

On the bidual of quasinormable Fréchet algebras

It is known that the bidual of a quasinormable Fréchet space E with local Banach spaces $(E_n)_{n\in {\mathbb N}}$ is topologically isomorphic to the inverse limit of $\big (E_n^{\prime \prime }\big )_{n\in {\mathbb N}}$. With the aid of the Arens product and by homological means, we prove that the previous result is equally valid for quasinormable Fréchet m‐convex algebras. This allows showing that the bidual of a σ‐C*‐algebra equipped with the Arens product is a σ‐C*‐algebra and presenting a new direct proof of a result on acyclic spectra due to Palamodov.     

Measures of pseudorandomness for finite sequences: typical values

Mauduit and Sárközy introduced and studied certainnumerical parameters associated to finite binary sequences E_N {–1, 1}^N in order to measure their ‘level of randomness’.Those parameters, the normality measure (E_N), the well-distributionmeasure W(E_N), and the correlation measure C_k(E_N) of order k,focus on different combinatorial aspects of E_N. In their work,amongst others, Mauduit and Sárközy (i) investigatedthe relationship among those parameters and their minimal possiblevalue, (ii) estimated (E_N), W(E_N) and C_k(E_N) for certain explicitlyconstructed sequences E_N suggested to have a ‘pseudorandomnature’, and (iii) investigated the value of those parametersfor genuinely random sequences E_N. In this paper, we continue the work in the direction of (iii)above and determine the order of magnitude of (E_N), W(E_N) andC_k(E_N) for typical E_N. We prove that, for most E_N {–1,1}^N, both W(E_N) and (E_N) are of order N, while C_k(E_N) is oforder for any given 2 k N/4.     

Multiplicative Error Analysis of Matrix Transformation Algorithms

The author's recently introduced relative error measure forvectors is applied to the error analysis of algorithms whichproceed by successive transformation of a matrix. Instead ofmodelling the roundoff errors at each stage by A: = T(A)+E onemodels them by A: =e^E T(A) where E is a small linear transformation.This can simplify analyses considerably. Applications to theparallel Jacobi method for eigenvalues, and to Gaussian elimination,are given.     

Characterization of the Mod 3 Cohomology of the Compact, Connected, Simple, Exceptional Lie Groups of Rank 6

It is shown that the mod 3 cohomology of a 1-connected, homotopyassociative mod 3 H-space that is rationally equivalent to theLie group E₆ is isomorphic to that of E₆ as an algebra. Moreover,it is shown that the mod 3 cohomology of a nilpotent, homotopy-associativemod 3 H-space that is rationally equivalent to E₆, and whosefundamental group localized at 3 is non-trivial, is isomorphicto that of the Lie group Ad E₆ as a Hopf algebra over the mod3 Steenrod algebra. It is also shown that the mod 3 cohomologyof the universal cover of such an H-space is isomorphic to thatof E₆ as a Hopf algebra over the mod 3 Steenrod algebra. 2000Mathematics Subject Classification 57T05, 57T10, 57T25.