The Dunford-Pettis property in Jb*-Triples

JB*-triples occur in the study of bounded symmetric domainsin several complex variables and in the study of contractiveprojections on C*-algebras. These spaces are equipped with aternary product {·,·,·}, the Jordan tripleproduct, and are essentially geometric objects in that the linearisometries between them are exactly the linear bijections preservingthe Jordan triple product (cf. [23]).     

Subdifferentiability of the norm and the Banach-Stone theorem for real and complex JB*-triples

We study the points of strong subdifferentiability for the norm of a real JB*-triple. As a consequence we describe weakly compact real JB*-triples and rediscover the Banach-Stone Theorem for complex JB*-triples.Authors Partially supported by I+D MCYT projects no. BFM2002-01529 and BFM2002-01810, and Junta de Andalucía grant FQM 0199Mathematics Subject Classification (2000): 46B04, 46L05, 46L70Revised version: 2 May 2004Acknowledgements. The authors would like to thank A. Rodr{\i}guez Palacios for fruitful comments and discussions during the preparation of this paper and to the referee for his or her interesting suggestions.     

Closed Tripotents and Weak Compactness in the Dual Space of a Jb*-Triple

We revise the concept of compact tripotent in the bidual spaceof a JB*-triple. This concept was introduced by Edwards andRüttimann generalizing the ideas developed by Akemann forcompact projections in the bidual of a C*-algebra. We also obtainsome characterizations of weak compactness in the dual spaceof a JC*-triple, showing that a bounded subset in the dual spaceof a JC*-triple is relatively weakly compact if and only ifits restriction to any abelian maximal subtriple C is relativelyweakly compact in the dual of C. This generalizes a very usefulresult by Pfitzner in the setting of C*-algebras. As a consequencewe obtain a Dieudonné theorem for JC*-triples which generalizesthe one obtained by Brooks, Saitô and Wright for C*-algebras.     

Von Neumann Regularity and Quadratic Conorms in JB^＊-triples and C^＊-algebras

We revise the notion of von Neumann regularity in JB^＊-triples by finding a new characterisation in terms of the range of the quadratic operator Q（a）. We introduce the quadratic conorm of an element a in a JB^＊-triple as the minimum reduced modulus of the mapping Q（a）. It is shown that the quadratic conorm of a coincides with the infimum of the squares of the points in the triple spectrum of a. It is established that a contractive bijection between JBW^＊-triples is a triple isomorphism if, and only if, it preserves quadratic conorms. The continuity of the quadratic conorm and the generalized inverse are discussed. Some applications to C^＊-algebras and von Neumann algebras are also studied.     

Little Grothendieck`s theorem for real JB*-triples

We prove that given a real JB*-triple E, and a real Hilbert space H, then the set of those bounded linear operators T from E to H, such that there exists a norm one functional and corresponding pre-Hilbertian semi-norm on E such that for all , is norm dense in the set of all bounded linear operators from E to H. As a tool for the above result, we show that if A is a JB-algebra and is a bounded linear operator then there exists a state such that for all . Received June 28, 1999; in final form January 28, 2000 / Published online March 12, 2001     

On real forms of JB*-triples

We introduce real JB^*-triples as real forms of (complex) JB^*-triples and give an algebraic characterization of surjective linear isometries between them. As main result we show: A bijective (not necessarily continuous) linear mapping between two real JB^*-triples is an isometry if and only if it commutes with the cube mappinga→a ³={aaa}. This generalizes a result of Dang for complex JB^*-triples. We also associate to every tripotent (i.e. fixed point of the cube mapping) and hence in particular to every extreme point of the unit ball in a real JB^*-triple numerical invariants that are respected by surjective linear isometries.     

Relatively weakly open sets in closed balls of Banach spaces,and real <Emphasis Type="Italic">JB</Emphasis><Superscript>*</Superscript>-triples of finite rank

We prove that, given a real JB^*-triple X, there exists a nonempty relatively weakly open subset of the closed unit ball of X with diameter less than 2 (if and) only if the Banach space of X is isomorphic to a Hilbert space. Moreover we give the structure of real JB^*-triples whose Banach spaces are isomorphic to Hilbert spaces. Such real JB^*-triples are also characterized in two different purely algebraic ways.Mathematics Subject Classification (2000): 46B04, 46B22, 46L05, 46L70Partially supported by Junta de Andalucía grant FQM 0199.Revised version: 30 September 2003     

Separate weak*-continuity of the triple product in dual real JB*-triples

We prove that, if E is a real JB*-triple having a predual then is the unique predual of E and the triple product on E is separately $\sigma (E,E_{*_{}})-$continuous. Received February 1, 1999; in final form March 29, 1999 / Published online May 8, 2000     

On JB*-triples defined by fibre bundles

We associate JB^*-triples to certain fibre bundles and describe their automorphisms, conjugations and real forms. In particular, we construct a JB^*-triple that does not admit a single conjugation.     

Non-commutative generalisations of Urysohn's lemma and hereditary inner ideals

We establish several generalisations of Urysohn's lemma in the setting of JB^∗-triples which provide full answers to Problems 1.12 and 1.13 in Fernández-Polo and Peralta (2007) [22]. These results extend the previous generalisations obtained by C.A. Akemann, G.K. Pedersen and L.G. Brown in the setting of C^∗-algebras. A generalised Kadison's transitivity theorem is established for finite sums of pairwise orthogonal compact tripotents in JBW^∗-triples. We introduce the notion of positively open tripotent in the bidual of a JB^∗-triple as an extension of a concept which was already considered in the setting of ternary rings of operators. We investigate the connections appearing between positively open tripotents and hereditary inner ideals.     

Special jordan H*-triple systems

This work, jointly with [9], completes the structure theory and classification of the Jordan H ^*- triple systems. The problem of describing the Jordan H ^*-triple systems is reduced in [5] to that of describing the topologically simple ones. Ruling out the finite-dimensional case, we have that any of these H ^*-triples has an underlying triple system structure of quadratic type (and these can be fully described), or it is the H ^*-triple system associated to the odd part of a topologically simple Z₂-graded Jordan H ^*-algebra, whose classification is given in [13].     

The Ubiquity of Thompson's Group F in Groups of Piecewise Linear Homeomorphisms of the Unit Interval

In the 1960s, Richard J. Thompson introduced a triple of groupsF T G which, among them, supplied the first examples of infinite,finitely presented, simple groups [14] (see [6] for publisheddetails), a technique for constructing an elementary exampleof a finitely presented group with an unsolvable word problem[12], the universal obstruction to a problem in homotopy theory[8], and the first examples of torsion free groups of type FPand not of type FP [5]. In abstract measure theory, it has beensuggested by Geoghegan (see [3] or [9, Question 13]) that Fmight be a counterexample to the conjecture that any finitelypresented group with no non-cyclic free subgroup is amenable(admits a bounded, non-trivial, finitely additive measure onall subsets that is invariant under left multiplication). Recently,F has arisen in the theory of groups of diagrams over semigrouppresentations [10], and as the object of questions in the algebraof string rewriting systems [7]. For more extensive bibliographiesand more results on Thompson's groups and their generalizationssee [1, 4, 6]. A persistent peculiarity of Thompson's groups is their abilityto pop up in diverse areas of mathematics. This suggests thatthere might be something very natural about Thompson's groups.We support this idea by showing (Theorem 1.1 below) that PL_o(I),the group of piecewise linear (finitely many changes of slope),orientation-preserving, self-homeomorphisms of the unit interval,is riddled with copies of F: a very weak criterion implies thata subgroup of PL_o(I) must contain an isomorphic copy of F.     

The fixed point property in -triples and preduals of -triples

We prove that for every member X in the class of real or complex JB^*-triples or preduals of JBW^*-triples, the following assertions are equivalent:

(1) X has the fixed point property.

(2) X has the super fixed point property.

(3) X has normal structure.

(4) X has uniform normal structure.

(5) The Banach space of X is reflexive.

As a consequence, a real or complex C^*-algebra or the predual of a real or complex W^*-algebra having the fixed point property must be finite-dimensional.

Contractivity of Projections Commuting with Inner Derivations on JBW*-triples

It is shown that if P is a weak*-continuous projection on a JBW*-triple A with predual A _*, such that the range PA of P is an atomic subtriple with finite-dimensional Cartan-factors, and P is the sum of coordinate projections with respect to a standard grid of PA, then P is contractive if and only if it commutes with all inner derivations of PA. This provides characterizations of 1-complemented elements in a large class of subspaces of A _* in terms of commutation relations.     

Hyers–Ulam stability of derivations and linear functions

In this paper the following implication is verified for certain basic algebraic curves: if the additive real function f approximately (i.e., with a bounded error) satisfies the derivation rule along the graph of the algebraic curve in consideration, then f can be represented as the sum of a derivation and a linear function. When, instead of the additivity of f, it is assumed that, in addition, the Cauchy difference of f is bounded, a stability theorem is obtained for such characterizations of derivations.     

Faithful Inner Ideals in Jbw*-Triples

The kernel Ker(J) and the annihilator J^⊥ of a weak*-closed inner ideal J in a JBW*-triple A consist of the sets of elements a in A for which {J a J} and {J a A} are zero, respectively, and J is said to be faithful if, for every non-zero ideal I in A, I ∩ Ker (J) is non-zero. It is shown that every weak*-closed inner ideal J in A has a unique orthogonal decomposition into a faithful weak*-closed inner ideal f(J) and a weak*-closed ideal f (J)^⊥ ∩ J of A. The central structure of f ( J) is investigated and used to show that J has zero annihilator if and only if it coincides with the multiplier of f (J). The results are applied to the cases in which J is the Peirce-two or Peirce-zero space A₂(v) or A₀(v) corresponding to a tripotent v in A, and to the case in which the JBW*-triple A is a von Neumann algebra.     

Weak limits of the extreme dual ball in JB*-triples

We study weak limits of the extreme points, ∂ _e (E ^* ₁), of the dual ball of a JB*-triple, E. We show that all such weak limits, except possibly the zero functional, are weak sequential limits and we discuss implications for the structure of E. Received: 9 April 2001