Absolutely continuous representations and a Kaplansky density theorem for free semigroup algebras

We introduce notions of absolutely continuous functionals and representations on the non-commutative disk algebra A_n. Absolutely continuous functionals are used to help identify the type L part of the free semigroup algebra associated to a *-extendible representation σ. A *-extendible representation of A_n is regular if the absolutely continuous part coincides with the type L part. All known examples are regular. Absolutely continuous functionals are intimately related to maps which intertwine a given *-extendible representation with the left regular representation. A simple application of these ideas extends reflexivity and hyper-reflexivity results. Moreover the use of absolute continuity is a crucial device for establishing a density theorem which states that the unit ball of σ(A_n) is weak-* dense in the unit ball of the associated free semigroup algebra if and only if σ is regular. We provide some explicit constructions related to the density theorem for specific representations. A notion of singular functionals is also defined, and every functional decomposes in a canonical way into the sum of its absolutely continuous and singular parts.     

Ghost ideals in uniform Roe algebras of coarse spaces

Let be the uniform Roe algebra of a coarse space X with uniformly locally finite coarse structure. We show that an operator G in is a ghost element if and only if the finite propagation operators in the principal ideal G are all compact operators. In contrast, if X is a discrete metric space with Yus property (A), then any ideal in is the closure of the finite propagation operators in the ideal.Received: 9 June 2004     

Remarks on ghost projections and ideals in the Roe algebras of expander sequences

We use repeating sequences of expander graphs or small perturbations of expanders to present examples of ideals in the Roe algebras of bounded geometry discrete metric spaces which cannot be expressed as the sum of a ghost ideal and an ideal in which finite propagation operators are dense. This gives a negative answer to a question in [1, 3]. Received: 9 December 2006, Revised: 18 April 2007     

Stable ranks of split extensions of Banach algebras

In this paper, we estimate the stable ranks of a Banach algebra in terms of the stable ranks of its quotient algebra and ideal under the assumption that the quotient map splits. As an application, several results about the Bass and the connected stable ranks of nest algebras are obtained.     

Duality and operator algebras II: operator algebras as Banach algebras

We answer, by counterexample, several questions concerning algebras of operators on a Hilbert space. The answers add further weight to the thesis that, for many purposes, such algebras ought to be studied in the framework of operator spaces, as opposed to that of Banach spaces and Banach algebras. In particular, the ‘nonselfadjoint analogue’ of a w*-algebra resides naturally in the category of dual operator spaces, as opposed to dual Banach spaces. We also show that an automatic w*-continuity result in the preceding paper of the authors, is sharp.     

Topological and bornological characterisations of ideals in von Neumann algebras: I

Suppose is a von Neumann algebra on a Hilbert space and is any ideal in . We determine a topology on , for which the members of that are to norm continuous are exactly those in ; and a bornology on such that the elements of which map the unit ball to an element of , equivalently those members of that are norm to bounded, are exactly those in . This is achieved via analogues of the notions of injectivity and surjectivity in the theory of operator ideals on Banach spaces.     

Weakly closed Jordan ideals in nest algebras on Banach spaces

We show that weakly closed Jordan ideals in nest algebras on Banach spaces are associative ideals. The decomposability of finite-rank operators in Jordan ideals and the commutants of bimodules are also investigated. Author’s address: J. Li and F. Lu, Department of Mathematics, Suzhou University, Suzhou 215006, People’s Republic of China This research was supported by NNSFC (No. 10771154) and PNSFJ (NO. BK2007049).     

Jordan derivations of triangular algebras

In this note, it is shown that every Jordan derivation of triangular algebras is a derivation.     

All-derivable points of operator algebras

Let A be an operator subalgebra in B(H), where H is a Hilbert space. We say that an element Z∈A is an all-derivable point of A for the norm-topology (strongly operator topology, etc.) if, every norm-topology (strongly operator topology, etc.) continuous derivable linear mapping φ at Z (i.e. φ(ST)=φ(S)T+Sφ(T) for any S,T∈A with ST=Z) is a derivation. In this paper, we show that every invertible operator in the nest algebra is an all-derivable point of the nest algebra for the strongly operator topology. We also prove that every nonzero element of the algebra of all 2×2 upper triangular matrixes is an all-derivable point of the algebra.     

Invariant subspaces of operator Lie algebras and Lie algebras with compact adjoint action

It is proved that a Lie algebra of compact operators with a non-zero Volterra ideal is reducible (has a nontrivial invariant subspace). A number of other criteria of reducibility for collections of operators is obtained. The results are applied to the structure theory of Lie algebras of compact operators and normed Lie algebras with compact adjoint action.     

On an Algebra of Toeplitz Operators With Piecewise Continuous Symbols

Let and be a finite collection of smooth curves in D. Given k points consider the family of all bounded and continuous functions on with finite limits at and radial limits at z_k. We study the Toeplitz operator algebra corresponding to M_r and we prove that its Calkin algebra is isomorphic to the algebra of all continuous functions on some compact set. This fact implies that the commutator of two Toeplitz operators with this kind of symbols is compact. We also prove that the semi-commutator of such Toeplitz operators is not compact, in general.     

Jordan higher derivations on triangular algebras

In this paper, we show that any Jordan higher derivation on a triangular algebra is a higher derivation. This extends the main result in [13] to the case of higher derivations.     

Local derivations on certain CSL algebras

In this paper, it is shown that every norm continuous linear local derivation from an arbitrary CSL algebra whose lattice is generated by finitely many independent nests into any ultraweakly closed subalgebra which contains the algebra is an inner derivation, and that every norm continuous linear local derivation from an arbitrary CSL algebra whose lattice is completely distributive into any ultraweakly closed subalgebra which contains the algebra is a derivation.     

Reflexive ideals in Iwasawa algebras

Let G be a torsionfree compact p-adic analytic group. We give sufficient conditions on p and G which ensure that the Iwasawa algebra ΩG of G has no non-trivial two-sided reflexive ideals. Consequently, these conditions imply that every non-zero normal element in ΩG is a unit. We show that these conditions hold in the case when G is an open subgroup of SL₂(Zp) and p is arbitrary. Using a previous result of the first author, we show that there are only two prime ideals in ΩG when G is a congruence subgroup of SL₂(Zp): the zero ideal and the unique maximal ideal. These statements partially answer some questions asked by the first author and Brown.     

On the structure of inner ideals of nest algebras

IfA is a nest algebra andA _s=A ∩ A* , whereA* is the set of the adjoints of the operators lying inA, then the pair (A, A _s) forms a partial Jordan *-triple. Important tools when investigating the structure of a partial Jordan *-triple are its tripotents. In particular, given an orthogonal family of tripotents of the partial Jordan *-triple (A, A _s), the nest algebraA splits into a direct sum of subspaces known as the Peirce decomposition relative to that family. In this paper, the Peirce decomposition relative to an orthogonal family of minimal tripotents is used to investigate the structure of the inner ideals of (A, A _s), whereA is a nest algebra associated with an atomic nest. A property enjoyed by inner ideals of the partial Jordan *-triple (A, A _s) is presented as the main theorem. This result is then applied in the final part of the paper to provide examples of inner ideals. A characterization of the minimal tripotents as a certain class of rank one operators is also obtained as a means to deduce the principal theorem.     

Generalized biderivations of nest algebras

Let N be a nest on a complex separable Hilbert space H, and τ(N) be the associated nest algebra. In this paper, we prove that every biderivation of τ(N) is an inner biderivation if and only if dim 0₊ ≠ 1 or , and that every generalized biderivation of τ(N) is an inner generalized biderivation if dim 0₊ ≠ 1 and .     

Tensor products on quiver representations

The aim of this article is to translate the well-known tensor product of representations of a group given by diagonal action to the case of representations of a quiver. We provide three different approaches and exhibit their close relationship to the point-wise tensor product, which is considered in [M. Herschend, Solution to the Clebsch-Gordan problem for Kronecker representations. U.U.D.M Project Report 2003:P1, Uppsala University, 2003; M. Herschend, Solution to the Clebsch-Gordan problem for representations of quivers of type , J. Algebra Appl. 4 (5) (2005) 481-488; M. Herschend, On the Clebsch-Gordan problem for quiver representations. U.U.D.M Report 2005:43, Uppsala University, 2005].