No Arabic abstract
We study residually finite-dimensional (or RFD) operator algebras which may not be self-adjoint. An operator algebra may be RFD while simultaneously possessing completely isometric representations whose generating C*-algebra is not RFD. This has provided many hurdles in characterizing residual finite-dimensionality for operator algebras. To better understand the elusive behaviour, we explore the C*-covers of an operator algebra. First, we equate the collection of C*-covers with a complete lattice arising from the spectrum of the maximal C*-cover. This allows us to identify a largest RFD C*-cover whenever the underlying operator algebra is RFD. The largest RFD C*-cover is shown to be similar to the maximal C*-cover in several different facets and this provides supporting evidence to a previous query of whether an RFD operator algebra always possesses an RFD maximal C*-cover. In closing, we present a non self-adjoint version of Hadwins characterization of separable RFD C*-algebras.
The generalized state space $ S_{mathcal{H}}(mathcal{mathcal{A}})$ of all unital completely positive (UCP) maps on a unital $C^*$-algebra $mathcal{A}$ taking values in the algebra $mathcal{B}(mathcal{H})$ of all bounded operators on a Hilbert space $mathcal{H}$, is a $C^ast$-convex set. In this paper, we establish a connection between $C^ast$-extreme points of $S_{mathcal{H}}(mathcal{A})$ and a factorization property of certain algebras associated to the UCP map. In particular, this factorization property of some nest algebras is used to give a complete characterization of those $C^ast$-extreme maps which are direct sums of pure UCP maps. This significantly extends a result of Farenick and Zhou [Proc. Amer. Math. Soc. 126 (1998)] from finite to infinite dimensional Hilbert spaces. Also it is shown that normal $C^ast$-extreme maps on type $I$ factors are direct sums of normal pure UCP maps if and only if an associated algebra is reflexive. Further, a Krein-Milman type theorem is established for $C^ast$-convexity of the set $ S_{mathcal{H}}(mathcal{A})$ equipped with bounded weak topology, whenever $mathcal{A}$ is a separable $C^ast$-algebra or it is a type $I$ factor. As an application, we provide a new proof of a classical factorization result on operator valued Hardy algebras.
I. Raeburn and J. Taylor have constructed continuous-trace C*-algebras with a prescribed Dixmier-Douady class, which also depend on the choice of an open cover of the spectrum. We study the asymptotic behavior of these algebras with respect to certain refinements of the cover and appropriate extension of cocycles. This leads to the analysis of a limit groupoid G and a cocycle sigma, and the algebra C*(G, sigma) may be regarded as a generalized direct limit of the Raeburn-Taylor algebras. As a special case, all UHF C*-algebras arise from this limit construction.
We analyze the dichotomy amenable/paradoxical in the context of (discrete, countable, unital) semigroups and corresponding semigroup rings. We consider also F{o}lners type characterizations of amenability and give an example of a semigroup whose semigroup ring is algebraically amenable but has no F{o}lner sequence. In the context of inverse semigroups $S$ we give a characterization of invariant measures on $S$ (in the sense of Day) in terms of two notions: $domain$ $measurability$ and $localization$. Given a unital representation of $S$ in terms of partial bijections on some set $X$ we define a natural generalization of the uniform Roe algebra of a group, which we denote by $mathcal{R}_X$. We show that the following notions are then equivalent: (1) $X$ is domain measurable; (2) $X$ is not paradoxical; (3) $X$ satisfies the domain F{o}lner condition; (4) there is an algebraically amenable dense *-subalgebra of $mathcal{R}_X$; (5) $mathcal{R}_X$ has an amenable trace; (6) $mathcal{R}_X$ is not properly infinite and (7) $[0] ot=[1]$ in the $K_0$-group of $mathcal{R}_X$. We also show that any tracial state on $mathcal{R}_X$ is amenable. Moreover, taking into account the localization condition, we give several C*-algebraic characterizations of the amenability of $X$. Finally, we show that for a certain class of inverse semigroups, the quasidiagonality of $C_r^*left(Xright)$ implies the amenability of $X$. The converse implication is false.
Let $A$ be a unital operator algebra and let $alpha$ be an automorphism of $A$ that extends to a *-automorphism of its $ca$-envelope $cenv (A)$. In this paper we introduce the isometric semicrossed product $A times_{alpha}^{is} bbZ^+ $ and we show that $cenv(A times_{alpha}^{is} bbZ^+) simeq cenv (A) times_{alpha} bbZ$. In contrast, the $ca$-envelope of the familiar contractive semicrossed product $A times_{alpha} bbZ^+ $ may not equal $cenv (A) times_{alpha} bbZ$. Our main tool for calculating $ca$-envelopes for semicrossed products is the concept of a relative semicrossed product of an operator algebra, which we explore in the more general context of injective endomorphisms. As an application, we extend a recent result of Davidson and Katsoulis to tensor algebras of $ca$-correspondences. We show that if $T_{X}^{+}$ is the tensor algebra of a $ca$-correspondence $(X, fA)$ and $alpha$ a completely isometric automorphism of $T_{X}^{+}$ that fixes the diagonal elementwise, then the contractive semicrossed product satisfies $ cenv(T_{X}^{+} times_{alpha} bbZ^+)simeq O_{X} times_{alpha} bbZ$, where $O_{X}$ denotes the Cuntz-Pimsner algebra of $(X, fA)$.
We study the structure of certain classes of homologically trivial locally C*-algebras. These include algebras with projective irreducible Hermitian A-modules, biprojective algebras, and superbiprojective algebras. We prove that, if A is a locally C*-algebra, then all irreducible Hermitian A-modules are projective if and only if A is a direct topological sum of elementary C*-algebras. This is also equivalent to A being an annihilator (dual, complemented, left quasi-complemented, or topologically modular annihilator) topological algebra. We characterize all annihilator $sigma$-C*-algebras and describe the structure of biprojective locally C*-algebras. Also, we present an example of a biprojective locally C*-algebra that is not topologically isomorphic to a Cartesian product of biprojective C*-algebras. Finally, we show that every superbiprojective locally C*-algebra is topologically *-isomorphic to a Cartesian product of full matrix algebras.