Bell's inequality forC *-algebras

Bell's inequality dealing with local hidden variables is given two formulations in terms ofC ^*-algebras. In particular, Bell's inequality holds for all states onAB wheneverA andB are unitalC ^*-algebras at least one of which is Abelian, i.e., at least one corresponds to a classical physical system.     

Complements to various Stone-Weierstrass theorems forC *-algebras and a theorem of Shultz

J. Glimm's Stone-Weierstrass theorem states that ifA is aC ^*-algebra,P(A) is the set of pure states ofA, andB is aC ^*-subalgebra which separates , thenB=A. We show that ifB is aC ^*-subalgebra ofA andx an element ofA such that any two elements of which agree onB agree also onx, thenxB. Similar complements are given to other Stone-Weierstrass theorems. A theorem of F. Shultz states that ifxA ^**, the enveloping von Neumann algebra ofA, and ifx, x ^*, x, andxx ^* are uniformly continuous onP(A){0}, then there is an element ofA which agrees withx onP(A). We show that the hypotheses onx ^*x andxx ^* can be dropped.     

Nets ofC*-algebras and classification of states

The concept of locality in quantum physics leads to mathematical structures in which the basic object is an operator algebra with a net of distinguished subalgebras (the local subalgebras). Such nets provide a classification of the states of this algebra in equivalence classes determined by local or asymptotic properties. The corresponding equivalence relations are natural generalizations of the (more stringent) standard quasiequivalence relation (they are also useful for classifying states by their properties with respect to automorphism groups). After discussing general nets from this point of view we investigate in the last section more specialized nets (funnels of von Neumann algebras) with special emphasis on their locally normal states.The research in this paper was supported in part by the N.S.F. and the Ministère de l'Education Nationale.     

Spectra of states,and asymptotically abelianC*-algebras

For each state of aC*-algebra its spectrum is defined and shown to coincide with the spectrum of the naturally associated modular operator. For strongly clustering states of asymptotically abelianC*-algebras the spectrum is minimal among the states in the same quasi-equivalence class, hence is a *-isomorphic invariant for the weak closure of the G.N.S.-representation. Furthermore, the non-zero elements in the spectrum of strongly clustering states form a multiplicative group.     

A remark onC*-algebras

It is shown that any complex Banach algebra with hermitean involution and the weakC*-property |x|²=|x ²| for allx=x* is aC*-algebra.The research in this paper was partially supported by the U. S. Army Research Office, Durham.     

Extremal decomposition of Wightman functions and of states on nuclear *-algebras by Choquet theory

We give a short proof for the decomposability of states on nuclear *-algebras into extremal states by using the integral decompositions of Choquet and the nuclear spectral theorem, recovering a recent result by Borchers and Yngvason. The decomposition of Wightman fields into irreducible fields is a special case of this. We also indicate a quick solution of the moment problem on nuclear spaces.     

Unbounded derivations of commutativeC*-algebras

It is shown that an unbounded *-derivation of a unital commutativeC*-algebraA is quasi well-behaved if and only if there is a dense open subsetU of the spectrum ofA such that, for anyf in the domain of , (f) vanishes at any point ofU wheref attains its norm. An example is given to show that even if is closed it need not be quasi well-behaved. This answers negatively a question posed by Sakai for arbitraryC*-algebras.It is also shown that there are no-zero closed derivations onA if the spectrum ofA contains a dense open totally disconnected subset.     

C*-algebras and automorphism groups

Let (A,G, α) be aC*-dynamical system withG a topological group. Let π be a representation ofA. We will show that there exists a quasiequivalent representation \(\hat \pi \) to π which is a covariant representation, if and only if the folium of π is invariant under the action ofG and this action is strongly continuous.     

Unbounded derivations ofC*-algebras II

It is demonstrated that a closed symmetric derivation δ of aC?-algebra \(\mathfrak{A}\) generates a strongly continuous one-parameter group of automorphisms of aC?-algebra \(\mathfrak{A}\) if and only if, it satisfies one of the following three conditions

1. (αδ+1)(D(δ))= \(\mathfrak{A}\) , α∈?\{0}.
2. δ possesses a dense set of analytic elements.
3. δ possesses a dense set of geometric elements.

Together with one of the following two conditions

1. ∥(αδ+1)(A)∥≧∥A∥, α∈IR,A∈D(δ).
2. If α∈IR andA∈D(δ) then (αδ+1)(A)≧0 impliesA≧0.

Other characterizations are given in terms of invariant states and the invariance ofD(δ) under the square root operation of positive elements.     

C*-algebras and Mackey's axioms

A non-commutative version of probability theory is outlined, based on the concept of a*-algebra of operators (sequentially weakly closedC*-algebra of operators). Using the theory of*-algebras, we relate theC*-algebra approach to quantum mechanics as developed byKadison with the probabilistic approach to quantum mechanics as axiomatized byMackey. The*-algebra approach to quantum mechanics includes the case of classical statistical mechanics; this important case is excluded by theW*-algebra approach. By considering the*-algebra, rather than the von Neumann algebra, generated by the givenC*-algebraA in its reduced atomic representation, we show that a difficulty encountered byGuenin concerning the domain of a state can be resolved.     

Unbounded derivations of C*-algebras

We study unbounded derivations ofC*-algebras and characterize those which generate one-parameter groups of automorphisms. We also develop a functional calculus for the domains of closed derivations and develop criteria for closeability. Some specialC*-algebras are considered \(\mathfrak{B}\mathbb{C}(\mathfrak{H}),\mathfrak{B}(\mathfrak{H})\) , UHF algebras, and in this last context we prove the existence of non-closeable derivations.     

On the Borel structure ofC*-algebras

We provide a method of embedding aC*-algebra in aC*-algebra called its -envelope. is contained in the enveloping algebra of but is generally much smaller, and if is commutative with identity then can be identified with the algebra of bounded Baire functions on the spectrum of. The main result is to completely determine the structure of for all separable G. C. R. algebras. This provides a good basis for a non-commutative theory of probability.We should like to thankJ. T. Lewis, G. W. Mackey andR. J. Plymen, who have given us considerable encouragement and insight into the quantum mechanical relevance of the ideas developed here.     

Amenability of crossed products ofC*-algebras

It is shown that the class of amenable (resp. strongly amenable)C*-algebras is closed under the process of taking crossed products with discrete amenable groups. Under certain circumstances, amenability is also preserved under taking a crossed product with an amenable semigroup of linear endomorphisms. These facts are used to show that certain simpleC*-algebras studied by J. Cuntz are amenable but not strongly amenable (thus answering a question of B. E. Johnson), yet are stably isomorphic to strongly amenable algebras.Partially supported by NSF     

Large groups of automorphisms of C*-algebras

Groups of *-automorphisms ofC*-algebras and their invariant states are studied. We assume the groups satisfy a certain largeness condition and then obtain results which contain many of those known for asymptotically abelianC*-algebras and for inner automorphisms and traces ofC*-algebras. Our key result is the construction in certain finite cases, where the automorphisms are spatial, of an invariant linear map of theC*-algebra onto the fixed point algebra carrying with it most of the relevant information.     

Topological entropy for endomorphisms of localC *-algebras

A notion of topological entropy for endomorphisms of localC ^*-algebras is introduced as a generalisation of the topological entropy of classical dynamical systems. The basic properties are derived and a series of calculations are presented.     

Dirichlet forms and Markov semigroups onC*-algebras

We extend the classical theory of Dirichlet forms and associated Markov semigroups to the case of aC*-algebra with a trace. Semigroups of completely positive maps are characterized by completely positive Dirichlet forms.Work supported by The Norwegian Research Council for Science and the Humanities