731 research outputs found
INVARIANT SUBSPACES AND HYPER-REFLEXIVITY FOR FREE SEMIGROUP ALGEBRAS
In this paper, we obtain a complete description of the invariant subspace structure of an interesting new class of algebras which we call free semigroup algebras. This enables us to prove that they are reflexive, and moreover to obtain a quantitative measure of the distance to these algebras in terms of the invariant subspaces. Such algebras are called hyper-reflexive. This property is very strong, but it has been established in only a very few cases. Moreover the prototypes of this class of algebras are the natural candidate for a non-commutative analytic Toeplitz algebra on n variables. The case we make for this analogy is very compelling. In particular, in this paper, the key to the invariant subspace analysis is a good analogue of the Beurling theorem for invariant subspaces of the unilateral shift. This leads to a notion of inner-outer factorization in these algebras. In a sequel to this paper [13], we add to this evidence by showing that there is a natural homomorphism of the automorphism group onto the group of conformal automorphisms of the ball in Cn
Noncommutative Choquet theory
We introduce a new and extensive theory of noncommutative convexity along
with a corresponding theory of noncommutative functions. We establish
noncommutative analogues of the fundamental results from classical convexity
theory, and apply these ideas to develop a noncommutative Choquet theory that
generalizes much of classical Choquet theory.
The central objects of interest in noncommutative convexity are
noncommutative convex sets. The category of compact noncommutative sets is dual
to the category of operator systems, and there is a robust notion of extreme
point for a noncommutative convex set that is dual to Arveson's notion of
boundary representation for an operator system.
We identify the C*-algebra of continuous noncommutative functions on a
compact noncommutative convex set as the maximal C*-algebra of the operator
system of continuous noncommutative affine functions on the set. In the
noncommutative setting, unital completely positive maps on this C*-algebra play
the role of representing measures in the classical setting.
The continuous convex noncommutative functions determine an order on the set
of unital completely positive maps that is analogous to the classical Choquet
order on probability measures. We characterize this order in terms of the
extensions and dilations of the maps, providing a powerful new perspective on
the structure of completely positive maps on operator systems.
Finally, we establish a noncommutative generalization of the
Choquet-Bishop-de Leeuw theorem asserting that every point in a compact
noncommutative convex set has a representing map that is supported on the
extreme boundary. In the separable case, we obtain a corresponding integral
representation theorem.Comment: 81 pages; minor change
Commutant Lifting for Commuting Row Contractions
If is a row contraction with commuting entries,
and the Arveson dilation is ,
then any operator commuting with each dilates to an operator of
the same norm which commutes with each .Comment: one section and references were adde
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