Duality and operator algebras: automatic weak* continuity and applications

We investigate some subtle and interesting phenomena in the duality theory of operator spaces and operator algebras, and give several applications of the surprising fact that certain maps are always weak∗-continuous on dual spaces. In particular, if X is a subspace of a C∗-algebra A, and if a ∈ A satisfies aX ⊂ X, then we show that the function x → ax on X is automatically weak∗ continuous if either (a) X is a dual operator space, or (b) a∗X ⊂ X and X is a dual Banach space. These results hinge on a generalization to Banach modules of Tomiyama’s famous theorem on contractive projections onto a C∗-subalgebra. Applications include a new characterization of the -weakly closed (possibly nonunital and nonselfadjoint) operator algebras, and a generalization of the theory of W∗-modules to the framework of modules over such algebras. We also give a Banach module characterization of -weakly closed spaces of operators which are invariant under the action of a von Neumann algebra. © 2004 Elsevier Inc. All rights reserved.