Matrix Compact Sets and Operator Approximation Properties

Abstract

The relationship between the operator approximation property and the strong operator approximation property has deep significance in the theory of operator algebras. The original definitions of Effros and Ruan, unlike the classical analogues, make no mention of compact operators or compact sets. In this paper we introduce ``compact matrix sets'' which correspond to the two different operator approximation properties, and show that a space has the operator approximation property if and only if the ``operator compact'' operators are contained in the closure of the finite rank operators. We also investigate when the two types of compactness agree, and introduce a natural condition which guarantees that they do.