Journal of Functional Analysis 179, 251308 (2001) doi:10.1006Âjfan.2000.3674, available online at http:ÂÂwww.idealibrary.com on Extension Properties for the Space of Compact Operators Timur Oikhberg and Haskell P. Rosenthal Department of Mathematics, The University of Texas at Austin, Austin, Texas 78712-1082 E-mail: timurÄmath.utexas.edu, rosenthlÄmath.utexas.edu Communicated by D. Sarason Received April 26, 1999; accepted August 15, 2000 Let Z be a fixed separable operator space, X/Y general separable operator spaces, and T: X Ä Z a completely bounded map. Z is said to have the Complete Separable Extension Property (CSEP) if every such map admits a completely bounded extension to Y and the Mixed Separable Extension Property (MSEP) if View metadata, citation and similar papers at core.ac.uk brought to you by CORE every such T admits a bounded extension to Y. Finally, Z is said to have the Complete Separable Complementation Property (CSCP) if Z is locally reflexive and T admits provided by Elsevier - Publisher Connector a completely bounded extension to Y provided Y is locally reflexive and T is a com- plete surjective isomorphism. Let K denote the space of compact operators on separable Hilbert space and K0 the c0 sum of Mn 's (the space of ``small compact operators''). It is proved that K has the CSCP, using the second author's previous result that K0 has this property. A new proof is given for the result (due to E. Kirchberg) that K0 (and hence K) fails the CSEP. It remains an open question if K has the MSEP; it is proved this is equivalent to whether K0 has this property. A new Banach space concept, Extendable Local Reflexivity (ELR), is introduced to study this problem. Further complements and open problems are discussed. 2001 Academic Press Contents Introduction. 1. Extending complete isomorphisms into B(H). 2. An operator space construction on certain subspaces of M . 3. The *-mixed separable extension property and extendably locally reflexive banach spaces. 4. K0 fails the CSEP: a new proof and generalizations. INTRODUCTION The space K of compact operators on a separable infinite dimension Hilbert space H is often that thought of as the non-commutative analogue of c0 , the space of sequences vanishing at infinity. Indeed, if one regards K as matrices with respect to a fixed orthonormal basis of H, the diagonal 251 0022-1236Â01 35.00 Copyright 2001 by Academic Press All rights of reproduction in any form reserved. 252 OIKHBERG AND ROSENTHAL matrices form a subalgebra isometric to c0 . In 1941, A. Sobcyk proved that c0 has the Separable Extension Property (SEP) [S]: If Z=c0 , then given X/Y separable Banach spaces and T: X Ä Z a bounded linear operator, there exists a bounded linear operator T : X Ä Z extending T. In 1977, M. Zippin proved the (much deeper!) converse to this result [Z]; any infinite-dimensional separable Banach space Z with the SEP is isomorphic to c0 . We continue here the study of operator space analogues of the SEP, initiated in [Ro2], with the goal in particular of specifying which of these analogues K satisfies. (For basic facts about operator spaces see [Pi3]; also see the Introduction to [Ro2] for a brief summary and orientation.) Thus we consider a fixed operator space Z, and consider the following diagram: Y ? _ T X wwwÄT Z Here, X and Y are (appropriately general) separable operator spaces and T is a completely bounded linear map. Z is said to have the Complete Separable Extension Property (CSEP) if every such T admits a completely bounded linear extension T ; the Mixed Separable Extension Property (MSEP) if T admits a bounded linear exten- sion T , and the Complete Separable Complementation Property (CSCP) if T admits a bounded linear extension T provided Z is separable locally reflexive, Y is also locally reflexive, and T is a complete surjective isomorphism. If 1* is such that T can be chosen with &T &cb* &T&cb in the CSEP-case, we say Z has the *-CSEP; if &T &* &T&cb in the MSEP- case, we say Z has the *-MSEP. It follows easily that if Z has the CSEP (resp. the MSEP), then X has the *-CSEP (resp. the *-MSEP) for some *1. Of course these properties are intimately connected with injectivity notions; thus Z is called (isomorphically) injective (resp. mixed injective) if this diagram admits a completely bounded solution (resp. bounded solu- tion) T for arbitrary (not necessarily separable) operator spaces X and Y. As in the separable setting, if Z is injective (resp. mixed injective), there is a *1 so that T may always be chosen with &T &cb* &T&cb (resp. &T &* &T&cb); if * works, we say Z is *-injective (resp. *-mixed injective). We say Z is isometrically injective (resp. isometrically mixed injective) when *=1. It is a fundamental theorem in operator space theory that B(H)is isometrically injective for any Hilbert space H, where B(H) denotes the space of bounded linear operators on H. It follows easily that if X is an EXTENSION PROPERTIES 253 operator space with X/B(H) for some Hilbert space H, then X is isomorphically injective (resp. mixed injective) if and only if X is com- pletely complemented (resp. complemented) in B(H). The separable extension properties we consider have their primary inter- est for *2. Indeed, if Z is separable, then if *<2 and Z has the *-CSEP, it is proved in [Ro2] that Z is *-injective; we show analogously here that if Z has the *-MSEP, Z is *-mixed injective (and moreover Z is reflexive, whence by a result of G. Pisier, Z is actually Hilbertian (cf. [R])). One of the main results of this work is that K has the CSCP. A result of E. Kirchberg yields that K fails the CSEP [Ki1]. We give a new proof and further complements in Section 4. It is proved in [Ro2] that K0 has the CSCP, where K0 denotes the space of ``small compact operators'', namely the c0 -sum of Mn 's, where Mn denotes the space of complex n_n matrices, identified with B(Cn) for all n, Cn being the standard n-dimensional complex Hilbert space. We obtain that K has the CSCP (Theorem 2.2) via the following route: in Section 1, we show that if X/Y are given separable operator spaces, then any com- plete isomorphism from X into B(H) admits a complete isomorphic extension from Y into B(H) (Theorem 1.1). It follows from this result that if X/B(H) is fixed with X separable locally reflexive, then X has the CSCP provided X is completely complemented in Y for any separable locally reflexive operator space Y with X/Y/B(H) (see Corollary 1.8). Now it follows from the main result of Section 2 (Theorem 2.1) that if K/Y/B(H) (where this is the natural embedding of K in B(H)) with Y separable, there is an absolute constant C and for all =>0, a projection P on B(H) with &P&cb<1+= with Y and K invariant under P so that (I&P) Y/K and dcb(PY, K0)C. It then easily follows that K is completely complemented in Y provided Y is locally reflexive, from the fact that then PK has this property by the result in [Ro2]. We do not know if K has the MSEP. However Theorem 2.1 also yields that K has the MSEP if K0 has this property (Proposition 2.3). After this paper was first submitted for publication, an alternate proof of Theorem 2.2 was obtained in [AR], based on Theorem 1.1 of the present paper. Another proof (along similar lines) has also been given by N. Ozawa [O]. We also obtain in Section 1 that if an operator space Z has the CSCP, it has the following stronger property: there is a completely bounded operator T completing the above diagram whenever Y is separable locally reflexive and X is locally complemented in Y (Theorem 1.6). As shown in [Ro2], X ``automatically'' is locally complemented provided X is com- pletely isomorphic to a nuclear C*-algebra, or more generally, if X** is isomorphically injective. (X is called locally complemented in Y provided there is a C< so that X is C-completely complemented in W for all 254 OIKHBERG AND ROSENTHAL X/W/Y with WÂX finite-dimensional). It was also previously proved in [Ro2] that K0 has this stronger property, and moreover one may drop the assumption that Y is locally reflexive. The MSEP is studied in Section 3, where we introduce the following con- cept: Given operator spaces X and Y, X is called completely semi- isomorphic to Y if there is a completely bounded surjective map T: X Ä Y which is a Banach isomorphism; X is called completely semi-isometric to Y &1 in case T can be chosen with &T&cb=1=&T &. We then have the simple permanence property: mixed injectivity and the MSEP are both preserved under complete semi-isomorphisms (Proposition 3.9). The finite-dimen- sional isometrically mixed injectives are known up to Banach isometry; they are the l-direct sums of Cartan factors of type IV (see Theorem A, following Problem 3.2). This result suggests a possible classification of the isometrically injective finite-dimensional operator spaces; are all such com- pletely semi-isometric to an l-direct sum of Cartan factors of types IIV? (Problem 3.3). A remarkable factorization result of M.