COMPLETION OF OPERATOR PARTIAL MATRICES TO PROJECTIONS

Let H and K be complex Hilbert spaces. For given operators A, B, and C on H,K and from K into H, respectively, necessary and sufficient cond itions are obtained for partial matrices ((C)(?) (C)(B)), ((C)*(?) (? )(C)), and ((A)(?) (?)(B)) to have projection completions. All such co mpletions are also characterized.