A Four Block Approach to Central Commutant Lifting A. Frazho Indiana University Mathematics Journal 1993 text pdf 10.1512/iumj.1993.42.42037 10.1512/iumj.1993.42.42037 en Indiana Univ. Math. J. 42 (1993) 821 - 838 state-of-the-art mathematics http://iumj.org/access/