Quasidiagonality, Similarity and Approximation by Nilpotent Operators Domingo Herrero Indiana University Mathematics Journal 1981 text pdf 10.1512/iumj.1981.30.30017 10.1512/iumj.1981.30.30017 en Indiana Univ. Math. J. 30 (1981) 199 - 233 state-of-the-art mathematics http://iumj.org/access/