Perturbations of Isometries on Hilbert Space Sue-Chin Lin Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21028 10.1512/iumj.1972.21.21028 en Indiana Univ. Math. J. 21 (1972) 363 - 372 state-of-the-art mathematics http://iumj.org/access/