The von Neumann Converse of Weyl's Theorem W. Sikonia Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21011 10.1512/iumj.1972.21.21011 en Indiana Univ. Math. J. 21 (1972) 121 - 124 state-of-the-art mathematics http://iumj.org/access/