Circular Operators W. ArvesonDonald HadwinT. HooverE. Kymala Indiana University Mathematics Journal 1984 text pdf 10.1512/iumj.1984.33.33031 10.1512/iumj.1984.33.33031 en Indiana Univ. Math. J. 33 (1984) 583 - 595 state-of-the-art mathematics http://iumj.org/access/