Subnormality of the Cesaro Operator and a Semigroup of Composition Operators C. Cowen Indiana University Mathematics Journal 1984 text pdf 10.1512/iumj.1984.33.33017 10.1512/iumj.1984.33.33017 en Indiana Univ. Math. J. 33 (1984) 305 - 318 state-of-the-art mathematics http://iumj.org/access/