Positive Operators on the Disk Algebra J. McDonald Indiana University Mathematics Journal 1977 text pdf 10.1512/iumj.1977.26.26085 10.1512/iumj.1977.26.26085 en Indiana Univ. Math. J. 26 (1977) 1057 - 1065 state-of-the-art mathematics http://iumj.org/access/