Polynomial Approximation and Spectral Properties of Composition Operators on H^2 James Caughran Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21006 10.1512/iumj.1972.21.21006 en Indiana Univ. Math. J. 21 (1972) 81 - 84 state-of-the-art mathematics http://iumj.org/access/