Universal Factorization of Positive Operators Kenneth DavidsonHouben Huang Indiana University Mathematics Journal 1994 text pdf 10.1512/iumj.1994.43.43006 10.1512/iumj.1994.43.43006 en Indiana Univ. Math. J. 43 (1994) 131 - 142 state-of-the-art mathematics http://iumj.org/access/