Singular Symmetric Positive First Order Differential Operators R. PhillipsLeonard Sarason Indiana University Mathematics Journal 1966 text pdf 10.1512/iumj.1966.15.15018 10.1512/iumj.1966.15.15018 en Indiana Univ. Math. J. 15 (1966) 235 - 271 state-of-the-art mathematics http://iumj.org/access/