Elliptic Regularization for Symmetric Positive Systems Leonard Sarason Indiana University Mathematics Journal 1967 text pdf 10.1512/iumj.1967.16.16053 10.1512/iumj.1967.16.16053 en Indiana Univ. Math. J. 16 (1967) 807 - 827 state-of-the-art mathematics http://iumj.org/access/