Weak and Strong Solutions in Two Dimensions Leonard Sarason Indiana University Mathematics Journal 1984 text pdf 10.1512/iumj.1984.33.33008 10.1512/iumj.1984.33.33008 en Indiana Univ. Math. J. 33 (1984) 159 - 178 state-of-the-art mathematics http://iumj.org/access/