Singular Sets of Solutions to Elliptic Equations Qing Han Indiana University Mathematics Journal 1994 text pdf 10.1512/iumj.1994.43.43043 10.1512/iumj.1994.43.43043 en Indiana Univ. Math. J. 43 (1994) 983 - 1002 state-of-the-art mathematics http://iumj.org/access/