Partial Regularity Results for Non-Linear Elliptic Systems Charles Morrey, Jr. Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17041 10.1512/iumj.1968.17.17041 en Indiana Univ. Math. J. 17 (1968) 649 - 670 state-of-the-art mathematics http://iumj.org/access/