Quasi-Linear Elliptic Boundary Value Problems Martin Schultz Indiana University Mathematics Journal 1969 text pdf 10.1512/iumj.1969.18.18003 10.1512/iumj.1969.18.18003 en Indiana Univ. Math. J. 18 (1969) 21 - 25 state-of-the-art mathematics http://iumj.org/access/