Some Existence Theorems for Quasi-Linear, Non-Uniformly Elliptic Equations in Divergence Form Neil Trudinger Indiana University Mathematics Journal 1969 text pdf 10.1512/iumj.1969.18.18066 10.1512/iumj.1969.18.18066 en Indiana Univ. Math. J. 18 (1969) 909 - 919 state-of-the-art mathematics http://iumj.org/access/