Interior Gradient Bounds for Non-Uniformly Elliptic Equations Leon Simon Indiana University Mathematics Journal 1976 text pdf 10.1512/iumj.1976.25.25066 10.1512/iumj.1976.25.25066 en Indiana Univ. Math. J. 25 (1976) 821 - 855 state-of-the-art mathematics http://iumj.org/access/