The Boundary Gradient Estimate for Quasilinear Elliptic and Parabolic Differential Equations Neil Trudinger Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21051 10.1512/iumj.1972.21.21051 en Indiana Univ. Math. J. 21 (1972) 657 - 670 state-of-the-art mathematics http://iumj.org/access/