On the Solvability of Nonlinear Elliptic Boundary Value Problems Peter Hess Indiana University Mathematics Journal 1976 text pdf 10.1512/iumj.1976.25.25036 10.1512/iumj.1976.25.25036 en Indiana Univ. Math. J. 25 (1976) 461 - 466 state-of-the-art mathematics http://iumj.org/access/