On the Existence of Positive Solutions of Nonlinear Elliptic Boundary value Problems Herbert Amann Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21012 10.1512/iumj.1972.21.21012 en Indiana Univ. Math. J. 21 (1972) 125 - 146 state-of-the-art mathematics http://iumj.org/access/