Existence of Multiple Solutions for Nonlinear Elliptic Boundary Value Problems Herbert Amann Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21074 10.1512/iumj.1972.21.21074 en Indiana Univ. Math. J. 21 (1972) 925 - 935 state-of-the-art mathematics http://iumj.org/access/