Positive Solutions of Nonlinear Elliptic Eigenvalue Problems Donald CohenR. Simpson Indiana University Mathematics Journal 1970 text pdf 10.1512/iumj.1970.19.19070 10.1512/iumj.1970.19.19070 en Indiana Univ. Math. J. 19 (1970) 895 - 910 state-of-the-art mathematics http://iumj.org/access/