Positive Solutions of Convex Nonlinear Eigenvalue Problems Herbert AmannTheodore Laetsch Indiana University Mathematics Journal 1976 text pdf 10.1512/iumj.1976.25.25021 10.1512/iumj.1976.25.25021 en Indiana Univ. Math. J. 25 (1976) 259 - 270 state-of-the-art mathematics http://iumj.org/access/