Nonlinear Sturm-Liouville Eigenvalue Problems and Topological Degree M. CrandallPaul Rabinowitz Indiana University Mathematics Journal 1970 text pdf 10.1512/iumj.1970.19.19086 10.1512/iumj.1970.19.19086 en Indiana Univ. Math. J. 19 (1970) 1083 - 1102 state-of-the-art mathematics http://iumj.org/access/