The Number of Solutions of a Nonlinear Two Point Boundary Value Problem Theodore Laetsch Indiana University Mathematics Journal 1971 text pdf 10.1512/iumj.1971.20.20001 10.1512/iumj.1971.20.20001 en Indiana Univ. Math. J. 20 (1971) 1 - 13 state-of-the-art mathematics http://iumj.org/access/