A Sublinear Sturm Liouville Problem George Pimbley, Jr. Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11008 10.1512/iumj.1962.11.11008 en Indiana Univ. Math. J. 11 (1962) 121 - 138 state-of-the-art mathematics http://iumj.org/access/