Sturm-Liouville Theory via Nonstandard Analysis Alan Macdonald Indiana University Mathematics Journal 1976 text pdf 10.1512/iumj.1976.25.25042 10.1512/iumj.1976.25.25042 en Indiana Univ. Math. J. 25 (1976) 531 - 540 state-of-the-art mathematics http://iumj.org/access/