Periodic Solutions of Singularly Perturbed Systems L. FlattoNorman Levinson Indiana University Mathematics Journal 1955 text pdf 10.1512/iumj.1955.4.54036 10.1512/iumj.1955.4.54036 en Indiana Univ. Math. J. 4 (1955) 943 - 950 state-of-the-art mathematics http://iumj.org/access/