On the Perturbation of a Periodic Solution when the Variational System has Non-Trivial Periodic Solutions D. Lewis Indiana University Mathematics Journal 1955 text pdf 10.1512/iumj.1955.4.54031 10.1512/iumj.1955.4.54031 en Indiana Univ. Math. J. 4 (1955) 795 - 815 state-of-the-art mathematics http://iumj.org/access/