1. A. Liapunov, Ann. Fac. Sci. Toulouse, 2 (1907), 203–474.
2. A. Weinstein, Inv. Math., 20 (1973), 47–57.
3. J. Moser, Comm. Pure Appl. Math., 29 (1976), 727–747.
4. E. Hopf, Ber Math-Phis. Sachs. Acad. Wiss. Leipzig, 94 (1942), 1–22.
5. J. Alexander and J. Yorke, Amer. J. Math., 100 (1978), 263–292.