1. N. G. Chetaev, Stability of Motion (Gostekhteorizdat, Moscow, 1946) [in Russian].
2. I. P. Andreichikov and V. I. Yudovich, Izv. Akad. Nauk SSSR Mekh. Tverd. Tela, No. 2, 78–87 (1974).
3. V. M. Lakhadanov, Prikl. Mat. Mekh. 39(1), 53–58 (1975).
4. W. Hauger, Trans. ASME, J. Appl. Mech. 42, 739–740 (1975).
5. V. I. Arnold, Additional Chapters of the Theory of Ordinary Differential Equations (Nauka, Moscow, 1978) [in Russian].