1. A. M. Lyapunov, The General Problem of Stability of Motion, Collected Works [in Russian], Vol. 2, Izd. AN SSSR, Moscow-Leningrad (1956).
2. I. G. Malkin, The Theory of Stability of Motion [in Russian], Gostekhizdat, Moscow-Leningrad (1952).
3. N. G. Chetaev, Stability of Motion [in Russian], Nauka, Moscow (1965).
4. N. N. Krasovskii, Some Problems in the Theory of Stability of Motion [in Russian], Fizmatgiz, Moscow (1959).
5. A. Poincaré, On Curves Defined by Differential Equations [Russian translation], Gostekhizdat, Moscow (1947).