1. A. M. Lyapunov, General Problem of Stability of Motion. Collection of Works [in Russian], Gostekhizdat, Moscow (1950)
2. English translation: A. M. Lyapunov, General Problem of Stability of Motion, CRC Press (1992).
3. J. La Salle and S. Lefschetz, Stability by Lyapunov’s Direct Method with Applications, Academic Press, New York (1961).
4. W. Hahn, Theorie und Anwendungen der Direkten Methodes von Lyapunov, Springer, Berlin (1959).
5. I. G. Malkin, Theory of Stability of Motion [in Russian], Nauka, Moscow (1966).