1. V. I. Zubov, Stability of Motion. Lyapunov Methods and Their Application (Vyssh. Shkola, Moscow, 1984) [in Russian].
2. A. M. Lyapunov, General Problem of the Stability of Motion (Gosudarstv. Izdat. Tekhn.-Teor. Lit., Moscow-Leningrad, 1950) [English translation: The General Problem of the Stability of Motion (Taylor & Francis Ltd., London, 1992)].
3. N. P. Bhatia and G. P. Szegö, Stability Theory of Dynamical Systems (Springer-Verlag, Berlin-Heidelberg-New York, 1970).
4. A. M. Samoilenko, (Elements of the Mathematical Theory of Multi-Frequency Oscillations (Nauka, Moscow, 1987; Kluwer Academic Publishers Group, Dordrecht, 1991).
5. N. G. Bulgakov and B. S. Kalitin, “A generalization of theorems of Ljapunov’s second method. I. Theory,” Vesci Akad. Navuk BSSR Ser. Fiz.-Mat. Navuk, No. 3, 32–36 (1978) [in Russian].