1. N. N. Bogolyubov and Yu. A. Mitropol'skii, Asymptotic Methods in the Theory of Nonlinear Vibrations, Fizmatgiz, Moscow (1963).
2. Z. C. Van-Dan', ?A Lyapunov criterion for stability of motion?, Differentsial'nye Uravneniya,4, No. 3 (1968).
3. V. I. Zubov, Mathematical Investigation Methods of Automatic Control Systems, Sudpromgiz, Leningrad (1959).
4. N. N. Krasovskii, Problems of Stability of Motion, Fizmatgiz, Moscow (1959).
5. V. V. Laricheva, ?Averaging a class of systems of nonlinear differential equations?, Differentsial'nye Uravneniya,2, No. 3 (1966).