1. R. V. Baluev, Celest. Mech. Dyn. Astron. 111, 235 (2011).
2. N. N. Bogolyubov and Yu. A. Mitropol’skii, Asymptotic Methods in the Theory of Nonlinear Oscillations (Gordon and Breach, London, 1961; Nauka, Moscow, 1974).
3. V. V. Chazov, Solar System. Res. 47, 99 (2013).
4. B. Cordani, Celest. Mech. Dyn. Astron. 89, 165 (2004).
5. B. P. Demidovich, Lectures on the Mathematical Theory of Stability (Nauka, Moscow, 1967) [in Russian].