1. N. N. Bogolyubov and Yu. A. Mitropol'skii, Asymptotic Methods in the Theory of Nonlinear Oscillations [in Russian], Fizmatgiz, Moscow (1963).
2. R. F. Ganiev, ?Resonance phenomena in the case of nonlinear oscillations of solid bodies,? Prikl. Mekh.,8, No. 12 (1972).
3. L. N. Grodko, Ground Resonance in helicopters [in Russian], Part 2, Mashinostroenie, Moscow (1967).
4. L. N. Grodko, Ground Resonance of Helicopter on Its Run. Proceedings of the N. E. Zhukovskii Central Aerohydrodynamic Institute [in Russian] (1970).
5. B. Ya. Zherebtsov, Dynamic Stability of the Main Rotor of a Helicopter, Taking into Account the Elexibility of Its Blade. Proceedings of the N. E. Zhukovskii Aerohydrodynamic Institute [in Russian] (1949).