1. Moiseev N.N., Asymptitic Methods of Nonlinear Mechanics, Nauka, Moscow (1981) (Russ)
2. Volosov V.M., Averaging in systems ordinary differential equations,Uspekhi Mat. Nauk.,17, N 6, 3–126 (1962) (Russ)
3. Grebennikov E.A., Method of average in apply problems, Nauka, Moscow (1986) (Russ)
4. Krylov N.M. & Bogolubov N.N., Applied of the nonlinear mechanics methods to the theory of stationary oscillations, Pub. House of AN UkSSR, Kiev, 3–26 (1934) (Russ)
5. Krylov N.M. & Bogolubov N.N., Introduction in nonlinear mechanics, Pub. House of AN UkSSR, Kiev (1937) (Ukrn)