1. Yu. A. Mitropolskii and B. I. Moseenkov, Lectures on the application of asymptotic methods to the solution of equations with partial derivatives (in Russian), Ac. of Sci. Ukr. SSR, Kiev, 1968
2. V. I. Fodchuk, Application of the averaging method to the differential-difference equations of the neutral type, in Proceedings of the fourth conference on nonlinear oscillations, Prague (1968), 161–165
3. Monofrequent oscillations in mechanical systems governed by second order hyperbolic differential equations with small non-linearities;Boyadžiev, G. N.;Internat. J. Non-Linear Mech.,1973
4. Z. Osinski, Longitudinal, torsional and bending vibrations of a uniform bar with nonlinear internal friction and relaxation, in Nonlinear vibration problems, P. Ac. of Sci. Warsaw 4, 159–166 (1962)
5. I. P. Popov and N. P. Paltov, Approximate methods for analyzing nonlinear automatic systems (in Russian), Gos. Izdat. Fiz.-Math. Lit., Moscow, 1960