1. S. Timoshenko, D. H. Young, and W. Weaver, Vibration Problems in Engineering, Wiley, New York (1974).
2. B. V. Bulgakov, Oscillations [in Russian], Gostekhteorizdat, Moscow (1954).
3. I. V. Babakov, Theory of Oscillations [in Russian], Nauka, Moscow (1968).
4. V. O. Kononenko, Nonlinear Oscillations of Mechanical Systems. Selected Works [in Russian], Naukova Dumka, Kiev (1980).
5. J. Hale, Theory of Functional Differential Equations, Springer, New York (1977).