1. N.N. Bogoliubov, Y.A. Mitropolsky, Asymptotic Methods in the Theory of Oscillations (Gordon and Breach, New York, 1961)
2. N. Minorsky, Nonlinear Oscillations (F. L. Krieger Publishing, Melbourne, 1974)
3. D.W. Jordan, P. Smith, Nonlinear Ordinary Differential Equations (Oxford University Press, Oxford, 1977)
4. S.H. Strogatz, Nonlinear Dynamics and Chaos: With Applications to Physics, Biology, Chemistry and Engineering, (Westview Press, USA, 1994)
5. R.H. Rand, Lecture Notes on Nonlinear Vibrations, The Internet-First University Press (2012). http://ecommons.library.cornell.edu/bitstream/1813/28989/2/NonlinearVibrations˙ver53.pdf