1. Mickens R.E.: Oscillations in Planar Dynamic Systems. Scientific, Singapore (1966)
2. Nayfeh, A.H., Mook, D.T.: Nonlinear Oscillations. Wiley-Interscience (1979)
3. Agarwal, R.P., Grace, S.R., O’Regan, D.: Oscillation Theory for Second Order Dynamic Equations. Series in Mathematical Analysis and Applications, vol. 5. Taylor & Francis, London (2003)
4. He, J.H.: Non-Perturbative Methods for Strongly Nonlinear Problems. Dissertation, de-Verlag im Internet GmbH, Berlin, Germany (2006)
5. Nayfeh A.H.: Perturbation Methods. Wiley, New York (1973)