1. Nonlinear Oscillations;Nayfeh,1979
2. Perturbation Methods;Nayfeh,1973
3. Perturbation Methods for Differential Equations;Shivamoggi,2003
4. Nonlinear Ordinary Differential Equations;Jordan,1999
5. Extraneous solutions predicted by the harmonic balance method;Hassan;Journal of Sound and Vibration,1995