1. J.D. Jackson, Classical Electrodynamics, 3rd edn. (Wiley, New York, 1998)
2. A. Yariv, Quantum Electronics, 3rd edn. (Wiley, New York, 1989)
3. A.H. Nayfeh, Introduction to Perturbation Techniques (Wiley, New York, 1981)
4. A.H. Nayfeh, D.T. Mook, Non-Linear Oscillations (Wiley, New York, 1979)
5. R. Bellman, Methods of Nonlinear Analysis, vol. 1 (Academic Press, New York, 1970), p. 198