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2. A.N. Nayfeh, D. Mook, Nonlinear Oscillations. (Wiley, 1979)
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4. A. Nayfeh, Problems in Perturbation (Wiley, New York, 1985)
5. F. Dincă, C. Teodosiu, Nonlinear and Random Vibrations (Editura Academiei R.S. România – Academic, New York, 1973)