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4. Arya, J. C., Bojadziev, G. N., Farooqui, A. S.: Response of a nonlinear vibrator under the influence of a time-dependent external force. IEEE Trans. Automat. Contr.AC-20, 233?234 (1975).
5. Arya, J. C., Bojadziev, G. N.: Response of a nonlinear vibrator governed by a certain partial differential equation. IEEE Trans. Automat. Contr.AC-24, 137?138 (1979).