The linear theory of oscillations traditionally operates with frequency representa tions based on the concepts of a transfer function and a frequency response. The universality of the critria of Nyquist and Mikhailov and the simplicity and obvi ousness of the application of frequency and amplitude - frequency characteristics in analysing forced linear oscillations greatly encouraged the development of practi cally important nonlinear theories based on various forms of the harmonic balance hypothesis 303]. Therefore mathematically rigorous frequency methods of investi gating nonlinear...