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3. Nonlinear vibration problems treated by the averaging method of W Ritz;Klotter,1951
4. Integration of ordinary differential equations by means of series;Perron;Sitzungsberichte der Heidelberger Akademie der Wissenschaften, Math.- naturw. Klasse,1919
5. Phase-lock automatic frequency control operation in the presence of noise;Tikhonov;Avtomatika i Telemekhanika,1960