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3. A. Sigmen, Masers [Russian translation], Mir, Moscow (1966).
4. V. B. Steinshleiger, G. S. Misezhnikov, and P. S. Lifanov, Ultra High Frequency Quantum Amplifiers (Masers) [in Russian], Sov. Radio, Moscow (1971).
5. N. T. Cherpak, Izv. Vyssh. Uchebn. Zaved., Radiofiz.,19, No. 2, 280 (1976).