1. C. Bell and G. Wade, IRE Trans., CT-7, 4, 1960.
2. S. D. Gvozdover, A. S. Gorshkov, and V. F. Marchenko, Izv. VUZ. Radiofizika,6, no. 1, 126, 1963.
3. J. Armstrong, N. Bloembergen, J. Ducuing, and P. Pershan, Phys. Rev.,127, 1918, 1962.
4. P. Franken and J. Ward, Rev. Mod. Phys.,35, 23, 1963.
5. S. A. Akhmanov, A. S. Gorshkov, and V. F. Marchenko, Papers from the 4th All-Union Conference on Radioelectronics [in Russian], Kharkov, 1960.