1. Neudatchin, V. G., Novoskol'tseva, G. A., and Smirnov, Yu. F., J. exp. theor. Phys., 28, 540 (1969).
2. Glassgold, A. E., and Ialongo, G., Phys. Rev., 175, 151 (1968).
3. Smirnov, Yu. F., and Neudatchin, V. G., J. exp. theor. Phys. Lett., 3, 192 (1966).
4. Levin, V. G., Neudatchin, V. G., and Smirnov, Yu. F., Phys. stat. sol., (6), 49, 489 (1972).
5. Levin, V. G., Phys. Lett., 39, A, 125 (1972).