1. A. Einstein, Phys. Zs.,18, no. 5, 121, 1917.
2. V. M. Fain, Izv. VUZ. Radiofizika,6, no. 2, 207, 1963.
3. I. I. Sobel'man and I. V. Tyutin, UFN,79, no. 4, 595, 1963.
4. V. M. Fain and Ya. I. Khanin, Quantum Radiophysics [in Russian], izd. Sov. radio, Moscow, 1965.
5. H. Barkhausen and K. Kurz, Phys. Zs.,21, 1, 1920.