1. I. A. Andreyev and V. V. Shapkin, USSR Inventor’s Certificate No. 527675, Byull. Izobret., No. 33 (1977).
2. I. A. Andreyev and V. V. Shapkin, USSR Inventor’s Certificate No. 716498, (1979).
3. J. Tolman, in Proceedings of the National Conference on Frequency Stability, A. C. Popov VUST, Prague, 1961, p. 29.
4. K. Hruska, Czech. J. Phys., Sect. B 12, 338 (1962).
5. J. Zelenka, Czech. J. Phys., Sect. B 80, 458 (1980).