1. Robinson F. S.: Proc. Phys. Soc.B 65 (1952), 910.
2. Jahoda F. C.: Thesis, Cornell University, not published.
3. Wolf E.: private communication.
4. Toll J. S.: Phys. Rev.104 (1956), 1760.
5. Landau L. D., Lifšic E. M.: Elektrodinamika splošnych sred, Fizmatgiz, Moscow 1959.