1. A. G. Aronov:Sov. Phys. Solid State,5, 402 (1963).
2. Q. H. F. Vrehen andB. Lax:Phys. Rev. Lett.,12, 471 (1964).
3. Q. H. F. Vrehen:Phys. Rev. Lett.,14, 558 (1965).
4. B. Lax:Proc. of VII International Conf. on Physics of Semiconductors (Paris, 1964), p. 253.
5. L. D. Landau andE. M. Lifshitz:Classical Theory of Fields (New York, 1951), Chapter 3, Sect. 3.8, 54 and Problem 2 and its solution, 59, Sect. 3.10, 60 and Sect. 3.11, 64.