1. G. Wentzel, Helv. Phys. Acta,13, 269 (1940); S. Tomonaga, Prog. Theor. Phys.1, 83 (1946).
2. H. Fröhlich, H. Pelzer, and S. Zienau, Phil. Mag.,41, 221 (1950); S. I. Pekar, Investigations into the Electronic Theory of Crystals [in Russian], Gostekhizdat (1951).
3. N. N. Bogolyubov, Ukr. Mat. Zh.,2, 3 (1950); Selected Works, Vol. 2, Naukova Dumka, Kiev (1970), p. 499.
4. E. N. Solodovnikova, A. N. Tavkhelidze, and O. A. Khrustalev, Teor. Mat. Fiz.,8, 256 (1971);11, 317 (1972);12, 164 (1972).
5. N. E. Tyurin and A. V. Shurgaya, Preprint IFVÉ 72-15, 72-32, Serpukhov (1972);