1. Abe, R.: 1959, Prog. Theor. Phys. Kyoto
22, 213.
2. Barker, B. M., Bhatia, M. S., and Szamosi, G.: 1967, Nuovo Cimento
52B, 355.
3. Bludman, S. A. and Ruderman, M. A.: 1968, Phys. Rev.
170, 1176.
4. Brout, R. and Carruthers, P.: 1963, Lectures on Many-Electron Problems, Interscience, New York.
5. Gartenhaus, S. and Stranahan, G.: 1965a, Phys. Rev. Letters
14, 341.