1. N. N. Bogolyubov, Suppl. Physica (Utrecht),26, 1 (1960).
2. N. N. Bogolyubov, ?On a model Hamiltonian in the theory of superconductivity?, Preprint R-511 [in Russian], JINR, Dubna (1960).
3. N. N. Bogolyubov (Jr.), Method of Investigating Model Hamiltonians [in Russian], Nauka, Moscow (1974), p. 176.
4. K. Hepp and E. H. Lieb, Ann. Phys. (N. Y.),76, 360 (1973).
5. I. P. Brankov, V. A. Zagrebnov, and N. S. Tonchev, Teor. Mat. Fiz.,22, 20 (1975).