1. Yu. G. Rudoi, Teor. Mat. Fiz.,38, 101 (1979).
2. S. V. Tyablikov, Methods of Quantum Theory of Magnetism, Plenum Press, New York (1967). (The Russian reference is to the second edition published by Nauka (1975)).
3. N. N. Bogolyubov, Preprint JINR D-781 [in Russian], Dubna (1961) (see also Selected Works, Vol. 3 [in Russian], Naukova Dumka, Kiev (1971)).
4. H. Wagner, J. Phys.,195, 273 (1966).
5. N. N. Bogolyubov (Jr) and B. I. Sadovnikov, Some Problems in Statistical Mechanics [in Russian], Vysshaya Shkola (1975).