1. N. N. Bogolubov, Jr.,A Method for Studying Model Hamiltonians (Pergamon Press, Oxford, 1972).
2. V. A. Zagrebnov,Teoret. Mat. Fiz. 22:20 (1975).
3. N. N. Bogolubov, Jr., J. G. Brankov, V. A. Zagrebnov, A. M. Kurbatov, and M. S. Tonchev,Approximate Hamiltonian Method in Statical Physics (Sofia, 1981).
4. D. Ya. Petrina,Teoret. Mat. Fiz. 4:394 (1970).
5. N. G. Duffield and J. V. Pule,Lett. Math. Phys. 14:329 (1987).