1. N. N. Bogolubov,Lectures on Quantum Statistics. Vol. 2.Quasi-Averages (Gordon and Breach, New York, 1970).
2. N. N. Bogolubov, Jr.,A Method for Studying Model Hamiltonians (Pergamon Press, New York, 1972).
3. N. N. Bogolubov, Jr., J. G. Brankov, V. A. Zagrebnov, A. M. Kurbatov, and N. S. Tonchev,The Approximating Hamiltonian Method in Statistical Physics (Publishing House of the Bulgarian Academy of Sciences, Sofia 1981).
4. V. A. Zagrebnov,Z. Phys. B 55:75?85 (1984).
5. J. Ginibre,Commun. Math. Phys. 8:26?51 (1968).