1. L. A. Pastur and M. V. Shcherbina, Teor. Mat. Fiz.,61, 3 (1984).
2. N. N. Bogolyubov (Jr), Method of Investigation of Model Hamiltonians [in Russian], Nauka, Moscow (1974).
3. N. N. Bogolyubov (Jr), I. G. Brankov, V. A. Zagrebnov, A. M. Kurbatov, and N. S. Tonchev, The Approximating Hamiltonian Method in Statistical Physics [in Russian], Izd-vo BAN, Sofiya (1981).
4. R. Bruinsma, Phys. Rev. B,30, 289 (1984).
5. P. A. Pearce and G. J. Thompson, Prog. Theor. Phys.,55, 665 (1976).