1. T. Schneider, E. Stoll, and H. Beck, Physica (Utrecht),79A, 201 (1975).
2. S. Sarbach and T. Schneider, Z. Phys. B,20, 399 (1975).
3. R. Schäfer, H. Beck, and H. Thomas, Z. Phys. B,30, 223 (1978).
4. C. S. Joyce, ?Properties of spherical model,? in: Phase Transition and Critical Phenomena (eds.) C. Domb and M. S. Green, Vol. 2, Academic Press, New York (1972), pp. 375?442.
5. N. N. Bogolyubov (Jr), Method of Investigating Model Hamiltonians [in Russian], Nauka, Moscow (1974); N. N. Bogolyubov (Jr), I. G. Brankov, V. A. Zagrebnov, A. M. Kurbatov, and H. C. Tonchev, Approximating Hamiltonian Method in Statistical Physics [in Russian], BAN, Sofiya (1981).