1. 1. For a review see B. R. Cooper, in Magnetic Properties of Rare Earth Metals, R. J. Elliott, ed., Plenum, N.Y. (1972)
2. 2. P. Fulde, in Handbook on Physics and Chemistry of Rare Earths, K. A. Gschneider and L. Eyring, ed. North Holland (1978).
3. J. Hubbard, Proc. Roy. Soc. A 285, 542 (1965); S. B. Haley and P. Erdos, Phys. Rev. B 5, 1106 (1972).
4. Phase Transitions and Critical Phenomena, Vol. 3, C. Domb and M. S. Green, ed. Academic, N.Y. (1974).
5. D. M. Saul, M. Wortis, and D. Stauffer, Phys. Rev. Bl, 4964 (1974); J. Oitmaa, J. Phys. C 5, 435 (1972); J. G. Brankov J. Przystawa, and E. Praveczki, J. Phys. C 5, 3387 (1972).