1. F. Dyson, Phys. Rev.,102, 1217 (1956).
2. Yu. G. Rudoi and Yu. A. Tserkovnikov, Supplement to: S. V. Tyablikov, Methods of the Quantum Theory of Magnetism, Second Edition [in Russian], Nauka, Moscow (1975), Chap. 3, §4.
3. V. G. Bar'yakhtar, V. N. Krivoruchko, and D. A. Yablonskii, Green's Functions in the Theory of Magnetism [in Russian], Naukova Dumka, Kiev (1984), Chaps. 4 and 5.
4. V. G. Bar'yakhtar, V. N. Krivoruchko, and D. A. Yablonskii, ?Dynamic and kinematic interaction of magnons. The spin Green's functions of the Heisenberg ferromagnet,? Preprint ITF-82-148R [in Russian], Institute of Theoretical Physics, Ukrainian SSR Academy of Sciences, Kiev (1982); Zh. Eksp. Teor. Fiz.,85, 602 (1983).
5. A. B. Harris, D. Kumar, B. I. Halperin, and P. C. Hohenberg, Phys. Rev. B,3, 961 (1971).