1. V. Degiorgio in Nonlinear Phenomena at Phase Transitions and Instabilities(ed. T. Riste), Plenum Press, New York and London (1982), p. 181.
2. G.S. Agarwal and S. Dattagupta, Phys. Rev. 26A(1982) 880
3. J.C. Antoranz, L.L. Bonilla, J. Gea, and M.G. Velarde, Phys. Rev. Lett. 49(1982) 35
4. H.R. Brand, P.C. Hohenberg, and V. Steinberg, Phys Rev. 27A(1983) 591
5. T. Riste, K. Otnes and H.B. Miller, Neutron Inelastic Scattering
1977, International Atomic Energy Agency, Vienna (1978), Vol. I, p. 511.