1. Bruce, A.D., Cowley, R.A.: Structural phase transitions. London: Taylor and Francis 1981
2. M�ller, K.A.: In: Statics and dynamics of nonlinear systems. p. 68. Berlin, Heidelberg, New York: Springer 1983
3. G�tze, W.: In: Recent developments in condensed matter physics. Devreese, T.T. (ed.), Vol. 1, p. 133. New York: Plenum Press 1981
4. Aksenov, V.L., Bobeth, M., Plakida, N.M., Schreiber, J.: J. Phys. C20, 375 (1987)
5. Tserkovnikov, Yu.A.: Theor. Math. Phys. (in Russian)19, 219 (1981)