2) See papers by A. P. Levanyuk, by D. G. Sannikov and by T. Jannsen in Incommensurate Phases in Dielectrics— Fundamentals, ed. R. Blinc and A. P. Levanyuk (North-Holland, Amsterdam, 1986).
3) H. Shiba and K. Nakanishi: Structural Phase Transitions in Layered Transition Metal Compounds, ed. K. Motizuki (Reidel, Dordrecht, 1986) p. 175.
4) J. Yeomans: Solid State Physics41, ed. H. Ehrenreich and D. Turnbull (Academic Press, New York, 1988) p. 151.
5) W. Selke: Phys. Reports 170 (1988) 213; See also W. Selke: Modulated Structure Materials, ed. T. Tsakalakos (Nijhoff, Boston, 1984) p. 23.