Analytical Dynamics of Modulated Systems

In theoretical physics one faces the dilemma of whether to work with simple model systems which are analytically tractable or with more complicated and realistic systems which are only soluble, if at all, by numerical methods. Of course both approaches have their value. The purpose of this brief review is to deal with analytical methods applied to the calculation of dynamical properties of some simple models which exhibit two independent periodicities. These periodicities provide two independent length scales for the model. In principle the two periodicities could be incommensurable and much of the interest in this topic is the investigation of the dependence of calculated quantities on their ratio. We give results which show the general features of the dynamics of such models, which are probably present in more complicated systems. In addition, some results which are of relevance to experiments on particular systems are given.