1. L.D. Landau, E.M. Lifshitz: Statistical Physics (Addison-Wesley, Reading, Mass. 1969 )
2. H.E. Stanley: Introduction to Phase Transitions and Critical Phenomena ( Clarendon Press, Oxford 1971 )
3. H. Thomas: In Structural and Soft Modes, ed. by EJ. Samuelsen, E. Andersen, J. Feder ( Universitetsforlaget, Oslo 1971 )
4. M.E. Lines, A.M. Glass: Principles and Applications of Ferroelectrics and Related Materials ( Clarendon Press, Oxford 1977 )
5. B.I. Halperin, P.C. Hohenberg: Rev. Mod. Phys. 49, 435 (1977)