1. Halperin, B.I., Varma, C.M.: Phys. Rev. B14, 4030 (1976)
2. Riste, T., Samuelsen, E.J., Otnes, K.: In ?Structural Phase Transitions and Soft Modes?, (edited by E.J. Samuelsen, E.J. Andersen, J. Feder) Oslo: Universitetsforlaget 1971, p. 395
3. See articles by M�ller, K.A., von Waldkirch, T., Borsa, F., Blinc, R., Rigamonti, A. In: Proceedings of the International School of Physics Enrico Fermi, Course LIX 1976 on ?Local Properties at Phase Transitions? (edited by K.A. M�ller and A. Rigamonti) Amsterdam: North-Holland 1976
4. Senn, W.: Diplomarbeit (Basel 1974)
5. Thomas, H.: In ?Structural Phase Transitions and Soft Modes?, edited by E.J. Samuelsen, E.J. Andersen, J. Feder (Universitetsforlaget, Oslo, 1971), p. 15. See also page 3-42 in the Proceedings mentioned in [3]