1. M.L. Klein, J.A. Venables (Eds.), Rare Gas Solids, Vol. 1, Academic Press, New York, 1976, p. 382.
2. Theory of Quantum Crystals
3. The Physics of Quantum Crystals
4. H. Horner, in: G.K. Horton, A.A. Maradudin (Eds.), Dynamical Properties of Solids, Vol. 1, North-Holland, Amsterdam, 1974, p. 451.
5. T.R. Koehler, in: G.K. Horton, A.A. Maradudin (Eds.), Dynamical Properties of Solids, Vol. II, North-Holland, Amsterdam, 1975 (Chapter 1).