1. See, e.g., J. Wilks,Liquid and Solid Helium (Oxford University Press, Oxford, England, 1967).
2. See, e.g., A. A. Maradudin, E. W. Montroll, and G. H. Weiss,Theory or Lattice Dynamics in the Hamonic Approximation (Academic Press, New York, 1963).
3. See review articles, e.g., R. A. Guyer,Solid State Physics (Academic Press, New York, 1969), Vol. 23; N. R. Werthamer,Am. J. Phys. 37, 763 (1969), and references therein.
4. H. Horner,Phys. Rev. 1, 1712 (1970).
5. C. Ebner and C. C. Sung, preprint, 1970.