1. Lehman G. W., Wolfram T., Dewames R. E.: Phys. Rev.128 (1962) 1593.
2. Maradudin A. A., Montroll E. W., Weiss G. H., Ipatova I. P.: Theory of lattice dynamics in the harmonic approximation. Academic Press, New York, 1971.
3. De Launay J.: Solid State Phys.2 (1956) 220.
4. Clark B. C., Gazis D. C., Wallis R. F.: Phys. Rev.134 (1964) 1486.
5. Khanna R. N., Rathore R. P. S.: Nuovo Cimento B54 (1979) 171.