1. A.D.B. Woods and E.C. Svensson, Phys. Rev. Letters 41, 974 (1978).
2. E.F. Talbot, H.R. Glyde, W.G. Stirling and E.C. Svensson, Phys. Rev. B38, 11 229 (1988).
3. W.G. Stirling and H.R. Glyde, Phys. Rev. B41, 4224 (1990).
4. H.R. Clyde and A. Griffin, Phys. Rev. Letters, 65, September 17 issue (1990).
5. N.N. Bogoliubov, J. Phys.(U.S.S.R.) 11, 23 (1947).