1. Madelung, E.: Phys. ZS.XIX, 524 (1918)
2. Born, M.: Problems of atomic dynamics, p. 168?170. Cambridge: MIT Press 1926
3. Khan, M.A.: J. Phys. C9, 81 (1976)
4. Ashcroft, N.W., Mermin, N.D.: Solid State Physics, p. 407, 555. Tokyo: CBS Publishing Japan 1981
5. Kr�ger, F.A.: The chemistry of imperfect crystals, p. 249. Amsterdam: North-Holland 1974