1. J. Hubbard, Proc. Roy. Soc. (London) A276:238 (1963);
2. J. Hubbard, Proc. Roy. Soc. (London) A277:237 (1964).
3. P.W. Anderson, Phys. Rev. 124:41 (1961).
4. J.B. Goodenough, in “Progress in Solid State Chemistry”, Vol. 5, edited by H. Reiss (Pergamon, Oxford, 1972).
5. J.A. Wilson and D. Pitt, Philos. Mag. 23:1297 (1971).