1. P. Langevin: J. Phys. (France) 4, 678 (1905).
2. P. Weiss: J. Phys. (France) 6, 667 (1907).
3. J. H. Van Vleck: The Theory of Electric and Magnetic Susceptibilities (Clarendon Press, Oxford, 1932).
4. W. Heisenberg: Z. Phys. 49, 619 (1928).
5. P. W. Anderson: Phys. Rev. 115, 2 (1959).