1. F. D. M. Haldane,Phys. Lett. 93A:464 (1983);Phys. Rev. Lett. 50:1153 (1983);J. Appl. Phys. 57:3359 (1985).
2. I. Affleck, T. Kennedy, E. Lieb, and H. Tasaki, Valence bond ground states in isotropic quantum antiferromagnets,Commun. Math. Phys., to appear.
3. F. D. M. Haldane,Phys. Rev. Lett. 51:605 (1983).
4. D. P. Arovas, A. Auerbach, and F. D. M. Haldane, Extended Heisenberg models of antiferromagnetism: Analogies to the fractional quantum Hall effect, preprint.
5. E. H. Lieb, T. Schultz, and D. Mattis,Ann. Phys. (N.Y.)16:407 (1961).