1. W. F. Brinkman, in lectures at the Scottish Universities Summer School, St. Andrews, 1974;
2. published in P. W. Anderson and W. F. Brinkman, in The Helium Liquids (Farquhar, ed.), 1975, p. 315.
3. G. Toulouse and M. Kleman, J. de Physique Letters 37, 2149 (1976).
4. S. Blaha, Phys. Rev. Letters, 36, 874 (1976).
5. K. Maki, various preprints (1976).