1. For a review of the theoretical literature see T. M. Rice, Solid State Phys. 32, ed. F. Seitz, D. Turnbull, H. Ehrenreich ( Academic, New York, 1977 ), p. 1.
2. For a review of the experimental literature see J. C. Hensel, T. G. Phillips, and G. A. Thomas, Solid State Phys. 32, ed. F. Seitz, D. Turnbull, and H. Ehrenreich ( Academic, New York, 1977 ), p. 88.
3. The possibility of a Bose-Einstein condensation of excitons or of a condensation of molecular excitons in semiconductors has occasionally been raised. To date there is no convincing evidence for them (see Refs. 1,2, R.J. Elliott, this volume.)
4. See for example David Pines, The Many Body Problem (W. A. Benjamin Inc., New York, 1962).
5. Ref. 2, p. 298.