1. T. Banks, W. Fischler, S. Shenker and L. Susskind, hep-th/9810043.
2. J.M. Maldacena, hep-th/9711200.
3. S.S. Gubser, I.R. Klebanov and A.M. Polyakov, hep-th/9802109; E. Witten, hep-th/9802150.
4. M. Li, hep-th/9807196.
5. E. Witten, hep-th/9803131.