1. N. Seiberg, E. Witten, JHEP 32, 9909 (1999)
2. A. Connes, M.R. Douglas, A. Schwarz, JHEP 3, 9802 (1998); M.R. Douglas, C.M. Hull, JHEP 8, 9802 (1998)
3. F. Ardalan, H. Arfaei, M.M. Sheikh-Jabbari, JHEP 016, 9902 (1999)
4. T. Curtright, D. Fairlie, C. Zachos, Phys. Rev. D 58, 25002 (1998); L. Mezincescu, hep-th/0007046
5. S.-C. Chu, P.-M. Ho, Nucl. Phys. B 550, 151 (1999); Nucl. Phys. B 568, 447 (2000)