-
[1]
M. D. Freeman and D. I. Olive, Phys. Lett, B175(1986), 151.[2] H. Sonoda, Phys. Lett., B184(1987), 336.[3] L. Alvarez and P. Nelson, Preprint., CERN-TH 4615/86[4] A. A. Belavin, A. M. Polyakov and A. B. Zamolodochikov, Nucl. Phys., B241(1984), 333.