T-DUALITY AND SCATTERING OF STRINGY STATES

I shall first review the salient features of T-duality, paying special attentions to the worldsheet perspective. The construction of vertex operators for Moduli will be discussed. The KLT formalism will be applied to show that the S-matrix for scattering of massless state arising from toroidal compactification of closed bosonic string is T-duality invariant.