Duality and the PolyakovN-point Green's function

Recently Polyakov proposed an $N$-point Green's function $G(p_1,\dots ,p_N)$ for closed strings. We consider the problem of finding poles in the external momenta and extracting on-shell scattering amplitudes from $G$. Moreover, we find that $G$ is invariant under complex Möbius transformations, and is presumably dual.