On Factorizing Correlation Functions in String Theory Using Loop Variables

Factorization of string amplitudes is one way of constructing string interaction vertices. We show that correlation functions in string theory can be conveniently factorized using loop variables representing delta functionals. We illustrate this construction with some examples where one particle is off-shell. These vertices are ``correct'' in the sense that they are guaranteed, by construction, to reproduce S-matrix elements when combined with propagators in a well defined way.