We give the field representation of the canonical forms for the operators which generate the projective transformation. This representation is also applicable in formulating the N-string vertex of Lovelace and Olive. It shows that they have the properties of a transition operator. Requiring the same properties for the ghost sector in bosonized form we construct the BRST invariant canonical form and the N-string vertex. We consider the (N - 3) b-ghost insertion. We show that the resulting vertex with (N - 3) parameters corresponding to the location of the ghosts is symmetric with respect to the N strings and it gives the proper integration measure.
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