  We consider an exremal three-point correlator of three heavy vertex operators
for the circular winding string state with one large spin and one windining
number in AdS5 and one large spin and one winding number in S5. We use a
Schwarz-Christoffel transformation to compute semiclassically the extremal
three-point correlator on a stationary string trajectory which is mapped to the
complex plane with three punctures. It becomes a 4d conformal invariant
three-point correlator on the boundary. We discuss the marginality condition of
vertex operator.
