Here
⟨ikϕ(x)ikϕ(x)⟩=α′k2πln(a/2R),
where a is an UV cutoff.
Now we can write (as all the ϕ's are located at x i.e. Time ordered {ϕn(x)}=ϕn(x) )
{ikϕ}n(x) = :{ikϕ}n(x):+∑all contractions= :{ikϕ}n(x):+ nC2(α′k2πln(a/2R)):{ikϕ}n−2(x):+nC2 n−2C22(α′k2πln(a/2R))2:{ikϕ}n−4(x):+⋯= :{ikϕ}n(x):+n(n−1)(α′k22πln(a/2R)):{ikϕ}n−2(x):+n(n−1)(n−2)(n−3)2!(α′k22πln(a/2R))2:{ikϕ}n−4(x):+⋯
We expand the vertex operator,
eikϕ(x)=∞∑n=0(ikϕ)n(x)n!=∞∑n=0:{ikϕ}n(x):n!+(α′k22πln(a/2R))∞∑n=2:{ikϕ}n−2(x):(n−2)!+12!(α′k22πln(a/2R))2∞∑n=4:{ikϕ}n−4(x):(n−4)!+⋯=∞∑n=0:{ikϕ}n(x):n![1+(α′k22πln(a/2R))+12!(α′k22πln(a/2R))2+⋯]= :eikϕ(x):e(α′k22πln(a/2R)).
Q.E.D.
This post imported from StackExchange Physics at 2015-05-06 11:41 (UTC), posted by SE-user layman