Consider the following Feynman diagram for a gluon (there is momentum k flowing between the two points):

Normally, the factor that goes along with this is Gabμν(k) = δab−ik2−iϵ+(ημν+(ξ−1)kμkνk2).
Using t'Hooft's double line formalism, the above diagram can be drawn as:

My task is to give a guess as to what the corresponding factor should be. I think that it's
G(ˉi,j)(ˉl,k)μν(k) = δjlδik−ik2−iϵ+(ημν+(ξ−1)kμkνk2)
However, this seems a little too simple and I'm worried that I'm missing out on a detail. When I google the "t'Hooft double line formalism" I find myself a lot of material, however I find that the discussion on the vertex factors is lacking (the same goes for the three- and four- gauge boson vertices). Can somebody either help me understand if I have the correct factor or point me in the direction of some literature that can?
(I should mention that the parameter ξ describes the gauge we're working in)
EDIT: I forgot to say that I'm talking about the U(N) gauge group.
This post imported from StackExchange Physics at 2017-02-15 08:33 (UTC), posted by SE-user Greg.Paul