Figure 6.

The exact G contains amplitudes from all possible paths between 1 and 2. Amplitudes from all of these paths are represented by the rectangle placed between the field operators, the action of which is represented by * symbols. These terms can be calculated order-by-order with perturbation theory. At a given order n, we must connect n interaction lines at internal times in all possible − and allowed − ways. Concrete examples of diagrams are in Figure 7. All terms of the topology which can be inserted between two G₀ lines form the reducible self-energy.