Figure 12: Graphical representation of the singlephoton Bloch equation (66) (c.f. Figure 3). The last diagram represents the “folded” term, that is, the last term of the equation. This equation can be compared with the BetheSalpeter equation, valid only in the singlereference case, where there is no folded contribution. The orderbyorder expansion of this equation is illustrated in Figure 11 [6, Figure 8.12]. 
