Figure 1: (a) Folded diagrams appear in the standard (Goldstone) graphical formulation of MBPT and represent the finite remainder after eliminating singularities due to intermediate model-space state, . In a relativistic treatment, Feynman diagrams are normally used, and then the corresponding diagrams will be drawn straight (right). (b) The second-order diagram is separated into a regular part with the intermediate state in the model space and a remainder, due to the intermediate model-space state, represented by the folded diagram.