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Fig. 5. For the case of Δi=0, the Feynmann diagram representing Δi(n) is a line with n dots corresponding to the n interactions. The rightmost and leftmost sides of a diagram have short lines, and they both have the same i representing the initial state. Diagrams like this are called self-energy diagrams in quantum field theories.
New Phys.: Sae Mulli 2022;72:537~543