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unrenormalized vertex diagram and unrenormalized self-energy diagram are as below: Now, let me summarize what Peskin and Schroeder try to express. Feynman propagators for electron $$S_F=\frac{i}{\require{cancel}\cancel p-m}$$ end photon $$D_F=\frac{-ig_{\mu\nu}}{q^2}$$ and propagator must hold $$S_F(p)S_F^{-1}=\mathbb{1}$$ simply differentiate ...