= One-loop diagram classes for Compton scattering
At order $e^4$, the open electron line can carry an internal-electron <self-energy>, a correction at either photon vertex, or a virtual photon spanning both vertices. Include the crossed external-photon order for each class. External-electron self-energies and external-photon <vacuum polarization> insertions appear in unamputated functions and are treated by <LSZ reduction> and field residues in physical amplitudes. A closed loop with three photon vertices cancels between orientations by <Furry's theorem>. Renormalized calculations also include the mass, field and vertex <counterterms> of the same perturbative order.
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