= Feynman rules
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<Feynman rules> translate a <Feynman diagram> into a contribution to a <scattering amplitude> or correlation function in <perturbation theory>. They specify factors for external states, interaction vertices, internal <propagators>, momentum-conservation constraints and loop integrals, with statistics-dependent signs and symmetry factors. For <quantum electrodynamics>, a photon-fermion vertex contains the coupling $e$, while a photon propagator supplies an inverse squared four-momentum. The complete amplitude also contains currents, so propagator powers alone do not determine the energy dependence of a <scattering cross-section>.
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