= 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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