= Proper scalar-QED seagull-vertex diagrams
{title2=$V_C+2V_S=4$}
With zero quartic coupling, a proper one-loop vertex with two scalar and two labelled <photon> legs has two boxes, five triangles and two bubbles. A directed scalar chain connects the charged external ends; the internal <photon> joins its endpoints. Cubic vertices have one <photon> slot, seagulls two. All nine graphs have <Feynman-diagram symmetry factor> one with external labels fixed. At zero external <momentum> a transverse internal <photon> kills any graph with a cubic endpoint, leaving only the two seagull bubbles.
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