In 'an introduction to quantum field theory' by peskin, he writes: To analyze the photon one-point function, note that the external photon must be attached to a QED vertex. Neglecting the external photon propagator, this amplitude is therefore:(adsbygoogle = window.adsbygoogle || []).push({});

I really cannot justify this equation. Can anyone explain it to me?

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# Photon one point function

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