It is said that so called tadpole graph gets eliminated from perturbation expansion if the normal-ordered interaction is adopted. How can it be proved? Can anybody provide some links or any other references about this problems?(adsbygoogle = window.adsbygoogle || []).push({});

[tex]L_{int}=-e:\bar{\psi}\gamma_{\mu}\psi A^{\mu}:[/tex]

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# Tadpole graph & normal ordering

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