Vertex function

In quantum electrodynamics, the vertex function is the one particle irreducible correlation function involving ψ, \bar{\psi} and the vector potential A.

\Gamma^\mu\equiv -{1\over e}{\delta^3 \Gamma_{eff}\over \delta \bar{\psi} \delta \psi \delta A_\mu}

It is unfortunate that the effective action Γeff and the vertex function Γμ happen to be described by the same letter.

The dominant (and classical) contribution to Γμ is the gamma matrix γμ, which explains the choice of the letter.

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The one-loop correction to the vertex function. This is the dominant contribution to the anomalous magnetic moment of the electron.
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See also: Vertex function, Effective action, One particle irreducible correlation function, Physics, Quantum electrodynamics, Vector potential, Gamma matrix