Vertex parity of a QED amplitude
= Vertex parity of a QED amplitude
{title2=$V=2I_\gamma+E_\gamma$}
Each elementary <QED> vertex has one photon leg, so counting these legs gives $V=2I_\gamma+E_\gamma$. An amplitude with two external photons has even $V$. In particular <Compton scattering> has no cubic-order elementary-vertex contribution. With four external legs and $V=4$, a connected diagram has four internal lines and one loop.