Path-integral perturbation by source derivatives
= Path-integral perturbation by source derivatives
{title2=$Z[J]\propto e^{iS_{\rm int}[(1/i)\delta/\delta J]}Z_0[J]$}
An interaction depending on the field can be represented by substituting $(1/i)\delta/\delta J$ into its action and acting on the free <generating functional>. Expanding that exponential produces <Feynman diagrams>, contractions and vertex weights. Normalize by the resulting value at $J=0$ to cancel vacuum diagrams.