= Scalar-field cubic and quartic Feynman vertices
{title2=$i\mu,\quad-i\lambda$}
With interaction density $\mu\phi^3/3!-\lambda\phi^4/4!$, the <Dyson series> contains $i\mathcal L_{\rm int}$ and hence cubic vertex $i\mu$ and quartic vertex $-i\lambda$. Wick contractions into the labelled slots cancel each factorial. Position integration gives the four-momentum delta function, and equivalent unlabelled contractions leave the <Feynman-diagram symmetry factor>. The free line carries $i/(p^2-m^2+i0)$ in the mostly-minus convention.
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