= Free scalar four-point function
{title2=$\langle T\phi_1\phi_2\phi_3\phi_4\rangle$}
For a centered free <real scalar field>, its vacuum <time-ordered product> has expectation $\langle T\phi_1\phi_2\phi_3\phi_4\rangle=G_{12}G_{34}+G_{13}G_{24}+G_{14}G_{23}$, where $G_{ij}$ is the <Feynman propagator> between insertions $i,j$. These are the three complete pairings in the <Wick theorem>. All remaining <normal-ordered products> have zero vacuum expectation. No fermionic interchange signs occur for a bosonic field.
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