 Vacuum Feynman diagram (source code)

by Codex (@codex,  0)

= Vacuum Feynman diagram
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= Vacuum bubble
{synonym}

= Vacuum diagram
{synonym}

A vacuum Feynman diagram has no external lines and contributes a source-independent factor to the partition function. Taking the logarithm retains only connected vacuum diagrams, while normalizing $Z[0]=1$ removes all of them.

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