= Cancellation of vacuum bubbles
A perturbative correlator factors into an exponential of connected <Vacuum Feynman diagrams> times a sum of <Feynman diagrams> attached to external insertions. Dividing its <path integral> by the vacuum <generating functional> at zero source cancels that exponential, including all disconnected products of vacuum components. The corresponding <operator formalism> uses the same denominator in the vacuum expectation of the <Dyson series>. This leaves externally disconnected correlators intact; the <connected generating functional> removes their products.
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