Srednicki page 65 it says "Let us compute the vacuum expectation value of the field $$\phi(x)$$ which is given by $$\langle 0| \phi (x)|0 \rangle = \frac{\delta}{\delta J(x)} Z_{1}(J) |_{J=0}$$ This expression is then the sum of all diagrams that have a single source, with the source removed."(adsbygoogle = window.adsbygoogle || []).push({});

Why only the single source diagrams? Is it because it's for a single field?

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# Vacuum expectation value

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