# Vacuum expectation value

1. Dec 11, 2015

### Higgsy

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."

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

2. Dec 16, 2015