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Srednicki has [tex] \langle 0 \mid \phi(x) \mid 0 \rangle =\frac{1}{i} \frac{\delta}{\delta J(x)} W_1 (J) \mid_{J=0}[/tex]

Which is thesum of all diagrams with that started with a single source (before the differentiation) now with the source removed.

Since we want this to be zero for the validity of the LSZ formula, we introduce counterm [tex] Y \phi [/tex] in the Lagrangian. We can choose this Y appropriatley at various orders of g now such that the vacuum expectation value is zero as we require.

I'm OK with all so far.

Now Srendicki says that because we have done this the sum of all connected diagrams with a single source is zero. Shouldn't this be the sum all diagrams with a single sourceis zero?with the source removed

Then he also goes on to say if we replace the single source in each of these diagrams with another subdiagram (any subdiagram), the sum of these is still zero. I do not understand why this is the case?

Could anyone please explain, thanks alot if so.

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# Srednicki P67

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