What are the disconnected Feynman diagrams in QFT?

[omephy]

I am learning QFT from Srednicki's book.
However, in Figure 9.2 Srednicki has given all connected diagrams with $E=0$ and $V=4$. Are there any way to know the disconnected diagrams? Can you tell me what they are.

 

[Bill_K]

Sure, see Eq. (9.12).

 

[omephy]

Can you please explain this for me?

 

[Bill_K]

Not really much to explain. The point is, we only need to calculate the contributions for the connected graphs, because... Because any disconnected graph consists of one or more connected components, and we get its contribution just by taking the product of its pieces.