# Srednicki 9.17, 9.18

1. Oct 15, 2009

### kexue

Why is the expression 9.17 the sum of all diagrams with a single source removed?

I thought diagrams stand for terms in double taylor expansion of Z_1(J).

2. Oct 15, 2009

### Avodyne

The expression in 9.17 is

$${\delta\over\delta J(x)}Z_1(J)\Bigr|_{J=0}$$

The derivative with respect to J removes one factor of J, and then all other factors of J are set to zero. So the only surviving diagrams are those that had just one J to begin with.

3. Oct 16, 2009

### kexue

Thanks, Avodyne!

Actually, when I stared at it again this morning, I already figured it out by myself.

How come your such an expert on Srednick's book? Always when someone ask a question to his book, you give the answer right away. I remember you gave me great answers like one year back ago, which i have written as side notes in the book.

4. Oct 16, 2009

### Avodyne

Srednicki had posted rough drafts of Spin Zero and Spin One Half at arxiv.org in 2004, and I thought they were much better than the existing textbooks, so by now I know the material pretty well.