# A Srednicki's QFT: Feynman Rules and Feynman Diagrams

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1. Jul 20, 2016

### Junaid456

I'm reading Srednicki's Quantum Field Theory. I 'm trying to read Srednicki's presentation of Feynman Diagrams in the chapter Path Integral for the Interacting Field Theory. Link to the book:

The path integral for the phi-cubed theory is equation 9.11 in the book. Please read that.

I get the following:

I get the following:

1. Feynman Diagrams are a away to to organize the terms in the aforementioned mammoth of an expression;
2. I understand the rules. See Srednicki for more details.
3. A diagram may represent a lot of different terms -- that is, those terms would be equivalent. That factor is given by the term: $V!P!(3!)^P(2!)^V$
4. Note that the coefficient from the Taylor Expansion is: $\frac{\displaystyle 1}{\displaystyle V!P!(3!)^P(2!)^V}$. It seems our counting factor exactly cancels the Taylor Expansion coefficient. Let's say that the numerical factor, after cancellation is, 1. But we may have over counted -- that is, a combination of permutations, described in the text, gives the same diagram. This is called the symmetry factor of the diagram. So, we must divide the numerical factor by the symmetry factor.

My question is as follows:

> Given my understanding of the Feynman Rules and Feynman diagrams, I am not sure how to figure out how many diagrams correspond to a fixed values of $V$ and $P$, say $V = V_{0}$ and say $P = P_{0}$. Let's say I have made a diagram, and I have computed its symmetry factor. I'm not sure how to figure how do I know how many different other diagrams are there and when have exhausted all possibilities.

It'd be great if someone could help me on this front.

Last edited by a moderator: Jul 23, 2016
2. Jul 20, 2016

### Staff: Mentor

Tip: for inline LaTeX, use ## as the delimiter, not \$. If you hurry, I think you can you can still edit your post.

3. Jul 21, 2016

### Junaid456

Okay. It'd be great if either you or someone else could answer the question though.

4. Jul 22, 2016

### houlahound

I don't think anyone can read the question, see post 2.

5. Jul 23, 2016

### Staff: Mentor

Because it's probably now too late for you to edit your post, I've changed the LaTeX delimiters for you. Maybe this will help.