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Could somebody explain how one can derive the symmetry factor both by counting possible contractions and by looking at the symmetry of a diagram. Consider for example this diagramenter image description here

in $\phi^4$-theory with $\mathcal{L}_\text{int} = -\frac{\lambda}{4!}\phi^4$.

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  • $\begingroup$ More on symmetry factors of Feynman diagrams: physics.stackexchange.com/… $\endgroup$ – Qmechanic Dec 2 '14 at 21:17
  • $\begingroup$ I've seen the other three questions. None of them helped me to understand how to tackle a diagram such as the above. $\endgroup$ – Casimir Dec 2 '14 at 21:39
  • $\begingroup$ Usually you never get anything beyond a two for a symmetry factor for a Feynman diagram. That does not answer your question, I know. So here is a site that will help you. I will give you a site to go to that explains it better than I could in the little space I have. Google, Feynman diagrams Radovan Dermisek, he explains it two different ways. He uses Srednicki's book, but a draft of that book is online for free. After you see it, you will understand why it is not so easy to explain in one post. I think he does a pretty good job of explaining it. Hope it helps! $\endgroup$ – Reddy15 Dec 3 '14 at 13:44
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    $\begingroup$ I assume, you are referring to these lecture notes. I read parts of them. However, I find that often 'powerpoint style' notes aren't very informative without the person who created them explaining their contents. After all, their main purpose is to back up the person's lecture and allow listeners to write down notes in context during the talk. $\endgroup$ – Casimir Dec 4 '14 at 12:12
  • $\begingroup$ I will give Srednicki's book a closer look this weekend. $\endgroup$ – Casimir Dec 4 '14 at 12:13
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Keep the external points x_i fixed and look at the exchanging of propagators for the virtual particles. In this case there the factor is 2 since you can only exchange them once and get the same diagram.

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