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as you can tell from the question I am new to Feynman diagrams. I am trying to understand ways a diagram could be disconnected (besides vacuum bubbles). I am mainly trying to understand what an external vertex would look like? I have been trying to find a definition or some examples to get an idea but couldn't see anything helpful.

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  • $\begingroup$ Is this from a reference? Which page? $\endgroup$ – Qmechanic Dec 6 '20 at 21:41
  • $\begingroup$ @Qmechanic Sorry its not, just in my notes from my lecture course I see diagrams being talked about as connected, and am interested in being able to distinguish them from one another. I had seen vacuum bubbles mentioned as one reason for disconnect (which I can recognise) but did not follow any other like external vertex. $\endgroup$ – milkcookie Dec 6 '20 at 21:58
  • $\begingroup$ @Qmechanic Not the lecture notes I have but provides an example of what I am talking about. damtp.cam.ac.uk/user/tong/qft/three.pdf Like fig 17. $\endgroup$ – milkcookie Dec 6 '20 at 22:00
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External vertices in a Feynman diagram presumably refer to

  • 1-vertices/external sources

and/or

  • vertices with amputated legs.
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