I am trying to calculate the 2-loop correction for a basic vertex in QED
So I need to figure out what diagrams I need to consider. My literature says (without much explanation), that there are 7 contributing diagrams (including pictures). But naively it is possible to draw many more 2-loop diagrams with two electron-legs and one photon leg. Most of them are excluded by simple rules such as
- All legs should be amputated.
- The diagram should be connected (i.e. no vacuum bubbles).
But what about the diagram
? It seems to obey all rules, but yet it is not listed as one of the contributing diagrams. Why is that? What rule am I missing?