Could anyone help/advise me please how to draw graphs for self-energy and Vertex functions given in attachments one and two.(adsbygoogle = window.adsbygoogle || []).push({});

In attachment one Sigma is self energy in coordinate space, S - fermion propagator, here I have one question: I was reading something where I found these exppresions, but how it can be that self-energy in attachment one is expressed only in terms of Propagator S(0), it's a just a vacuum bubble, just circle isn't it?

Irealy need someones help.

Thks a lot

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# Draw graphs for self-energy and Vertex functions

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