# Calculating Feynman amplitude for closed two loop diagrams

Could someone explain or post a link for how one calculates the Feynman amplitudes for closed two loop diagrams?

This is in QED with the same fermion in both loops and a photon connecting them.

• Well what have you tried on your own? – Kyle Kanos Mar 18 '17 at 19:37
• I think I do get how to set up the different factors now, but I am a little unsure how the energy-momentum conservation (delta distributions) should look in diagram A. Say the fermion line on the left has 4-momentum p (going into the vertex from the top) and the photon k (going out of the vertex), should the argument in the delta distribution be (p-k) or (p-p-k)? I know that diagram A will become zero, but am just curious how it would be. – MasterDesperate Mar 19 '17 at 20:41

A loop is one internal fermion and one internal antifermion line; here you must involve an integral over $d^4q$ where $q$ is the positron 4-momentum. The integral goes over the product of fermion and antifermion contribution. Imply energy-momentum conservation in delta distribution.