Hi everyone, sorry if this is not the right place to post that question but i'm new to this forum, i'll delete if necessary. I am currently trying to learn QFT from Matthew Schwartz's "Quantum field theory and the standard model", quite clear during the first chapters, but i have been completely lost by the chapter about the feynman rules (can be found here : http://isites.harvard.edu/fs/docs/icb.topic1097985.files/I-7-Feynman.pdf ) I understand the first examples and the 2 derivations, but once he starts adding the derivative coupling (p.17) i'm completely lost : i don't understand how he gets the diagrams. Is it that he decides to study 2->2 scattering of the first 2 fields through the third given the interaction? If this is the case, why are there 4 diagrams instead of 1 after the integration by parts? And one last question : why is there only the p2 momentum in the first amplitude? Since the two particles enter the first vertice (and exit the second), shouldn't they both give a factor?