Tuesday, July 31, 2018

quantum field theory - How to count and 'see' the symmetry factor of Feynman diagrams?


Could somebody explain how one can derive the symmetry factor both by counting possible contractions and by looking at the symmetry of a diagram. Consider for example this diagramenter image description here


in $\phi^4$-theory with $\mathcal{L}_\text{int} = -\frac{\lambda}{4!}\phi^4$.




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