In Mark Srednicki's Quantum field theory(page 89) it says
This is a general result for tree diagrams (those with no closed loops): once the sources have been stripped off and the endpoints labeled, each diagram with a distinct endpoint labeling has an overall symmetry factor of one. The tree diagrams for a given process represent the lowest-order (in g) nonzero contribution to that process.
So the question is why the symmetry factor of the tree diagram is one with each endpoint labeled? Could you kindly provided a proof or some links including the proof?
