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Don't know if anyone can help me but have a bit of confusion with Srednicki ch75 p466 just above (75.55). I understand why in non-Abelian gauge theory we get extra factors [itex]Tr(T^aT^bT^c)[/itex] and so on, but I don't understand why the [itex] P_{L}\to1/2 [/itex] diagrams then end up with the extra factor [itex] 1/2Tr([T^a,T^b],T^c) [/itex], does anyone know?

Also then in ch77, Srednicki says the triangle diagrams analyzed now come with the extra factor [itex] Tr(T^a T^b) [/itex], why not [itex] Tr(T^a T^b T^c) [/itex]?, after all they are the same diagrams he talks about of p466 (except for some changes he notes on p470 that don't seem to make a difference to this argument)

Thanks, would be really grateful if anyone is familiar with this...