Im getting crazy trying to derive this simple expression.
Say fabc are structure constants of a Lie algebra of SU(N) with [Ta,Tb]=ifabcTc. Then chosing normalization such that
∑c,dfacdfbcd=Nδab
Show that for fundamental rep of SU(N): tr(TaTb)=12δab
And for adjoint rep: tr(TaTb)=Nδab
Let me know if you have any suggestions, all much appreciated.
EDIT: Adjoint representation case is simply a matter of definition (Taadj)bc=−ifabc
tr(TaTb)=−famnfbnm=famnfbmn=Nδab
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