- #1
CAF123
Gold Member
- 2,948
- 88
Homework Statement
Consider a semi simple lie algebra. Show that if ##T_a## are the generators of a semi simple Lie algebra then ##\text{Tr}T_a=0##.
2. Homework Equations
The commutation relations of the generators and the cyclic properties of the trace.
The Attempt at a Solution
$$[T_a, T_b] = ic^c_{ab}T_c \Rightarrow \text{Tr}[T_a, T_b] = ic^c_{ab}\text{Tr}(T_c) = 0.$$
I also know that ##g_{ab} = -c^c_{ad}c^{d}_{bc}\,\,(1)##. So since the l.h.s of the equation above is zero, I could multiply by another structure constant, thereby introducing the metric and then since the lie algebra is semi simple, multiply by the inverse of the metric and thus just have ##\text{Tr}(T_a)## on the l.h.s meaning it has to vanish.
That is my argument but I can't write it down because i don't see a way to introduce another structure constant because in the definition ##(1)##, ##d## is contracted but in my expression ##d## is also contracted with the generator. So the placements of the indices mean I can't just multiply by another structure constant with the required indices without violating the summation convention.
Any hints on how to make progress? Thanks!