In this wikipedia article:
https://en.wikipedia.org/wiki/Lie_algebra-valued_differential_form
the bracket of Lie algebra-valued forms is defined. At one point it mentions that it is the bilinear product [⋅,⋅] on Ω∗(g) such that,
[(g1⊗α),(g2⊗β)]=[g1,g2]⊗(α∧β)
for all g1,g2∈g and all α,β∈Ω∗(M).
From this expression it looks like for an odd form α, [α,α]=0 since the second term is 0. But this is false according to the definition
[α,β](X1,…,Xp+q):=∑σ∈Sp+qsgn(σ)[α(Xσ(1),…,Xσ(p)),β(Xσ(p+1),…,Xσ(p+q))]
What am I missing?
Answer
It's true that the Lie bracket of things of the form g⊗α with themselves are always zero. (This is really no more than the statement that [g,g]=0, as you note.)
But most Lie algebra-valued forms do not look like this. They're sums of such terms. If gi is a basis of the Lie algebra, then a Lie algebra valued form actually looks like ∑gi⊗αi. You can see how the brackets of such terms can fail to be zero. (Pick some nice 2-dinensional Lie algebra to play with.)
So things are only interesting when you're working with non-pure tensors.
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