Friday, July 22, 2016

Bracket of Lie algebra-valued differential form



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,

[(g1α),(g2β)]=[g1,g2](αβ)


for all g1,g2g and all α,βΩ(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):=σSp+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 gα 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 giαi. 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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