Lie Algebra: Why is 2D Nilpotent Lie Algebra Abelian?

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SUMMARY

A 2-dimensional nilpotent Lie algebra is always abelian, as established in Knapp's book on Lie algebras. This is due to the fact that nilpotent algebras have a lower central series that terminates at zero, which implies commutativity in two dimensions. There are exactly two 2-dimensional Lie algebras up to isomorphism: one is abelian and the other is nonabelian, with the latter being non-nilpotent. Proving that the nonabelian algebra cannot be nilpotent is a key exercise in understanding the structure of Lie algebras.

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lion8172
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I read in Knapp's book on Lie algebras that "a 2-dimensional nilpotent Lie algebra is abelian." Why is this the case? Can somebody who knows please tell me?
 
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How many 2-dimensional Lie algebras are there (up to isomorphism)? There are two: one's abelian and the other's not. Try proving that the nonabliean one cannot be nilpotent.
 

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