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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?
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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