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Lie algebras are algebraic structures which were introduced to study the concept of infinitesimal transformations. The term "Lie algebra" (after Sophus Lie) was introduced by Hermann Weyl in the 1930s. In older texts, the name "infinitesimal group" is used. Related mathematical concepts include Lie groups and differentiable manifolds.

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reductive Lie subalgebra

Sorry, this started as a comment, but got too long. If $G$ is semisimple, then every derivation of $G$ is inner, so that the normalizer $N_L(G)=C_L(G)+G$ where $C_L(G)$ is the centralizer. In this si …
Robin Chapman's user avatar
4 votes
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Reductive Lie algebra

A reductive Lie algebra $L$ is the direct sum of a semisimple Lie algebra $L_1$ and an abelian Lie algebra $L_2$. Let's consider the case where $L_2$ is one-dimensional. We can embed $L$ into a larger …
Robin Chapman's user avatar
4 votes

Exceptional Lie algebras

I expect if you did construct a Lie algebra with relations built from a "forbidden" Cartan matrix then you would get an infinite-dimensional Kac-Moody algebra or something similar. Also for the excep …
Robin Chapman's user avatar
9 votes

Root systems and sums of squares

If a quadratic form in $n$ variables is the sum of the squares of $n$ integer linear forms, it's the sum of the squares of $n$ rational linear forms. Thus it's equivalent as a rational quadratic form …
Robin Chapman's user avatar