# Radical of a Lie algebra

In the mathematical field of Lie theory, the radical of a Lie algebra $\mathfrak{g}$ is the largest solvable ideal of $\mathfrak{g}.$
Let $k$ be a field and let $\mathfrak{g}$ be a finite-dimensional Lie algebra over $k$. A maximal solvable ideal, which is called the radical, exists for the following reason.
Firstly let $\mathfrak{a}$ and $\mathfrak{b}$ be two solvable ideals of $\mathfrak{g}$. Then $\mathfrak{a}+\mathfrak{b}$ is again an ideal of $\mathfrak{g}$, and it is solvable because it is an extension of $(\mathfrak{a}+\mathfrak{b})/\mathfrak{a}\simeq\mathfrak{b}/(\mathfrak{a}\cap\mathfrak{b})$ by $\mathfrak{a}$. Therefore we may also define the radical of $\mathfrak{g}$ as the sum of all the solvable ideals of $\mathfrak{g}$, hence the radical of $\mathfrak{g}$ is unique. Secondly, as $\{0\}$ is always a solvable ideal of $\mathfrak{g}$, the radical of $\mathfrak{g}$ always exists.
• A Lie algebra is semisimple if and only if its radical is $0$.