Radical of a Lie algebra

Radical of a Lie algebra

The radical of a Lie algebra mathfrak{g} is a particular ideal of mathfrak{g}.

Definition

Let mathfrak{g} be a Lie algebra. The radical of mathfrak{g} is defined as the largest solvable ideal of mathfrak{g}.

Such an ideal exists for the following reason. 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}simeqmathfrak{b}/(mathfrak{a}capmathfrak{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}.

Relation with semisimple Lie algebras

A Lie algebra is semisimple if its radical is 0.

ee also

*Semisimple Lie algebra


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