Radical of an algebraic group

The radical of an algebraic group is the identity component of its maximal normal solvable subgroup. For example, the radical of the general linear group $GL_{n}(K)$ (for a field K) is the subgroup consisting of scalar matrices, i.e. matrices $(a_{ij})$ with $a_{11}=\dots =a_{nn}$ and $a_{ij}=0$ for $i\neq j$ .

An algebraic group is called semisimple if its radical is trivial, i.e., consists of the identity element only. The group $SL_{n}(K)$ is semi-simple, for example.

The subgroup of unipotent elements in the radical is called the unipotent radical, it serves to define reductive groups.

References

"Radical of a group", Encyclopaedia of Mathematics

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