# Reductive group

A linear algebraic group (over an algebraically closed field ) that satisfies one of the following equivalent conditions: 1) the radical of the connected component of the unit element of is an algebraic torus; 2) the unipotent radical of the group is trivial; or 3) the group is a product of closed normal subgroups and that are a semi-simple algebraic group and an algebraic torus, respectively. In this case is the commutator subgroup of and coincides with the radical of as well as with the connected component of the unit element of its centre; is finite, and any semi-simple or unipotent subgroup of the group is contained in .

A linear algebraic group is called linearly reductive if either of the two following equivalent conditions is fulfilled: a) each rational linear representation of is completely reducible (cf. Reducible representation); or b) for each rational linear representation and any -invariant vector there is a -invariant linear function on such that . Any linearly reductive group is reductive. If the characteristic of the field is 0, the converse is true. This is not the case when : A connected linearly reductive group is an algebraic torus. However, even in the general case, a reductive group can be described in terms of its representation theory. A linear algebraic group is called geometrically reductive (or semi-reductive) if for each rational linear representation and any -invariant vector there is a non-constant -invariant polynomial function on such that . A linear algebraic group is reductive if and only if it is geometrically reductive (see Mumford hypothesis).

The generalized Hilbert theorem on invariants is true for reductive groups. The converse is also true: If is a linear algebraic group over an algebraically closed field and if for any locally finite-dimensional rational representation by automorphisms of a finitely-generated associative commutative -algebra with identity the algebra of invariants is finitely generated, then is reductive (see [4]).

Any finite linear group is reductive and if its order is not divisible by , then it is also linearly reductive. Connected reductive groups have a structure theory that is largely similar to the structure theory of reductive Lie algebras (root system; Weyl group, etc., see [2]). This theory extends to groups where is a connected reductive group defined over a subfield and is the group of its -rational points (see [3]). In this case the role of Borel subgroups (cf. Borel subgroup), maximal tori (cf. Maximal torus) and Weyl groups is played by minimal parabolic subgroups (cf. Parabolic subgroup) defined over , maximal tori split over , and relative Weyl groups (see Weyl group), respectively. Any two minimal parabolic subgroups of that are defined over are conjugate by an element of ; this is also true for any two maximal -split tori of .

If is a connected reductive group defined over a field , then is a split group over a separable extension of finite degree of ; if, in addition, is an infinite field, then is dense in in the Zariski topology. If is a reductive group and is a closed subgroup of it, then the quotient space is affine if and only if is reductive. A linear algebraic group over a field of characteristic 0 is reductive if and only if its Lie algebra is a reductive Lie algebra (cf. Lie algebra, reductive). If , this is also equivalent to being the complexification of a compact Lie group (see Complexification of a Lie group).

#### References

[1] | T.A. Springer, "Invariant theory" , Lect. notes in math. , 585 , Springer (1977) |

[2] | J.E. Humphreys, "Linear algebraic groups" , Springer (1975) |

[3] | A. Borel, J. Tits, "Groupes réductifs" Publ. Math. IHES , 27 (1965) pp. 55–150 |

[4] | V.L. Popov, "Hilbert's theorem on invariants" Soviet Math. Dokl. , 20 : 6 (1979) pp. 1318–1322 Dokl. Akad. Nauk SSSR , 249 : 3 (1979) pp. 551–555 |

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