In mathematics, a reductive group is a type of linear algebraic group over a field. One definition is that a connected linear algebraic group G over a perfect field is reductive if it has a representation that has a finite kernel and is a direct sum of irreducible representations. Reductive groups include some of the most important groups in mathematics, such as the general linear group GL(n) of invertible matrices, the special orthogonal group SO(n), and the symplectic group Sp(2n). Simple algebraic groups and (more generally) semisimple algebraic groups are reductive.

Claude Chevalley showed that the classification of reductive groups is the same over any algebraically closed field. In particular, the simple algebraic groups are classified by Dynkin diagrams, as in the theory of compact Lie groups or complex semisimple Lie algebras. Reductive groups over an arbitrary field are harder to classify, but for many fields such as the real numbers R or a number field, the classification is well understood. The classification of finite simple groups says that most finite simple groups arise as the group G(k) of k-rational points of a simple algebraic group G over a finite field k, or as minor variants of that construction.

Reductive groups have a rich representation theory in various contexts. First, one can study the representations of a reductive group G over a field k as an algebraic group, which are actions of G on k-vector spaces. But also, one can study the complex representations of the group G(k) when k is a finite field, or the infinite-dimensional unitary representations of a real reductive group, or the automorphic representations of an adelic algebraic group. The structure theory of reductive groups is used in all these areas.

A linear algebraic group over a field k is defined as a smooth closed subgroup scheme of GL(n) over k, for some positive integer n. Equivalently, a linear algebraic group over k is a smooth affine group scheme over k.

A connected linear algebraic group ${\displaystyle G}$ over an algebraically closed field is called semisimple if every smooth connected solvable normal subgroup of ${\displaystyle G}$ is trivial. More generally, a connected linear algebraic group ${\displaystyle G}$ over an algebraically closed field is called reductive if the largest smooth connected unipotent normal subgroup of ${\displaystyle G}$ is trivial. This normal subgroup is called the unipotent radical and is denoted ${\displaystyle R_{u}(G)}$. (Some authors do not require reductive groups to be connected.) A group ${\displaystyle G}$ over an arbitrary field k is called semisimple or reductive if the base change ${\displaystyle G_{\overline {k}}}$ is semisimple or reductive, where ${\displaystyle {\overline {k}}}$ is an algebraic closure of k. (This is equivalent to the definition of reductive groups in the introduction when k is perfect.) Any torus over k, such as the multiplicative group G_m, is reductive.

Over fields of characteristic zero another equivalent definition of a reductive group is a connected group ${\displaystyle G}$ admitting a faithful semisimple representation which remains semisimple over its algebraic closure ${\displaystyle k^{al}}$ ^{page 424}.

A linear algebraic group G over a field k is called simple (or k-simple) if it is semisimple, nontrivial, and every smooth connected normal subgroup of G over k is trivial or equal to G. (Some authors call this property "almost simple".) This differs slightly from the terminology for abstract groups, in that a simple algebraic group may have nontrivial center (although the center must be finite). For example, for any integer n at least 2 and any field k, the group SL(n) over k is simple, and its center is the group scheme μ_n of nth roots of unity.

A central isogeny of reductive groups is a surjective homomorphism with kernel a finite central subgroup scheme. Every reductive group over a field admits a central isogeny from the product of a torus and some simple groups. For example, over any field k,