# Wedderburn-Mal'tsev theorem

Let $A$ be a finite-dimensional associative algebra (cf. Associative rings and algebras) over a field $F$ with radical $N$, and let the quotient algebra $A/N$ be a separable algebra (for algebras over a field of characteristic zero this is always true). Then $A$ can be decomposed (as a linear space) into a direct sum of the radical $N$ and some semi-simple subalgebra $S$:

$$A = N \oplus S,$$

and if there exists another decomposition $A = N \oplus {S _ {1} }$, where $S _ {1}$ is a semi-simple subalgebra, then there exists an automorphism $\phi$ of the algebra $A$ which maps $S$ onto $S _ {1}$( the automorphism $\phi$ is inner, i.e. there exist elements $a, a ^ \prime \in A$ such that $a \cdot a ^ \prime = a ^ \prime \cdot a = 0$ and $x \phi = a \cdot x \cdot a ^ \prime$ for all $x \in A$, where $x \cdot y = x + y + xy$). The existence of this decomposition was shown by J.H.M. Wedderburn [1] and the uniqueness, up to an automorphism of the semi-simple term, was proved by A.I. Mal'tsev [2]. This theorem, together with Wedderburn's theorem (cf. Associative rings and algebras) on the structure of semi-simple algebras constitutes the central part of the classical theory of finite-dimensional algebras.

#### References

 [1] J.H.M. Wedderburn, "On hypercomplex numbers" Proc. London Math. Soc. (2) , 6 (1908) pp. 77–118 [2] A.I. Mal'tsev, "On the representation of an algebra as a direct sum of the radical and a semi-simple subalgebra" Dokl. Akad. Nauk SSSR , 36 : 1 (1942) pp. 42–45 (In Russian) [3] A.A. Albert, "Structure of algebras" , Amer. Math. Soc. (1939) [4] C.W. Curtis, I. Reiner, "Representation theory of finite groups and associative algebras" , Interscience (1962)

A similar theorem holds for Lie algebras. Let $\mathfrak g$ be a finite-dimensional Lie algebra over a field of characteristic zero with radical $\mathfrak r$. Then there exists a semi-simple subalgebra $\mathfrak h$ of $\mathfrak g$ such that $\mathfrak g = \mathfrak h \oplus \mathfrak r$. Such a decomposition is called a Levi decomposition and $\mathfrak h$ is called a Levi factor or Levi subalgebra. It is unique up to inner automorphisms.