# Cartan subalgebra

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of a finite-dimensional Lie algebra over a field A nilpotent subalgebra of which is equal to its normalizer in . For example, if is the Lie algebra of all complex square matrices of a fixed order, then the subalgebra of all diagonal matrices is a Cartan subalgebra in . A Cartan subalgebra can also be defined as a nilpotent subalgebra in which is equal to its Fitting null-component (cf. Weight of a representation of a Lie algebra) where denotes the adjoint representation (cf. Lie algebra) of .

Suppose further that is of characteristic zero. Then for any regular element , the set of all elements of which are annihilated by powers of is a Cartan subalgebra of , and every Cartan subalgebra of has the form for some suitable regular element . Each regular element belongs to one and only one Cartan subalgebra. The dimension of all the Cartan subalgebras of are the same and are equal to the rank of . The image of a Cartan subalgebra under a surjective homomorphism of Lie algebras is a Cartan subalgebra. If is algebraically closed, then all Cartan subalgebras of are conjugate; more precisely, they can be transformed into another by operators of the algebraic group of automorphisms of whose Lie algebra is the commutator subalgebra of . If is solvable, then the above assertion holds without the hypothesis that be algebraically closed.

Let be either a connected linear algebraic group over an algebraically closed field of characteristic zero, or a connected Lie group, and let be its Lie algebra. Then a subalgebra of is a Cartan subalgebra if and only if it is the Lie algebra of a Cartan subgroup of .

Let be a subalgebra of the Lie algebra of all endomorphisms of a finite-dimensional vector space over , and let be the smallest algebraic Lie algebra in containing (cf. Lie algebra, algebraic). If is a Cartan subalgebra of , then is a Cartan subalgebra of , and if is a Cartan subalgebra of and is the smallest algebraic subalgebra of containing , then is a Cartan subalgebra of and .

Let be a field extension. A subalgebra of is a Cartan subalgebra if and only if is a Cartan subalgebra of .

Cartan subalgebras play an especially important role when is a semi-simple Lie algebra (this was used by E. Cartan ). In this case, every Cartan subalgebra of is Abelian and consists of semi-simple elements (see Jordan decomposition), and the restriction of the Killing form to is non-singular.