Solvable Lie algebra

In mathematics, a Lie algebra is solvable if its derived series terminates in the zero subalgebra. The derived Lie algebra of the Lie algebra is the subalgebra of, denoted
that consists of all linear combinations of Lie brackets of pairs of elements of. The derived series is the sequence of subalgebras
If the derived series eventually arrives at the zero subalgebra, then the Lie algebra is called solvable. The derived series for Lie algebras is analogous to the derived series for commutator subgroups in group theory, and solvable Lie algebras are analogs of solvable groups.
Any nilpotent Lie algebra is a fortiori solvable but the converse is not true. The solvable Lie algebras and the semisimple Lie algebras form two large and generally complementary classes, as is shown by the Levi decomposition. The solvable Lie algebras are precisely those that can be obtained from semidirect products, starting from 0 and adding one dimension at a time.
A maximal solvable subalgebra is called a Borel subalgebra. The largest solvable ideal of a Lie algebra is called the radical.

Characterizations

Let be a finite-dimensional Lie algebra over a field of characteristic. The following are equivalent.

is solvable.
, the adjoint representation of, is solvable.
There is a finite sequence of ideals of :
:
is nilpotent.
For -dimensional, there is a finite sequence of subalgebras of :
:
There is a finite sequence of subalgebras of,
:
The Killing form of satisfies for all in and in. This is Cartan's criterion for solvability.
Properties

states that if is a finite-dimensional vector space over an algebraically closed field of characteristic zero, and is a solvable Lie algebra, and if is a representation of over, then there exists a simultaneous eigenvector of the endomorphisms for all elements.

Every Lie subalgebra and quotient of a solvable Lie algebra are solvable.
Given a Lie algebra and an ideal in it,
: is solvable if and only if both and are solvable.
A solvable nonzero Lie algebra has a nonzero abelian ideal, the last nonzero term in the derived series.
If are solvable ideals, then so is. Consequently, if is finite-dimensional, then there is a unique solvable ideal containing all solvable ideals in. This ideal is the radical of.
A solvable Lie algebra has a unique largest nilpotent ideal, called the nilradical, the set of all such that is nilpotent. If is any derivation of, then.
Completely solvable Lie algebras

A Lie algebra is called completely solvable or split solvable if it has an elementary sequence of ideals in from to. A finite-dimensional nilpotent Lie algebra is completely solvable, and a completely solvable Lie algebra is solvable. Over an algebraically closed field a solvable Lie algebra is completely solvable, but the -dimensional real Lie algebra of the group of Euclidean isometries of the plane is solvable but not completely solvable.
A solvable Lie algebra is split solvable if and only if the eigenvalues of are in for all in.

Examples

Abelian Lie algebras

Every abelian Lie algebra is solvable by definition, since its commutator. This includes the Lie algebra of diagonal matrices in, which are of the form

for. The Lie algebra structure on a vector space given by the trivial bracket for any two matrices gives another example.

Nilpotent Lie algebras

Another class of examples comes from nilpotent Lie algebras since the adjoint representation is solvable. Some examples include the upper-diagonal matrices, such as the class of matrices of the form

called the Lie algebra of strictly upper triangular matrices. In addition, the Lie algebra of upper diagonal matrices in form a solvable Lie algebra. This includes matrices of the form

and is denoted.

Solvable but not split-solvable

Let be the set of matrices on the form

Then is solvable, but not split solvable. It is isomorphic with the Lie algebra of the group of translations and rotations in the plane.

Non-example

A semisimple Lie algebra is never solvable since its radical, which is the largest solvable ideal in, is trivial. ^{page 11}

Solvable Lie groups

Because the term "solvable" is also used for solvable groups in group theory, there are several possible definitions of solvable Lie group. For a Lie group, there is

termination of the usual derived series of the group ;
termination of the closures of the derived series;
having a solvable Lie algebra