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In mathematics, a **toral Lie algebra** is a Lie subalgebra of a general linear Lie algebra all of whose elements are semisimple (or diagonalizable over an algebraically closed field). Equivalently, a Lie algebra is toral if it contains no nonzero nilpotent elements. Over an algebraically closed field, every toral Lie algebra is abelian; thus, its elements are simultaneously diagonalizable.

**Mathematics** includes the study of such topics as quantity, structure (algebra), space (geometry), and change. It has no generally accepted definition.

In linear algebra, a square matrix is called **diagonalizable** or **nondefective** if it is similar to a diagonal matrix, i.e., if there exists an invertible matrix such that is a diagonal matrix. If is a finite-dimensional vector space, then a linear map is called **diagonalizable** if there exists an ordered basis of with respect to which is represented by a diagonal matrix. **Diagonalization** is the process of finding a corresponding diagonal matrix for a diagonalizable matrix or linear map. A square matrix that is not diagonalizable is called *defective.*

In mathematics, an element *x* of a ring *R* is called **nilpotent** if there exists some positive integer *n* such that *x*^{n} = 0.

A subalgebra *H* of a semisimple Lie algebra *L* is called toral if the adjoint representation of *H* on *L*, *ad*(*H*)⊂*gl*(*L*) is a toral Lie algebra. A maximal toral Lie subalgebra of a finite-dimensional semisimple Lie algebra, or more generally of a finite-dimensional reductive Lie algebra, over an algebraically closed field of characteristic 0 is a Cartan subalgebra and vice versa. In particular, a maximal toral Lie subalgebra in this setting is self-normalizing, coincides with its centralizer, and the Killing form of *L* restricted to *H* is nondegenerate.

In mathematics, a Lie algebra is **semisimple** if it is a direct sum of simple Lie algebras, i.e., non-abelian Lie algebras whose only ideals are {0} and itself. It is important to emphasize that a one-dimensional Lie algebra is by definition not considered a simple Lie algebra, even though such an algebra certainly has no nontrivial ideals. Thus, one-dimensional algebras are not allowed as summands in a semisimple Lie algebra.

In mathematics, a Lie algebra is **reductive** if its adjoint representation is completely reducible, whence the name. More concretely, a Lie algebra is reductive if it is a direct sum of a semisimple Lie algebra and an abelian Lie algebra: there are alternative characterizations, given below.

In mathematics, a **Cartan subalgebra**, often abbreviated as **CSA**, is a nilpotent subalgebra of a Lie algebra that is self-normalising. They were introduced by Élie Cartan in his doctoral thesis.

For more general Lie algebras, a Cartan algebra may differ from a maximal toral algebra.

- Maximal torus, in the theory of Lie groups

In mathematics, a **Lie algebra** is a vector space together with a non-associative operation called the **Lie bracket**, an alternating bilinear map , satisfying the Jacobi identity.

In algebraic geometry, an **algebraic group** is a group that is an algebraic variety, such that the multiplication and inversion operations are given by regular maps on the variety.

In the mathematical field of representation theory, a **weight** of an algebra *A* over a field **F** is an algebra homomorphism from *A* to **F**, or equivalently, a one-dimensional representation of *A* over **F**. It is the algebra analogue of a multiplicative character of a group. The importance of the concept, however, stems from its application to representations of Lie algebras and hence also to representations of algebraic and Lie groups. In this context, a **weight of a representation** is a generalization of the notion of an eigenvalue, and the corresponding eigenspace is called a **weight space**.

In the mathematical field of representation theory, a **Lie algebra representation** or **representation of a Lie algebra** is a way of writing a Lie algebra as a set of matrices in such a way that the Lie bracket is given by the commutator. In the language of physics, one looks for a vector space together with a collection of operators on satisfying some fixed set of commutation relations, such as the relations satisfied by the angular momentum operators.

In mathematics, a **linear algebraic group** is a subgroup of the group of invertible *n*×*n* matrices that is defined by polynomial equations. An example is the orthogonal group, defined by the relation M^{T}M = 1 where M^{T} is the transpose of M.

In mathematics, the term **Cartan matrix** has three meanings. All of these are named after the French mathematician Élie Cartan. Amusingly, the Cartan matrices in the context of Lie algebras were first investigated by Wilhelm Killing, whereas the Killing form is due to Cartan.

In mathematics, a **unipotent element***r* of a ring *R* is one such that *r* − 1 is a nilpotent element; in other words, (*r* − 1)^{n} is zero for some *n*.

In Lie theory and representation theory, the **Levi decomposition**, conjectured by Wilhelm Killing and Élie Cartan and proved by Eugenio Elia Levi (1905), states that any finite-dimensional real Lie algebra *g* is the semidirect product of a solvable ideal and a semisimple subalgebra. One is its **radical**, a maximal solvable ideal, and the other is a semisimple subalgebra, called a **Levi subalgebra**. The Levi decomposition implies that any finite-dimensional Lie algebra is a semidirect product of a solvable Lie algebra and a semisimple Lie algebra.

In mathematics, a Lie algebra is **solvable** if its derived series terminates in the zero subalgebra. The *derived Lie algebra* is the subalgebra of , denoted

In mathematics, **nilpotent orbits** are generalizations of nilpotent matrices that play an important role in representation theory of real and complex semisimple Lie groups and semisimple Lie algebras.

In mathematics, the **Jordan–Chevalley decomposition**, named after Camille Jordan and Claude Chevalley, expresses a linear operator as the sum of its commuting semisimple part and its nilpotent parts. The multiplicative decomposition expresses an invertible operator as the product of its commuting semisimple and unipotent parts. The decomposition is important in the study of algebraic groups. The decomposition is easy to describe when the Jordan normal form of the operator is given, but it exists under weaker hypotheses than the existence of a Jordan normal form.

In the theory of Lie algebras, an ** sl_{2}-triple** is a triple of elements of a Lie algebra that satisfy the commutation relations between the standard generators of the special linear Lie algebra

**Representation theory** is a branch of mathematics that studies abstract algebraic structures by *representing* their elements as linear transformations of vector spaces, and studies modules over these abstract algebraic structures. In essence, a representation makes an abstract algebraic object more concrete by describing its elements by matrices and the algebraic operations in terms of matrix addition and matrix multiplication. The algebraic objects amenable to such a description include groups, associative algebras and Lie algebras. The most prominent of these is the representation theory of groups, in which elements of a group are represented by invertible matrices in such a way that the group operation is matrix multiplication.

In the mathematical field of Lie theory, a **split Lie algebra** is a pair where is a Lie algebra and is a **splitting Cartan subalgebra**, where "splitting" means that for all , is triangularizable. If a Lie algebra admits a splitting, it is called a **splittable Lie algebra**. Note that for reductive Lie algebras, the Cartan subalgebra is required to contain the center.

In mathematics, **Borel–de Siebenthal theory** describes the closed connected subgroups of a compact Lie group that have *maximal rank*, i.e. contain a maximal torus. It is named after the Swiss mathematicians Armand Borel and Jean de Siebenthal who developed the theory in 1949. Each such subgroup is the identity component of the centralizer of its center. They can be described recursively in terms of the associated root system of the group. The subgroups for which the corresponding homogeneous space has an invariant complex structure correspond to parabolic subgroups in the complexification of the compact Lie group, a reductive algebraic group.

In algebra, a **linear Lie algebra** is a subalgebra of the Lie algebra consisting of endomorphisms of a vector space *V*. In other words, a linear Lie algebra is the image of a Lie algebra representation.

In mathematics, **semi-simplicity** is a widespread concept in disciplines such as linear algebra, abstract algebra, representation theory, category theory, and algebraic geometry. A **semi-simple object** is one that can be decomposed into a sum of *simple* objects, and simple objects are those that do not contain non-trivial proper sub-objects. The precise definitions of these words depends on the context.

In representation theory, a branch of mathematics, the **theorem of the highest weight** classifies the irreducible representations of a complex semisimple Lie algebra . There is a closely related theorem classifying the irreducible representations of a connected compact Lie group . The theorem states that there is a bijection

- Borel, Armand (1991),
*Linear algebraic groups*, Graduate Texts in Mathematics,**126**(2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-97370-8, MR 1102012 - Humphreys, James E. (1972),
*Introduction to Lie Algebras and Representation Theory*, Berlin, New York: Springer-Verlag, ISBN 978-0-387-90053-7

**Armand Borel** was a Swiss mathematician, born in La Chaux-de-Fonds, and was a permanent professor at the Institute for Advanced Study in Princeton, New Jersey, United States from 1957 to 1993. He worked in algebraic topology, in the theory of Lie groups, and was one of the creators of the contemporary theory of linear algebraic groups.

The **International Standard Book Number** (**ISBN**) is a numeric commercial book identifier which is intended to be unique. Publishers purchase ISBNs from an affiliate of the International ISBN Agency.

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