 Simple Lie group

Lie groups General linear group GL(n)
Special linear group SL(n)
Orthogonal group O(n)
Special orthogonal group SO(n)
Unitary group U(n)
Special unitary group SU(n)
Symplectic group Sp(n)Exponential map
Adjoint representation of a Lie group
Adjoint representation of a Lie algebra
Killing form
Lie point symmetryStructure of semisimple Lie groupsDynkin diagrams
Cartan subalgebra
Root system
Real form
Complexification
Split Lie algebra
Compact Lie algebraRepresentation of a Lie group
Representation of a Lie algebrav · group theory, a simple Lie group is a connected nonabelian Lie group G which does not have nontrivial connected normal subgroups. A simple Lie algebra is a nonabelian Lie algebra whose only ideals are 0 and itself. A direct sum of simple Lie algebras is called a semisimple Lie algebra.
An equivalent definition of a simple Lie group follows from the Lie correspondence: a connected Lie group is simple if its Lie algebra is simple. An important technical point is that a simple Lie group may contain discrete normal subgroups, hence being a simple Lie group is different from being simple as an abstract group.
Simple Lie groups include many classical Lie groups, which provide a grouptheoretic underpinning for spherical geometry, projective geometry and related geometries in the sense of Felix Klein's Erlangen programme. It emerged in the course of classification of simple Lie groups that there exist also several exceptional possibilities not corresponding to any familiar geometry. These exceptional groups account for many special examples and configurations in other branches of mathematics, as well as contemporary theoretical physics.
While the notion of a simple Lie group is satisfying from the axiomatic perspective, in applications of Lie theory, such as the theory of Riemannian symmetric spaces, somewhat more general notions of semisimple and reductive Lie groups proved to be even more useful. In particular, every connected compact Lie group is reductive, and the study of representations of general reductive groups is a major branch of representation theory.
Contents
Comments on the definition
Unfortunately there is no single standard definition of a simple Lie group. The definition given above is sometimes varied in the following ways:
 Connectedness: Usually simple Lie groups are connected by definition. This excludes discrete simple groups (these are zerodimensional Lie groups that are simple as abstract groups) as well as disconnected orthogonal groups.
 Center: Usually simple Lie groups are allowed to have a discrete center; for example, SL_{2}(R) has a center of order 2, but is still counted as a simple Lie group. If the center is nontrivial (and not the whole group) then the simple Lie group is not simple as an abstract group. Some authors require that the center of a simple Lie group be finite (or trivial); the universal cover of SL_{2}(R) is an example of a simple Lie group with infinite center.
 R: Usually the group R of real numbers under addition (and its quotient R/Z) are not counted as simple Lie groups, even though they are connected and have a Lie algebra with no proper nonzero ideals. Occasionally authors define simple Lie groups in such a way that R is simple, though this sometimes seems to be an accident caused by overlooking this case.
 Matrix groups: Some authors restrict themselves to Lie groups that can be represented as groups of finite matrices. The metaplectic group is an example of a simple Lie group that cannot be represented in this way.
 Complex Lie algebras: The definition of a simple Lie algebra is not stable under the extension of scalars. The complexification of a complex simple Lie algebra, such as sl_{n}(C) is semisimple, but not simple.
The most common definition is the one above: simple Lie groups have to be connected, they are allowed to have nontrivial centers (possibly infinite), they need not be representable by finite matrices, and they must be nonabelian.
Method of classification
Main article: list of simple Lie groupsSuch groups are classified using the prior classification of the complex simple Lie algebras: for which see the page on root systems. It is shown that a simple Lie group has a simple Lie algebra that will occur on the list given there, once it is complexified (that is, made into a complex vector space rather than a real one). This reduces the classification to two further matters.
Real forms
The groups SO(p,q,R) and SO(p+q,R), for example, give rise to different real Lie algebras, but having the same Dynkin diagram. In general there may be different real forms of the same complex Lie algebra.
Relationship of simple Lie algebras to groups
Secondly the Lie algebra only determines uniquely the simply connected (universal) cover G* of the component containing the identity of a Lie group G. It may well happen that G* isn't actually a simple group, for example having a nontrivial center. We have therefore to worry about the global topology, by computing the fundamental group of G (an abelian group: a Lie group is an Hspace). This was done by Élie Cartan.
For an example, take the special orthogonal groups in even dimension. With the nonidentity matrix −I in the center, these aren't actually simple groups; and having a twofold spin cover, they aren't simplyconnected either. They lie 'between' G* and G, in the notation above.
Classification by Dynkin diagram
Main article: root systemAccording to Dynkin's classification, we have as possibilities these only, where n is the number of nodes:
Infinite series
A series
A_{1}, A_{2}, ...
A_{r} corresponds to the special linear group, SL(r+1).
B series
B_{2}, B_{3}, ...
B_{r} corresponds to the special orthogonal group, SO(2r+1).
C series
C_{2}, C_{3}, ...
C_{r} corresponds to the symplectic group, Sp(2r).
D series
D_{4}, D_{5}, ...
D_{r} corresponds to the special orthogonal group, SO(2r). Note that SO(4) is not a simple group, though. The Dynkin diagram has two nodes that are not connected. There is a surjective homomorphism from SO(3)* × SO(3)* to SO(4) given by quaternion multiplication; see quaternions and spatial rotation. Therefore the simple groups here start with D_{3}, which as a diagram straightens out to A_{3}. With D_{4} there is an 'exotic' symmetry of the diagram, corresponding to socalled triality.
Exceptional cases
For the socalled exceptional cases see G_{2}, F_{4}, E_{6}, E_{7}, and E_{8}. These cases are deemed 'exceptional' because they do not fall into infinite series of groups of increasing dimension. From the point of view of each group taken separately, there is nothing so unusual about them. These exceptional groups were discovered around 1890 in the classification of the simple Lie algebras, over the complex numbers (Wilhelm Killing, redone by Élie Cartan). For some time it was a research issue to find concrete ways in which they arise, for example as a symmetry group of a differential system.
See also E_{7½}
Simply laced groups
A simply laced group is a Lie group whose Dynkin diagram only contain simple links, and therefore all the nonzero roots of the corresponding Lie algebra have the same length. The A, D and E series groups are all simply laced, but no group of type B, C, F, or G is simply laced.
See also
 Cartan matrix
 Coxeter matrix
 Weyl group
 Coxeter group
 Kac–Moody algebra
 Catastrophe theory
References
 Jacobson, Nathan (19710601). Exceptional Lie Algebras (1 ed.). CRC Press. ISBN 0824713265.
Categories: Lie groups
 Lie algebras
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Simple Lie group
 Simple Lie group

Lie groups General linear group GL(n)
Special linear group SL(n)
Orthogonal group O(n)
Special orthogonal group SO(n)
Unitary group U(n)
Special unitary group SU(n)
Symplectic group Sp(n)Exponential map
Adjoint representation of a Lie group
Adjoint representation of a Lie algebra
Killing form
Lie point symmetryStructure of semisimple Lie groupsDynkin diagrams
Cartan subalgebra
Root system
Real form
Complexification
Split Lie algebra
Compact Lie algebraRepresentation of a Lie group
Representation of a Lie algebra