List of simple Lie groups

In mathematics, the simple Lie groups were first classified by Wilhelm Killing and later perfected by Élie Cartan. This classification is often referred to as Killing-Cartan classification.

The list of simple Lie groups can be used to read off the list of simple Lie algebras and Riemannian symmetric spaces. See also the table of Lie groups for a smaller list of groups that commonly occur in theoretical physics, and the Bianchi classification for groups of dimension at most 3.

Simple Lie groups

Unfortunately, there is no generally accepted definition of a simple Lie group. In particular, it is not defined as a Lie group that is simple as an abstract group. Authors differ on whether a simple Lie group has to be connected, or on whether it is allowed to have a non-trivial center, or on whether R is a simple Lie group.

The most common definition is that a Lie group is simple if it is connected, non-abelian, and every closed connected normal subgroup is either the identity or the whole group. In particular, simple groups are allowed to have a non-trivial center.

In this article the connected simple Lie groups with trivial center are listed. Once these are known, the ones with non-trivial center are easy to list as follows. Any simple Lie group with trivial center has a universal cover, whose center is the fundamental group of the simple Lie group. The corresponding simple Lie groups with non-trivial center can be obtained as quotients of this universal cover by a subgroup of the center.

Simple Lie algebras

The Lie algebra of a simple Lie group is a simple Lie algebra. This is a one-to-one correspondence between connected simple Lie groups with trivial center and simple Lie algebras of dimension greater than 1. (Authors differ on whether the one-dimensional Lie algebra should be counted as simple.)

Over the complex numbers the simple Lie algebras are classified by their Dynkin diagrams, of types "ABCDEFG". If L is a real simple Lie algebra, its complexification is a simple complex Lie algebra, unless L is already the complexification of a Lie algebra, in which case the complexification of L is a product of two copies of L. This reduces the problem of classifying the real simple Lie algebras to that of finding all the real forms of each complex simple Lie algebra (i.e., real Lie algebras whose complexification is the given complex Lie algebra). There are always at least 2 such forms: a split form and a compact form, and there are usually a few others. The different real forms correspond to the classes of automorphisms of order at most 2 of the complex Lie algebra.

Symmetric spaces

Symmetric spaces are classified as follows.

First, the universal cover of a symmetric space is still symmetric, so we can reduce to the case of simply connected symmetric spaces. (For example, the universal cover of a real projective plane is a sphere.)

Second, the product of symmetric spaces is symmetric, so we may as well just classify the irreducible simply connected ones (where irreducible means they cannot be written as a product of smaller symmetric spaces).

The irreducible simply connected symmetric spaces are the real line, and exactly two symmetric spaces corresponding to each non-compact simple Lie group G, one compact and one non-compact. The non-compact one is a cover of the quotient of G by a maximal compact subgroup H, and the compact one is a cover of the quotient of the compact form of G by the same subgroup H. This duality between compact and non-compact symmetric spaces is a generalization of the well known duality between spherical and hyperbolic geometry.

Hermitian symmetric spaces

A symmetric space with a compatible complex structure is called Hermitian. The compact simply connected irreducible Hermitian symmetric spaces fall into 4 infinite families with 2 exceptional ones left over, and each has a non-compact dual. In addition the complex plane is also a Hermitian symmetric space; this gives the complete list of irreducible Hermitian symmetric spaces.

The four families are the types A III, B I and D I for p = 2, D III, and C I, and the two exceptional ones are types E III and E VII of complex dimensions 16 and 27.

Notation

$\mathbb {R, C, H, O}$  stand for the real numbers, complex numbers, quaternions, and octonions.

In the symbols such as E₆⁻²⁶ for the exceptional groups, the exponent −26 is the signature of an invariant symmetric bilinear form that is negative definite on the maximal compact subgroup. It is equal to the dimension of the group minus twice the dimension of a maximal compact subgroup.

The fundamental group listed in the table below is the fundamental group of the simple group with trivial center. Other simple groups with the same Lie algebra correspond to subgroups of this fundamental group (modulo the action of the outer automorphism group).

List

	Dimension	Outer automorphism group	Dimension of symmetric space	Symmetric space	Remarks
R (Abelian)	1	R^∗	1	R	^†

Compact

	Dimension	Real rank	Fundamental group	Outer automorphism group	Other names	Remarks
An (n ≥ 1) compact	n(n + 2)	0	Cyclic, order n + 1	1 if n = 1, 2 if n > 1.	projective special unitary group PSU(n + 1)	A1 is the same as B1 and C1
Bn (n ≥ 2) compact	n(2n + 1)	0	2	1	special orthogonal group SO2n+1(R)	B1 is the same as A1 and C1. B2 is the same as C2.
Cn (n ≥ 3) compact	n(2n + 1)	0	2	1	projective compact symplectic group PSp(n), PSp(2n), PUSp(n), PUSp(2n)	Hermitian. Complex structures of H^n. Copies of complex projective space in quaternionic projective space.
Dn (n ≥ 4) compact	n(2n − 1)	0	Order 4 (cyclic when n is odd).	2 if n > 4, S3 if n = 4	projective special orthogonal group PSO2n(R)	D3 is the same as A3, D2 is the same as A1^2, and D1 is abelian.
E6^−78 compact	78	0	3	2
E7^−133 compact	133	0	2	1
E8^−248 compact	248	0	1	1
F4^−52 compact	52	0	1	1
G2^−14 compact	14	0	1	1		This is the automorphism group of the Cayley algebra.

Split

	Dimension	Real rank	Maximal compact subgroup	Fundamental group	Outer automorphism group	Other names	Dimension of symmetric space	Compact symmetric space	Non-Compact symmetric space	Remarks
An I (n ≥ 1) split	n(n + 2)	n	Dn/2 or B(n−1)/2	Infinite cyclic if n = 1 2 if n ≥ 2	1 if n = 1 2 if n ≥ 2.	projective special linear group PSLn+1(R)	n(n + 3)/2	Real structures on C^n+1 or set of RP^n in CP^n. Hermitian if n = 1, in which case it is the 2-sphere.	Euclidean structures on R^n+1. Hermitian if n = 1, when it is the upper half plane or unit complex disc.
Bn I (n ≥ 2) split	n(2n + 1)	n	SO(n)SO(n+1)	Non-cyclic, order 4	1	identity component of special orthogonal group SO(n,n+1)	n(n + 1)			B1 is the same as A1.
Cn I (n ≥ 3) split	n(2n + 1)	n	An−1S^1	Infinite cyclic	1	projective symplectic group PSp2n(R), PSp(2n,R), PSp(2n), PSp(n,R), PSp(n)	n(n + 1)	Hermitian. Complex structures of H^n. Copies of complex projective space in quaternionic projective space.	Hermitian. Complex structures on R^2n compatible with a symplectic form. Set of complex hyperbolic spaces in quaternionic hyperbolic space. Siegel upper half space.	C2 is the same as B2, and C1 is the same as B1 and A1.
Dn I (n ≥ 4) split	n(2n - 1)	n	SO(n)SO(n)	Order 4 if n odd, 8 if n even	2 if n > 4, S3 if n = 4	identity component of projective special orthogonal group PSO(n,n)	n^2			D3 is the same as A3, D2 is the same as A1^2, and D1 is abelian.
E6^6 I split	78	6	C4	Order 2	Order 2	E I	42
E7^7 V split	133	7	A7	Cyclic, order 4	Order 2		70
E8^8 VIII split	248	8	D8	2	1	E VIII	128			@ E8
F4^4 I split	52	4	C3 × A1	Order 2	1	F I	28	Quaternionic projective planes in Cayley projective plane.	Hyperbolic quaternionic projective planes in hyperbolic Cayley projective plane.
G2^2 I split	14	2	A1 × A1	Order 2	1	G I	8	Quaternionic subalgebras of the Cayley algebra. Quaternion-Kähler.	Non-division quaternionic subalgebras of the non-division Cayley algebra. Quaternion-Kähler.

Complex

	Real dimension	Real rank	Maximal compact subgroup	Fundamental group	Outer automorphism group	Other names	Dimension of symmetric space	Compact symmetric space	Non-Compact symmetric space
An (n ≥ 1) complex	2n(n + 2)	n	An	Cyclic, order n + 1	2 if n = 1, 4 (noncyclic) if n ≥ 2.	projective complex special linear group PSLn+1(C)	n(n + 2)	Compact group An	Hermitian forms on C^n+1 with fixed volume.
Bn (n ≥ 2) complex	2n(2n + 1)	n	Bn	2	Order 2 (complex conjugation)	complex special orthogonal group SO2n+1(C)	n(2n + 1)	Compact group Bn
Cn (n ≥ 3) complex	2n(2n + 1)	n	Cn	2	Order 2 (complex conjugation)	projective complex symplectic group PSp2n(C)	n(2n + 1)	Compact group Cn
Dn (n ≥ 4) complex	2n(2n − 1)	n	Dn	Order 4 (cyclic when n is odd)	Noncyclic of order 4 for n > 4, or the product of a group of order 2 and the symmetric group S3 when n = 4.	projective complex special orthogonal group PSO2n(C)	n(2n − 1)	Compact group Dn
E6 complex	156	6	E6	3	Order 4 (non-cyclic)		78	Compact group E6
E7 complex	266	7	E7	2	Order 2 (complex conjugation)		133	Compact group E7
E8 complex	496	8	E8	1	Order 2 (complex conjugation)		248	Compact group E8
F4 complex	104	4	F4	1	2		52	Compact group F4
G2 complex	28	2	G2	1	Order 2 (complex conjugation)		14	Compact group G2

Others

	Dimension	Real rank	Maximal compact subgroup	Fundamental group	Outer automorphism group	Other names	Dimension of symmetric space	Compact symmetric space	Non-Compact symmetric space	Remarks
A2n−1 II (n ≥ 2)	(2n − 1)(2n + 1)	n − 1	Cn			SLn(H), SU^∗(2n)	(n − 1)(2n + 1)	Quaternionic structures on C^2n compatible with the Hermitian structure	Copies of quaternionic hyperbolic space (of dimension n − 1) in complex hyperbolic space (of dimension 2n − 1).
An III (n ≥ 1) p + q = n + 1 (1 ≤ p ≤ q)	n(n + 2)	p	Ap−1Aq−1S^1			SU(p,q), A III	2pq	Hermitian. Grassmannian of p subspaces of C^p+q. If p or q is 2; quaternion-Kähler	Hermitian. Grassmannian of maximal positive definite subspaces of C^p,q. If p or q is 2, quaternion-Kähler	If p=q=1, split If | p−q | ≤ 1, quasi-split
Bn I (n > 1) p+q = 2n+1	n(2n + 1)	min(p,q)	SO(p)SO(q)			SO(p,q)	pq	Grassmannian of R^ps in R^p+q. If p or q is 1, Projective space If p or q is 2; Hermitian If p or q is 4, quaternion-Kähler	Grassmannian of positive definite R^ps in R^p,q. If p or q is 1, Hyperbolic space If p or q is 2, Hermitian If p or q is 4, quaternion-Kähler	If | p−q | ≤ 1, split.
Cn II (n > 2) n = p+q (1 ≤ p ≤ q)	n(2n + 1)	min(p,q)	CpCq	Order 2	1 if p ≠ q, 2 if p = q.	Sp2p,2q(R)	4pq	Grassmannian of H^ps in H^p+q. If p or q is 1, quaternionic projective space in which case it is quaternion-Kähler.	H^ps in H^p,q. If p or q is 1, quaternionic hyperbolic space in which case it is quaternion-Kähler.
Dn I (n ≥ 4) p+q = 2n	n(2n − 1)	min(p,q)	SO(p)SO(q)		If p and q ≥ 3, order 8.	SO('p,q)	pq	Grassmannian of R^ps in R^p+q. If p or q is 1, Projective space If p or q is 2 ; Hermitian If p or q is 4, quaternion-Kähler	Grassmannian of positive definite R^ps in R^p,q. If p or q is 1, Hyperbolic Space If p or q is 2, Hermitian If p or q is 4, quaternion-Kähler	If p = q, split If | p−q | ≤ 2, quasi-split
Dn III (n ≥ 4)	n(2n − 1)	⌊n/2⌋	An−1R^1	Infinite cyclic	Order 2		n(n − 1)	Hermitian. Complex structures on R^2n compatible with the Euclidean structure.	Hermitian. Quaternionic quadratic forms on R^2n.
E6^2 II (quasi-split)	78	4	A5A1	Cyclic, order 6	Order 2	E II	40	Quaternion-Kähler.	Quaternion-Kähler.	Quasi-split but not split.
E6^−14 III	78	2	D5S^1	Infinite cyclic	Trivial	E III	32	Hermitian. Rosenfeld elliptic projective plane over the complexified Cayley numbers.	Hermitian. Rosenfeld hyperbolic projective plane over the complexified Cayley numbers.
E6^−26 IV	78	2	F4	Trivial	Order 2	E IV	26	Set of Cayley projective planes in the projective plane over the complexified Cayley numbers.	Set of Cayley hyperbolic planes in the hyperbolic plane over the complexified Cayley numbers.
E7^−5 VI	133	4	D6A1	Non-cyclic, order 4	Trivial		64	Quaternion-Kähler.	Quaternion-Kähler.
E7^−25 VII	133	3	E6S^1	Infinite cyclic	Order 2	E VII	54	Hermitian.	Hermitian.
E8^−24 IX	248	4	E7 × A1	Order 2	1	E IX	112	Quaternion-Kähler.	Quaternion-Kähler.
F4^−20 II	52	1	B4 (Spin9(R))	Order 2	1	F II	16	Cayley projective plane. Quaternion-Kähler.	Hyperbolic Cayley projective plane. Quaternion-Kähler.

Simple Lie groups of small dimension

The following table lists some Lie groups with simple Lie algebras of small dimension. The groups on a given line all have the same Lie algebra. In the dimension 1 case, the groups are abelian and not simple.

Dim	Groups		Symmetric space	Compact dual	Rank	Dim
1	R, S^1=U(1)=SO2(R)=Spin(2)	Abelian	Real line		0	1
3	S^3=Sp(1)=SU(2)=Spin(3), SO3(R)=PSU(2)	Compact
3	SL2(R)=Sp2(R), SO2,1(R)	Split, Hermitian, hyperbolic	Hyperbolic plane H^2	Sphere S^2	1	2
6	SL2(C)=Sp2(C), SO3,1(R), SO3(C)	Complex	Hyperbolic space H^3	Sphere S^3	1	3
8	SL3(R)	Split	Euclidean structures on R^3	Real structures on C^3	2	5
8	SU(3)	Compact
8	SU(1,2)	Hermitian, quasi-split, quaternionic	Complex hyperbolic plane	Complex projective plane	1	4
10	Sp(2)=Spin(5), SO5(R)	Compact
10	SO4,1(R), Sp2,2(R)	Hyperbolic, quaternionic	Hyperbolic space H^4	Sphere S^4	1	4
10	SO3,2(R),Sp4(R)	Split, Hermitian	Siegel upper half space	Complex structures on H^2	2	6
14	G2	Compact
14	G2	Split, quaternionic	Non-division quaternionic subalgebras of non-division octonions	Quaternionic subalgebras of octonions	2	8
15	SU(4)=Spin(6), SO6(R)	Compact
15	SL4(R), SO3,3(R)	Split	R^3 in R^3,3	Grassmannian G(3,3)	3	9
15	SU(3,1)	Hermitian	Complex hyperbolic space	Complex projective space	1	6
15	SU(2,2), SO4,2(R)	Hermitian, quasi-split, quaternionic	R^2 in R^2,4	Grassmannian G(2,4)	2	8
15	SL2(H), SO5,1(R)	Hyperbolic	Hyperbolic space H^5	Sphere S^5	1	5
16	SL3(C)	Complex		SU(3)	2	8
20	SO5(C), Sp4(C)	Complex		Spin5(R)	2	10
21	SO7(R)	Compact
21	SO6,1(R)	Hyperbolic	Hyperbolic space H^6	Sphere S^6
21	SO5,2(R)	Hermitian
21	SO4,3(R)	Split, quaternionic
21	Sp(3)	Compact
21	Sp6(R)	Split, hermitian
21	Sp4,2(R)	Quaternionic
24	SU(5)	Compact
24	SL5(R)	Split
24	SU4,1	Hermitian
24	SU3,2	Hermitian, quaternionic
28	SO8(R)	Compact
28	SO7,1(R)	Hyperbolic	Hyperbolic space H^7	Sphere S^7
28	SO6,2(R)	Hermitian
28	SO5,3(R)	Quasi-split
28	SO4,4(R)	Split, quaternionic
28	SO^∗8(R)	Hermitian
28	G2(C)	Complex
30	SL4(C)	Complex

Notes

^† The group R is not simple as an abstract group, and according to most (but not all) definitions this is not a simple Lie group. Most authors do not count its Lie algebra as a simple Lie algebra. It is listed here so that the list of irreducible simply connected symmetric spaces is complete. Note that R is the only such non-compact symmetric space without a compact dual (although it has a compact quotient S¹).

Further reading

• Besse, Einstein manifolds ISBN 0-387-15279-2
• Helgason, Differential geometry, Lie groups, and symmetric spaces. ISBN 0-8218-2848-7
• Fuchs and Schweigert, Symmetries, Lie algebras, and representations: a graduate course for physicists. Cambridge University Press, 2003. ISBN 0-521-54119-0