In
mathematics
Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
, G
2 is three simple
Lie group
In mathematics, a Lie group (pronounced ) is a group (mathematics), group that is also a differentiable manifold, such that group multiplication and taking inverses are both differentiable.
A manifold is a space that locally resembles Eucli ...
s (a complex form, a compact real form and a split real form), their
Lie algebra
In mathematics, a Lie algebra (pronounced ) is a vector space \mathfrak g together with an operation called the Lie bracket, an alternating bilinear map \mathfrak g \times \mathfrak g \rightarrow \mathfrak g, that satisfies the Jacobi ident ...
s
as well as some
algebraic group
In mathematics, an algebraic group is an algebraic variety endowed with a group structure that is compatible with its structure as an algebraic variety. Thus the study of algebraic groups belongs both to algebraic geometry and group theory.
Man ...
s. They are the smallest of the five exceptional
simple Lie group
In mathematics, a simple Lie group is a connected non-abelian Lie group ''G'' which does not have nontrivial connected normal subgroups. The list of simple Lie groups can be used to read off the list of simple Lie algebras and Riemannian symm ...
s. G
2 has rank 2 and dimension 14. It has two
fundamental representations, with dimension 7 and 14.
The compact form of G
2 can be described as the
automorphism group
In mathematics, the automorphism group of an object ''X'' is the group consisting of automorphisms of ''X'' under composition of morphisms. For example, if ''X'' is a finite-dimensional vector space, then the automorphism group of ''X'' is the g ...
of the
octonion algebra or, equivalently, as the subgroup of SO(7) that preserves any chosen particular vector in its 8-dimensional
real spinor
In geometry and physics, spinors (pronounced "spinner" IPA ) are elements of a complex numbers, complex vector space that can be associated with Euclidean space. A spinor transforms linearly when the Euclidean space is subjected to a slight (infi ...
representation (a
spin representation
In mathematics, the spin representations are particular projective representations of the orthogonal or special orthogonal groups in arbitrary dimension and signature (i.e., including indefinite orthogonal groups). More precisely, they are two equi ...
).
History
The Lie algebra
, being the smallest exceptional simple Lie algebra, was the first of these to be discovered in the attempt to classify simple Lie algebras. On May 23, 1887,
Wilhelm Killing
Wilhelm Karl Joseph Killing (10 May 1847 – 11 February 1923) was a German mathematician who made important contributions to the theories of Lie algebras, Lie groups, and non-Euclidean geometry.
Life
Killing studied at the University of M ...
wrote a letter to
Friedrich Engel saying that he had found a 14-dimensional simple Lie algebra, which we now call
.
In 1893,
Élie Cartan
Élie Joseph Cartan (; 9 April 1869 – 6 May 1951) was an influential French mathematician who did fundamental work in the theory of Lie groups, differential systems (coordinate-free geometric formulation of PDEs), and differential geometry. He ...
published a note describing an open set in
equipped with a 2-dimensional
distribution Distribution may refer to:
Mathematics
*Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations
*Probability distribution, the probability of a particular value or value range of a varia ...
—that is, a smoothly varying field of 2-dimensional subspaces of the tangent space—for which the Lie algebra
appears as the infinitesimal symmetries. In the same year, in the same journal, Engel noticed the same thing. Later it was discovered that the 2-dimensional distribution is closely related to a ball rolling on another ball. The space of configurations of the rolling ball is 5-dimensional, with a 2-dimensional distribution that describes motions of the ball where it rolls without slipping or twisting.
In 1900, Engel discovered that a generic antisymmetric trilinear form (or 3-form) on a 7-dimensional complex vector space is preserved by a group isomorphic to the complex form of G
2.
In 1908 Cartan mentioned that the automorphism group of the octonions is a 14-dimensional simple Lie group. In 1914 he stated that this is the compact real form of G
2.
In older books and papers, G
2 is sometimes denoted by E
2.
Real forms
There are 3 simple real Lie algebras associated with this root system:
*The underlying real Lie algebra of the complex Lie algebra G
2 has dimension 28. It has complex conjugation as an outer automorphism and is simply connected. The maximal compact subgroup of its associated group is the compact form of G
2.
*The Lie algebra of the compact form is 14-dimensional. The associated Lie group has no outer automorphisms, no center, and is simply connected and compact.
*The Lie algebra of the non-compact (split) form has dimension 14. The associated simple Lie group has fundamental group of order 2 and its
outer automorphism group
In mathematics, the outer automorphism group of a group, , is the quotient, , where is the automorphism group of and ) is the subgroup consisting of inner automorphisms. The outer automorphism group is usually denoted . If is trivial and has ...
is the trivial group. Its maximal compact subgroup is . It has a non-algebraic double cover that is simply connected.
Algebra
Dynkin diagram and Cartan matrix
The
Dynkin diagram
In the Mathematics, mathematical field of Lie theory, a Dynkin diagram, named for Eugene Dynkin, is a type of Graph (discrete mathematics), graph with some edges doubled or tripled (drawn as a double or triple line). Dynkin diagrams arise in the ...
for ''G''
2 is given by .
Its
Cartan matrix In mathematics, the term Cartan matrix has three meanings. All of these are named after the French mathematician Élie Cartan
Élie Joseph Cartan (; 9 April 1869 – 6 May 1951) was an influential French mathematician who did fundamental work in ...
is:
:
Roots of G2
A set of simple roots for can be read directly from the Cartan matrix above. These are (2,−3) and (−1, 2), however the integer lattice spanned by those is not the one pictured above (from obvious reason: the hexagonal lattice on the plane cannot be generated by integer vectors). The diagram above is obtained from a different pair roots:
and
.
The remaining
(positive) roots are
.
Although they do
span a 2-dimensional space, as drawn, it is much more symmetric to consider them as
vectors in a 2-dimensional subspace of a three-dimensional space. In this identification α corresponds to e₁−e₂, β to −e₁ + 2e₂−e₃, A to e₂−e₃ and so on. In euclidean coordinates these vectors look as follows:
The corresponding set of simple roots is:
:e₁−e₂ = (1,−1,0), and −e₁+2e₂−e₃ = (−1,2,−1)
Note: α and A together form root system ''identical'' to
A₂, while the system formed by β and B is ''isomorphic'' to
A₂.
Weyl/Coxeter group
Its
Weyl
Hermann Klaus Hugo Weyl (; ; 9 November 1885 – 8 December 1955) was a German mathematician, theoretical physicist, logician and philosopher. Although much of his working life was spent in Zürich, Switzerland, and then Princeton, New Jersey, ...
/
Coxeter group
is the
dihedral group
In mathematics, a dihedral group is the group (mathematics), group of symmetry, symmetries of a regular polygon, which includes rotational symmetry, rotations and reflection symmetry, reflections. Dihedral groups are among the simplest example ...
of
order
Order, ORDER or Orders may refer to:
* A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
...
12. It has minimal faithful degree
.
Special holonomy
G
2 is one of the possible special groups that can appear as the
holonomy
In differential geometry, the holonomy of a connection on a smooth manifold is the extent to which parallel transport around closed loops fails to preserve the geometrical data being transported. Holonomy is a general geometrical consequence ...
group of a
Riemannian metric
In differential geometry, a Riemannian manifold is a geometric space on which many geometric notions such as distance, angles, length, volume, and curvature are defined. Euclidean space, the N-sphere, n-sphere, hyperbolic space, and smooth surf ...
. The
manifold
In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or ''n-manifold'' for short, is a topological space with the property that each point has a N ...
s of G
2 holonomy are also called
G2-manifolds.
Polynomial invariant
G
2 is the automorphism group of the following two polynomials in 7 non-commutative variables.
:
:
(± permutations)
which comes from the octonion algebra. The variables must be non-commutative otherwise the second polynomial would be identically zero.
Generators
Adding a representation of the 14 generators with coefficients ''A'', ..., ''N'' gives the matrix:
:
It is exactly the Lie algebra of the group
:
There are 480 different representations of
corresponding to the 480 representations of octonions. The calibrated form,
has 30 different forms and each has 16 different signed variations. Each of the signed variations generate signed differences of
and each is an automorphism of all 16 corresponding octonions. Hence there are really only 30 different representations of
. These can all be constructed with Clifford algebra
using an invertible form
for octonions. For other signed variations of
, this form has remainders that classify 6 other non-associative algebras that show partial
symmetry. An analogous calibration in
leads to sedenions and at least 11 other related algebras.
Representations

The characters of finite-dimensional representations of the real and complex Lie algebras and Lie groups are all given by the
Weyl character formula
In mathematics, the Weyl character formula in representation theory describes the characters of irreducible representations of compact Lie groups in terms of their highest weights. It was proved by . There is a closely related formula for the ch ...
. The dimensions of the smallest irreducible representations are :
:1, 7, 14, 27, 64, 77 (twice), 182, 189, 273, 286, 378, 448, 714, 729, 748, 896, 924, 1254, 1547, 1728, 1729, 2079 (twice), 2261, 2926, 3003, 3289, 3542, 4096, 4914, 4928 (twice), 5005, 5103, 6630, 7293, 7371, 7722, 8372, 9177, 9660, 10206, 10556, 11571, 11648, 12096, 13090....
The 14-dimensional representation is the
adjoint representation, and the 7-dimensional one is action of G
2 on the imaginary octonions.
There are two non-isomorphic irreducible representations of dimensions 77, 2079, 4928, 30107, etc. The
fundamental representations are those with dimensions 14 and 7 (corresponding to the two nodes in the
Dynkin diagram
In the Mathematics, mathematical field of Lie theory, a Dynkin diagram, named for Eugene Dynkin, is a type of Graph (discrete mathematics), graph with some edges doubled or tripled (drawn as a double or triple line). Dynkin diagrams arise in the ...
in the order such that the triple arrow points from the first to the second).
described the (infinite-dimensional) unitary irreducible representations of the split real form of G
2.
The embeddings of the maximal subgroups of G
2 up to dimension 77 are shown to the right.
Finite groups
The group G
2(''q'') is the points of the algebraic group G
2 over the
finite field
In mathematics, a finite field or Galois field (so-named in honor of Évariste Galois) is a field (mathematics), field that contains a finite number of Element (mathematics), elements. As with any field, a finite field is a Set (mathematics), s ...
F
''q''. These finite groups were first introduced by
Leonard Eugene Dickson
Leonard Eugene Dickson (January 22, 1874 – January 17, 1954) was an American mathematician. He was one of the first American researchers in abstract algebra, in particular the theory of finite fields and classical groups, and is also rem ...
in for odd ''q'' and for even ''q''. The order of G
2(''q'') is . When , the group is
simple
Simple or SIMPLE may refer to:
*Simplicity, the state or quality of being simple
Arts and entertainment
* ''Simple'' (album), by Andy Yorke, 2008, and its title track
* "Simple" (Florida Georgia Line song), 2018
* "Simple", a song by John ...
, and when , it has a simple subgroup of
index
Index (: indexes or indices) may refer to:
Arts, entertainment, and media Fictional entities
* Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index''
* The Index, an item on the Halo Array in the ...
2 isomorphic to
2''A''
2(3
2), and is the automorphism group of a maximal order of the octonions. The Janko group
J1 was first constructed as a subgroup of G
2(11). introduced twisted
Ree groups
2G
2(''q'') of order for , an odd power of 3.
See also
*
Cartan matrix In mathematics, the term Cartan matrix has three meanings. All of these are named after the French mathematician Élie Cartan
Élie Joseph Cartan (; 9 April 1869 – 6 May 1951) was an influential French mathematician who did fundamental work in ...
*
Dynkin diagram
In the Mathematics, mathematical field of Lie theory, a Dynkin diagram, named for Eugene Dynkin, is a type of Graph (discrete mathematics), graph with some edges doubled or tripled (drawn as a double or triple line). Dynkin diagrams arise in the ...
*
Exceptional Jordan algebra
*
Fundamental representation
*
G2-structure
*
Lie group
In mathematics, a Lie group (pronounced ) is a group (mathematics), group that is also a differentiable manifold, such that group multiplication and taking inverses are both differentiable.
A manifold is a space that locally resembles Eucli ...
*
Seven-dimensional cross product
In mathematics, the seven-dimensional cross product is a bilinear operation on vectors in seven-dimensional Euclidean space. It assigns to any two vectors a, b in a vector also in .
Like the cross product in three dimensions, the seven-dime ...
*
Simple Lie group
In mathematics, a simple Lie group is a connected non-abelian Lie group ''G'' which does not have nontrivial connected normal subgroups. The list of simple Lie groups can be used to read off the list of simple Lie algebras and Riemannian symm ...
*
Star of David
The Star of David (, , ) is a symbol generally recognized as representing both Jewish identity and Judaism. Its shape is that of a hexagram: the compound of two equilateral triangles.
A derivation of the Seal of Solomon was used for decora ...
References
*
* .
::See section 4.1: G
2; an online HTML version of which is available at http://math.ucr.edu/home/baez/octonions/node14.html.
*
* Leonard E. Dickson reported groups of type G
2 in fields of odd characteristic.
* Leonard E. Dickson reported groups of type G
2 in fields of even characteristic.
*
*
{{String theory topics , state=collapsed
Algebraic groups
Lie groups
Octonions
Exceptional Lie algebras