Adjoint representation

Adjoint representation of a Lie algebra

Let ${\displaystyle \mathfrak{g}}$ be a Lie algebra over some field. Given an element x of a Lie algebra ${\displaystyle \mathfrak{g}}$, one defines the adjoint action of x on ${\displaystyle \mathfrak{g}}$ as the map

${\displaystyle {\mathrm{ad}}_x:\mathfrak{g}\to \mathfrak{g}\text{with}{\mathrm{ad}}_x(y)=[x,y]}$

for all y in ${\displaystyle \mathfrak{g}}$. It is called the adjoint endomorphism or adjoint action. (${\displaystyle {\mathrm{ad}}_x}$ is also often denoted as ${\displaystyle \mathrm{ad}(x)}$.) Since a bracket is bilinear, this determines the linear mapping

${\displaystyle \mathrm{ad}:\mathfrak{g}\to \mathfrak{g}\mathfrak{l}(\mathfrak{g})=(\mathrm{End}(\mathfrak{g}),[,])}$

given by x ↦ ad_x. Within End${\displaystyle (\mathfrak{g})}$, the bracket is, by definition, given by the commutator of the two operators:

${\displaystyle [T,S]=T\circ S-S\circ T}$

where ${\displaystyle \circ }$ denotes composition of linear maps. Using the above definition of the bracket, the Jacobi identity

${\displaystyle [x,[y,z]]+[y,[z,x]]+[z,[x,y]]=0}$

takes the form

${\displaystyle \left([{\mathrm{ad}}_x,{\mathrm{ad}}_y]\right)(z)=\left({\mathrm{ad}}_{[x,y]}\right)(z)}$

where x, y, and z are arbitrary elements of ${\displaystyle \mathfrak{g}}$.

This last identity says that ad is a Lie algebra homomorphism; i.e., a linear mapping that takes brackets to brackets. Hence, ad is a representation of a Lie algebra and is called the adjoint representation of the algebra ${\displaystyle \mathfrak{g}}$. In a more module-theoretic language, the construction says that ${\displaystyle \mathfrak{g}}$ is a module over itself.

The kernel of ad is the center of ${\displaystyle \mathfrak{g}}$ (that's just rephrasing the definition). On the other hand, for each element z in ${\displaystyle \mathfrak{g}}$, the linear mapping ${\displaystyle \delta ={\mathrm{ad}}_z}$ obeys the Leibniz' law:

${\displaystyle \delta ([x,y])=[\delta (x),y]+[x,\delta (y)]}$

for all x and y in the algebra (the restatement of the Jacobi identity). That is to say, ad_z is a derivation and the image of ${\displaystyle \mathfrak{g}}$ under ad is a subalgebra of Der${\displaystyle (\mathfrak{g})}$, the space of all derivations of ${\displaystyle \mathfrak{g}}$.

When ${\displaystyle \mathfrak{g}=\mathrm{Lie}(G)}$ is the Lie algebra of a Lie group G, ad is the differential of Ad at the identity element of G.

There is the following formula similar to the Leibniz formula: for scalars ${\displaystyle \alpha ,\beta }$ and Lie algebra elements ${\displaystyle x,y,z}$,

${\displaystyle ({\mathrm{ad}}_x-\alpha -\beta {)}^n[y,z]=\sum _{i=0}^n(\genfrac{}{}{0ex}{}{n}{i})\left[({\mathrm{ad}}_x-\alpha {)}^{i}y,({\mathrm{ad}}_x-\beta {)}^{n-i}z\right].}$

Structure constants

The explicit matrix elements of the adjoint representation are given by the structure constants of the algebra. That is, let {eⁱ} be a set of basis vectors for the algebra, with

${\displaystyle [e^i,e^j]=\sum _k{c^{ij}}_k{e}^k.}$

Then the matrix elements for ad_eⁱ are given by

${\displaystyle {{\left[{\mathrm{ad}}_{e^i}\right]}_k}^j={c^{ij}}_k.}$

Thus, for example, the adjoint representation of su(2) is the defining representation of so(3).

Examples

• If G is abelian of dimension n, the adjoint representation of G is the trivial n-dimensional representation.
• If G is a matrix Lie group (i.e. a closed subgroup of ${\displaystyle \mathrm{G}\mathrm{L}(n,\mathbb{C})}$), then its Lie algebra is an algebra of n×n matrices with the commutator for a Lie bracket (i.e. a subalgebra of ${\displaystyle {\mathfrak{g}\mathfrak{l}}_n(\mathbb{C})}$). In this case, the adjoint map is given by Ad_g(x) = gxg⁻¹.
• If G is SL(2, R) (real 2×2 matrices with determinant 1), the Lie algebra of G consists of real 2×2 matrices with trace 0. The representation is equivalent to that given by the action of G by linear substitution on the space of binary (i.e., 2 variable) quadratic forms.

Properties

The following table summarizes the properties of the various maps mentioned in the definition

The image of G under the adjoint representation is denoted by Ad(G). If G is connected, the kernel of the adjoint representation coincides with the kernel of Ψ which is just the center of G. Therefore, the adjoint representation of a connected Lie group G is faithful if and only if G is centerless. More generally, if G is not connected, then the kernel of the adjoint map is the centralizer of the identity component G₀ of G. By the first isomorphism theorem we have

${\displaystyle \mathrm{A}\mathrm{d}(G)\cong G/Z_G(G_0).}$

Given a finite-dimensional real Lie algebra ${\displaystyle \mathfrak{g}}$, by Lie's third theorem, there is a connected Lie group ${\displaystyle \mathrm{Int}(\mathfrak{g})}$ whose Lie algebra is the image of the adjoint representation of ${\displaystyle \mathfrak{g}}$ (i.e., ${\displaystyle \mathrm{Lie}(\mathrm{Int}(\mathfrak{g}))=\mathrm{ad}(\mathfrak{g})}$.) It is called the adjoint group of ${\displaystyle \mathfrak{g}}$.

Now, if ${\displaystyle \mathfrak{g}}$ is the Lie algebra of a connected Lie group G, then ${\displaystyle \mathrm{Int}(\mathfrak{g})}$ is the image of the adjoint representation of G: ${\displaystyle \mathrm{Int}(\mathfrak{g})=\mathrm{Ad}(G)}$.

Roots of a semisimple Lie group

If G is semisimple, the non-zero weights of the adjoint representation form a root system. (In general, one needs to pass to the complexification of the Lie algebra before proceeding.) To see how this works, consider the case G = SL(n, R). We can take the group of diagonal matrices diag(t₁, ..., t_n) as our maximal torus T. Conjugation by an element of T sends

${\displaystyle \left[\begin{array}{cccc}{a}_{11}& a_{12}& \cdots & a_{1n}\\ a_{21}& a_{22}& \cdots & a_{2n}\\ ⋮& ⋮& \ddots & ⋮\\ a_{n1}& a_{n2}& \cdots & a_{nn}\end{array}\right]\mapsto \left[\begin{array}{cccc}{a}_{11}& t_1{t}_2^{-1}{a}_{12}& \cdots & t_1{t}_n^{-1}{a}_{1n}\\ t_2{t}_1^{-1}{a}_{21}& a_{22}& \cdots & t_2{t}_n^{-1}{a}_{2n}\\ ⋮& ⋮& \ddots & ⋮\\ t_n{t}_1^{-1}{a}_{n1}& t_n{t}_2^{-1}{a}_{n2}& \cdots & a_{nn}\end{array}\right].}$

Thus, T acts trivially on the diagonal part of the Lie algebra of G and with eigenvectors t_it_j⁻¹ on the various off-diagonal entries. The roots of G are the weights diag(t₁, ..., t_n) → t_it_j⁻¹. This accounts for the standard description of the root system of G = SL_n(R) as the set of vectors of the form e_i−e_j.

Example SL(2, R)

When computing the root system for one of the simplest cases of Lie Groups, the group SL(2, R) of two dimensional matrices with determinant 1 consists of the set of matrices of the form:

${\displaystyle \left[\begin{array}{cc}a& b\\ c& d\end{array}\right]}$

with a, b, c, d real and ad − bc = 1.

A maximal compact connected abelian Lie subgroup, or maximal torus T, is given by the subset of all matrices of the form

${\displaystyle \left[\begin{array}{cc}{t}_1& 0\\ 0& t_2\end{array}\right]=\left[\begin{array}{cc}{t}_1& 0\\ 0& 1/t_1\end{array}\right]=\left[\begin{array}{cc}\exp (\theta )& 0\\ 0& \exp (-\theta )\end{array}\right]}$

with ${\displaystyle t_1{t}_2=1}$. The Lie algebra of the maximal torus is the Cartan subalgebra consisting of the matrices

${\displaystyle \left[\begin{array}{cc}\theta & 0\\ 0& -\theta \end{array}\right]=\theta \left[\begin{array}{cc}1& 0\\ 0& 0\end{array}\right]-\theta \left[\begin{array}{cc}0& 0\\ 0& 1\end{array}\right]=\theta (e_1-e_2).}$

If we conjugate an element of SL(2, R) by an element of the maximal torus we obtain

${\displaystyle \left[\begin{array}{cc}{t}_1& 0\\ 0& 1/t_1\end{array}\right]\left[\begin{array}{cc}a& b\\ c& d\end{array}\right]\left[\begin{array}{cc}1/t_1& 0\\ 0& t_1\end{array}\right]=\left[\begin{array}{cc}a{t}_1& b{t}_1\\ c/t_1& d/t_1\end{array}\right]\left[\begin{array}{cc}1/t_1& 0\\ 0& t_1\end{array}\right]=\left[\begin{array}{cc}a& b{t}_1^2\\ c{t}_1^{-2}& d\end{array}\right]}$

The matrices

${\displaystyle \left[\begin{array}{cc}1& 0\\ 0& 0\end{array}\right]\left[\begin{array}{cc}0& 0\\ 0& 1\end{array}\right]\left[\begin{array}{cc}0& 1\\ 0& 0\end{array}\right]\left[\begin{array}{cc}0& 0\\ 1& 0\end{array}\right]}$

are then 'eigenvectors' of the conjugation operation with eigenvalues ${\displaystyle 1,1,t_1^2,t_1^{-2}}$. The function Λ which gives ${\displaystyle t_1^2}$ is a multiplicative character, or homomorphism from the group's torus to the underlying field R. The function λ giving θ is a weight of the Lie Algebra with weight space given by the span of the matrices.

It is satisfying to show the multiplicativity of the character and the linearity of the weight. It can further be proved that the differential of Λ can be used to create a weight. It is also educational to consider the case of SL(3, R).

Variants and analogues

The adjoint representation can also be defined for algebraic groups over any field.

The co-adjoint representation is the contragredient representation of the adjoint representation. Alexandre Kirillov observed that the orbit of any vector in a co-adjoint representation is a symplectic manifold. According to the philosophy in representation theory known as the orbit method (see also the Kirillov character formula), the irreducible representations of a Lie group G should be indexed in some way by its co-adjoint orbits. This relationship is closest in the case of nilpotent Lie groups.