nLab
adjoint action

Idea

An adjoint action is an action by conjugation .

Definition

Of a group on itself

The adjoint action of a group $G$ on itself is the action $Ad : G \times G \to G$ given by

$$Ad : (g,h) \mapsto g^{-1} \cdot h \cdot g \,.$$

Of a Lie group on its Lie algebra

The adjoint action $ad : G \times \mathfrak{g} \to \mathfrak{g}$ of a Lie group $G$ on its Lie algebra $\mathfrak{g}$ is for each $g \in G$ the derivative $d Ad(g) : T_e G \to T_e G$ of this action in the second argument at the neutral element of $G$

$$ad : (g,x) \mapsto Ad(g)_*(x) \,.$$

This is often written as $ad(g)(x) = g^{-1} x g$ even though for a general Lie group the expression on the right is not the product of three factors in any way. But for a matrix Lie group $G$ it is: in this case both $g$ as well as $x$ are canonically identified with matrices and the expression on the right is the product of these matrices.

Since this is a linear action, it is called the adjoint representation of a Lie group. The associated bundles with respect to this representation are called adjoint bundles.

Of a Lie algebra on itself

Differentiating the above example also in the second argument, yields the adjoint action of a Lie algebra on itself

$$ad : \mathfrak{g} \times \mathfrak{g} \to \mathfrak{g}$$

which is simply the Lie bracket

$$ad_x : y \mapsto [x,y] \,.$$

Of a Hopf algebra on itself

Let $k$ be a commutative unital ring and $H = (H,m,\eta,\Delta,\epsilon, S)$ be a Hopf $k$-algebra with multiplication $m$, unit map $\eta$, comultiplication $\Delta$, counit $\epsilon$ and the antipode map $S: H\to H^{op}$. We can use Sweedler notation $\Delta(h) = \sum h_{(1)}\otimes_k h_{(2)}$. The adjoint action of $H$ on $H$ is given by

$$h\triangleright g = \sum h_{(1)} g S(h_{(2)})$$

and it makes $H$ not only an $H$-module, but in fact a monoid in the monoidal category of $H$-modules (usually called $H$-module algebra).

Related concepts

• coadjoint action

• adjoint representation, adjoint bundle

• conjugation action

• conjugacy class

Literature

• Sigurdur Helgason, Differential geometry, Lie groups, and symmetric spaces
• E. Meinrenken, Clifford algebras and Lie theory, Springer
• eom: adjoint representation of a Lie group, adjoint group