nLab equivariant group



Group Theory

Representation theory



For GG a fixed group, to be called the equivariance group, then by a GG-equivariant group we mean a group object internal to GG-actions, e.g. internal to G-sets, G-spaces, or G-manifolds, etc.

Beware that the term “equivariant group” for this notion is non-standard; or rather: there is no other established term for this notion at all. The term is meant to rhyme on the established terminology of equivariant principal bundles, in which context it serves to make nicely transparent the full (but often hidden) internal nature of the concept: equivariant principal bundles have, in general, equivariant groups as their structure groups.


We discuss equivariant groups in/as topological spaces, for definiteness and due to their relevance as models in equivariant homotopy theory. All of the discussion generalizes, say to smooth manifolds or general toposes (see at category of G-sets – For internal group actions).



(convenient category of topological spaces)
We write

TopologicalSpacesCategories TopologicalSpaces \;\in\; Categories

for any convenient category of topological spaces whose mapping space serves as an internal hom, such as

This means in particular that for X,Y,ATopologicalSpacesX,Y,A \,\in\, TopologicalSpaces, we have a natural bijection

(1)X×YAadjunctsXMaps(Y,A) X \times Y \longrightarrow A \;\;\;\;\;\;\; \overset{adjuncts}{\leftrightarrow} \;\;\;\;\;\;\; X \longrightarrow Maps(Y,A)

between maps (meaning: continuous functions) out of the product topological space with YY and maps into the mapping space.


(topological GG-spaces)
For GG be a topological group – to be called the equivariance group – we write

GActions(TopologicalSpaces)Categories G Actions(TopologicalSpaces) \;\in\; Categories

for the category whose


(further conditions on the equivariance group)
For purposes of equivariant homotopy theory one typically assumes the topological equivariance group GG in Def. to be that underlying a compact Lie group, such as a finite group (as that guarantees that G-CW-complexes are well-behaved and that the equivariant Whitehead theorem holds). But for the plain point-set topology of equivariant groups and their equivariant bundles this condition is not necessary.


(topological G-spaces are cartesian monoidal) The category of topological G-spaces (Def. ) is a Cartesian monoidal category: The Cartesian product of two topological G-spaces (X i,ρ i)(X_i, \rho_i) is the underlying product topological space equipped with the diagonal action by GG:

(X 1,ρ 1)×(X 2,ρ 2)=(X 1×X 2,ρ 1,2(g)(x 1,x 2)=(ρ 1(x 1),ρ 2(x 2))). (X_1, \rho_1) \times (X_2, \rho_2) \;=\; \Big( X_1 \times X_2, \, \rho_{1,2}(g)(x_1,x_2) \,=\, \big( \rho_1(x_1), \rho_2(x_2) \big) \Big) \,.

In a Cartesian monoidal category there is a notion of internal group objects:

Equivariant groups


(equivariant topological groups)
Given an equivariance group GG (Def. ), a GG-equivariant topological group (𝒢,α)(\mathcal{G}, \alpha) is a group object internal to topological G-spaces (Def. ):

(𝒢,α)Groups(GActions(TopologicalSpaces)). \big( \mathcal{G}, \, \alpha \big) \;\in\; Groups \big( G Actions ( TopologicalSpaces ) \big) \,.

(See Prop. below for the choice of notation used here.)


Since the forgetful functor from topological G-spaces (Def. ) to underlying topological spaces

GActions(TopologicalSpaces) TopologicalSpaces \array{ G Actions ( TopologicalSpaces ) & \overset{ }{\longrightarrow} & TopologicalSpaces }

preservesCartesian products (explicitly so by Remark ), it preserves group objects and hence sends GG-equivariant topological groups (Def. ) to underlying plain topological groups:

(2)Groups(GActions(TopologicalSpaces)) Groups(TopologicalSpaces) (𝒢,α) 𝒢. \array{ Groups \big( G Actions ( TopologicalSpaces ) \big) & \overset{ }{\longrightarrow} & Groups ( TopologicalSpaces ) \\ \big( \mathcal{G} , \alpha \big) &\mapsto& \mathcal{G} \,. }


Equivalence with semidirect products with GG


(Equivariant groups as semidirect product groups)
The category of GG-equivariant topological groups (Def. ) is equivalent

(3)Groups(GActions(TopologicalSpaces)) Groups /G G/ (𝒢,α) 𝒢 αG. \array{ Groups \big( G Actions ( TopologicalSpaces ) \big) & \overset{ \;\; \;\; }{\hookrightarrow} & Groups^{G/}_{/G} \\ \big( \mathcal{G}, \alpha \big) &\mapsto& \mathcal{G} \rtimes_{\alpha} G \,. }

to that of semidirect product groups of the form ()G(-) \rtimes G

(4)𝒢 αG(𝒢×G,(γ 1,g 1)(γ 2,g 2)(γ 1α(g 1)(γ 2),g 1g 2)) \mathcal{G} \rtimes_\alpha G \;\; \coloneqq \;\; \Big( \mathcal{G} \times G \,, \;\;\; (\gamma_1, g_1) \cdot (\gamma_2, g_2) \;\coloneqq\; \big( \gamma_1 \cdot \alpha(g_1)(\gamma_2) ,\, g_1 \cdot g_2 \big) \Big)

and regarded as pointed objects in the slice category of Groups over GG via the canonical homomorphisms

(5)G 𝒢× αG G (γ,g) g g (e 𝒢,g). \array{ G &\longrightarrow& \mathcal{G} \times_\alpha G &\longrightarrow& G \\ && (\gamma,g) &\mapsto& g \\ g &\mapsto& (e_{\mathcal{G}},g) \,. }

(which jointly witness the semidirect product as a split group extension of GG, see there).


This is a straightforward matter of unwinding the definitions:

First to see that we have a functor as claimed:

A group object in GActions(TopologicalSpaces)G Actions(TopologicalSpaces) is, by definition, a plain topological group 𝒢\mathcal{G} whose underlying topological space is equipped with a continuous GG-action and such such this GG-action preserves all its group operations. In other words, this is a group 𝒢\mathcal{G} and a homomorphism α:GAut Grp(𝒢)\alpha \;\colon\;G \longrightarrow Aut_{Grp}(\mathcal{G}) to the group-automorphism group (whose hom-adjunct (1) is continuous).

This is exactly the data that determines the semidirect product group (4).

Moreover, a homomorphism of equivariant groups (𝒢 1,α 1)(𝒢 2,α 2)(\mathcal{G}_1, \alpha_1) \longrightarrow (\mathcal{G}_2, \alpha_2) is a continuous group homomorphism ϕ:𝒢 1𝒢 2\phi \,\colon\, \mathcal{G}_1 \longrightarrow \mathcal{G}_2 whose underlying map is GG-equivariant in that

gGϕα 1(g)=α 2(g)ϕ. \underset{ g \in G }{\forall} \;\; \;\; \phi \circ \alpha_1(g) \;=\; \alpha_2(g) \circ \phi \,.

By (4) this means that ϕ\phi induces a group homomorphism of semidirect product groups of the form

𝒢 1 α 1G 𝒢 2 α 2G 𝒢 1×G ϕ×id G 𝒢 2×G \array{ \mathcal{G}_1 \rtimes_{\alpha_1} G & \overset{ }{\longrightarrow} & \mathcal{G}_2 \rtimes_{\alpha_2} G \\ \mathcal{G}_1 \times G & \overset{ \phi \times id_G }{\longrightarrow} & \mathcal{G}_2 \times G }

This construction

(6)(𝒢 1,α 1) 𝒢 1 α 1G ϕ ϕ×id G (𝒢 2,α 2) 𝒢 2 α 2G \array{ (\mathcal{G}_1, \alpha_1) && && \mathcal{G}_1 \rtimes_{\alpha_1} G \\ \big\downarrow {}^{_{\mathrlap{\phi}}} && \mapsto && \big\downarrow {}^{_{\mathrlap{\phi \times id_G}}} \\ (\mathcal{G}_2, \alpha_2) && && \mathcal{G}_2 \rtimes_{\alpha_2} G }

is clearly functorial.

It remains to see that this functor is a full subcategory-inclusion, hence a fully faithful functor, hence that it is a bijection on hom-sets for any pair of objects:

But by (6) the homomorphisms of semidirect product groups in its image are precisely those of the product form ϕ×id G\phi \times id_G, and this is exactly the form of the homomorphisms between semidirect product groups that is picked out by slicing over and under GG (by Remark ).


Here and in the following we use that a group homomorphism out of a semidirect product group (4) is fixed already by its restriction to the two canonical subgroups

𝒢 γϕ(γ,e G) 𝒢 αG ϕ 𝒢 αG gϕ(e 𝒢,g) G \array{ \mathcal{G} \\ \big\downarrow & \searrow^{ \mathrlap{ \gamma \mapsto \phi\big( \gamma, e_G \big) } } \\ \mathcal{G} \rtimes_{\alpha} G & \overset{\phi}{\longrightarrow} & \mathcal{G}' \rtimes_\alpha G \\ \big\uparrow & \nearrow_{ \mathrlap{ g \mapsto \phi\big(e_{\mathcal{G}},g\big) } } \\ G }

because every element of the semidirect product is equal to a product of elements from these subgroups:

(7)(γ,g)=(γ,e G)(e 𝒢,g). (\gamma,g) \;=\; (\gamma, e_G) \cdot ( e_{\mathcal{G}}, g ) \,.

This implies in particular that

  1. ϕ\phi being a morphism in Groups /GGroups_{/G} means equivalently that its restriction to 𝒢\mathcal{G} factors (via some group homomorphism 𝒢𝒢\mathcal{G} \to \mathcal{G}') through the canonical inclusion of 𝒢\mathcal{G}' (5);

  2. ϕ\phi being a morphism in Groups G/Groups^{G/} means equivalently that its restriction to GG is the canonical inclusion of GG (5).

Equivariant group actions


(equivariant group actions as semidirect product group actions)
Under the identification from Prop. of GG-equivariant groups (𝒢,α)\big(\mathcal{G}, \alpha \big) with semidirect product groups 𝒢 αG\mathcal{G} \rtimes_\alpha G, we have an equivalence of their actions, given by:

(8)(𝒢,α)Actions(GActions(TopologicalSpaces)) (𝒢G)Actions(TopologicalSpaces) (𝒢,α)RAut((X,ρ)) 𝒢 αG(R,ρ)Aut(X) \array{ \big( \mathcal{G}, \, \alpha \big) Actions \big( G Actions ( TopologicalSpaces ) \big) & \overset{\simeq}{ \longrightarrow } & ( \mathcal{G} \rtimes G ) Actions( TopologicalSpaces ) \\ ( \mathcal{G}, \alpha ) \overset{ R }{ \to } Aut \big( (X, \rho) \big) &\mapsto& \mathcal{G} \rtimes_\alpha G \overset{ (R,\rho) }{\longrightarrow} Aut(X) }


(9)γ,g,x(R,ρ)(γ,g)(x)R(γ)(ρ(g)(x)). \underset{\gamma,g,x}{\forall} \;\;\; (R,\rho)(\gamma, g)(x) \;\coloneqq\; R(\gamma)\big( \rho(g)(x) \big) \,.


We observe that the given formula in fact establishes a bijection between the two kinds of actions:

First, notice by the decomposition (7) in Remark , that any action of 𝒢 αG\mathcal{G} \rtimes_\alpha G can be written in the form (9) for some actions RR and ρ\rho of 𝒢\mathcal{G} and GG, respectively, satisfying some compatibility conditions:

  1. the action property of ρ\rho is equivalently the action property of (R,ρ)(R,\rho) on elements of the form (e,g)(e, g);

  2. the action property of RR on the underlying topological spaces is equivalently the action property of (R,ρ)(R,\rho) on elements of the form (γ,e)(\gamma,e);

  3. the GG-equivariance of RR

    (10)γ,g,xR(α(g)(γ))(ρ(g)(x))=ρ(g)(R(γ)(x)) \underset{ \gamma,g,x }{\forall} \;\;\; R \big( \alpha(g)(\gamma) \big) \big( \rho(g)(x) \big) \;=\; \rho(g) \big( R(\gamma)(x) \big)

    is equivalent to the action property of (R,ϕ)(R,\phi) on mixed pairs of elements of the form ((e 𝒢,g),(γ,e G))\big( (e_{\mathcal{G}},g), \; (\gamma,e_G) \big):

    γ,g,x(R,ρ)((e,g)(γ,e)(α(g)(γ),g))(x)=(R,ρ)(e,g)((R,ρ)(γ,e)(x)). \underset{ \gamma,g,x }{\forall} \;\;\; (R,\rho) \big( \underset{ \big( \alpha(g)(\gamma), g \big) }{ \underbrace{ (e,g) \cdot (\gamma,e) } } \big) (x) \;=\; (R,\rho)(e,g) \big( (R,\rho)(\gamma,e) (x) \big) \,.

These three conditions exhaust the conditions on RR to be a GG-equivariant action. Therefore it just remains to see that they also exhaust the conditions on (R,ρ)(R,\rho) to be a plain action:

But the remaining mixed action conditions on (R,ϕ)(R, \phi)

γ 1,γ 2,g,x(R,ρ)((γ 1,e)(γ 2,g)=(γ 1γ 2,g))(x)=(R,ρ)((γ 1,e))((R,ρ)((γ 2,g))(x)) \underset{ \gamma_1, \gamma_2, g, x }{\forall} \;\;\; (R,\rho) \big( \underset{ = \, (\gamma_1 \cdot \gamma_2,g) }{ \underbrace{ (\gamma_1,e) \cdot (\gamma_2,g) } } \big) (x) \;=\; (R,\rho) \big( (\gamma_1,e) \big) \Big( (R,\rho) \big( (\gamma_2,g) \big) (x) \Big)


γ,g 1,g 2,x(R,ρ)((e,g 1)(γ,g 2))(x) =(R,ρ)((α(g 1)(γ),g 1g 2))(x) =R(α(g 1)(γ))(ρ(g 1g 2)(x)) =!R(α(g 1)(γ))(ρ(g 1)(ρ(g 2)(x))) =!ρ(g 1)(R(γ)(ρ(g 2)(x))) =(R,ρ)((e,g 1))((R,ρ)((γ,g 2))(x)) \begin{aligned} \underset{ \gamma, g_1, g_2, x }{\forall} \;\;\; (R,\rho) \big( (e,g_1) \cdot (\gamma, g_2) \big) (x) & =\; (R,\rho) \Big( \big( \alpha(g_1)(\gamma), g_1 \cdot g_2 \big) \Big) (x) \\ & =\; R \big( \alpha(g_1)(\gamma) \big) \big( \rho(g_1 \cdot g_2) (x) \big) \\ & \overset{!}{=} \; R \big( \alpha(g_1)(\gamma) \big) \Big( \rho(g_1) \big( \rho(g_2) (x) \big) \Big) \\ & \overset{!}{=} \; \rho(g_1) \Big( R \big( \gamma \big) \big( \rho(g_2) (x) \big) \Big) \\ & =\; (R,\rho) \big( (e,g_1) \big) \Big( (R,\rho) \big( (\gamma, g_2) \big) (x) \Big) \end{aligned}

follow right from the definition (9) and using again (in the highlighted steps “=!\overset{!}{=}”):

In conclusion, (R,ρ)(R,\rho) is an action of the semidirect product group 𝒢 αG\mathcal{G} \rtimes_\alpha G on XX iff RR is a GG-equivariant action of 𝒢\mathcal{G} on XX.

This implies immediately that the condition for a map XXX \to X to be an action homomorphisms on both sides are the same.

And so the functor (8) is in fact an isomorphism on both objects as well as morphisms, hence in particular is an equivalence of categories.

Last revised on March 20, 2021 at 08:35:20. See the history of this page for a list of all contributions to it.