# Group action

{{#invoke:Hatnote|hatnote}} Given an equilateral triangle, the counterclockwise rotation by 120° around the center of the triangle maps every vertex of the triangle to another one. The cyclic group C3 consisting of the rotations by 0°, 120° and 240° acts on the set of the three vertices.

In algebra and geometry, a group action is a description of symmetries of objects using groups. The essential elements of the object are described by a set, and the symmetries of the object are described by the symmetry group of this set, which consists of bijective transformations of the set. In this case, the group is also called a permutation group (especially if the set is finite or not a vector space) or transformation group (especially if the set is a vector space and the group acts like linear transformations of the set).

A group action is an extension to the definition of a symmetry group in which every element of the group "acts" like a bijective transformation (or "symmetry") of some set, without being identified with that transformation. This allows for a more comprehensive description of the symmetries of an object, such as a polyhedron, by allowing the same group to act on several different sets of features, such as the set of vertices, the set of edges and the set of faces of the polyhedron.

If G is a group and X is a set, then a group action may be defined as a group homomorphism h from G to the symmetric group on X. The action assigns a permutation of X to each element of the group in such a way that the permutation of X assigned to:

Since each element of G is represented as a permutation, a group action is also known as a permutation representation.

The abstraction provided by group actions is a powerful one, because it allows geometrical ideas to be applied to more abstract objects. Many objects in mathematics have natural group actions defined on them. In particular, groups can act on other groups, or even on themselves. Despite this generality, the theory of group actions contains wide-reaching theorems, such as the orbit stabilizer theorem, which can be used to prove deep results in several fields.

## Definition

If G is a group and X is a set, then a (left) group action φ of G on X is a function

$\varphi :G\times X\to X:(g,x)\mapsto \varphi (g,x)$ that satisfies the following two axioms (where we denote φ(g, x) as g.x):

Compatibility
(gh).x = g.(h.x) for all g, h in G and all x in X. (Here, gh denotes the result of applying the group operation of G to the elements g and h.)
Identity
e.x = x for all x in X. (Here, e denotes the neutral element of the group G.)

The set X is called a (left) G-set. The group G is said to act on X (on the left).

From these two axioms, it follows that for every g in G, the function which maps x in X to g.x is a bijective map from X to X (its inverse being the function which maps x to g−1.x). Therefore, one may alternatively define a group action of G on X as a group homomorphism from G into the symmetric group Sym(X) of all bijections from X to X.

In complete analogy, one can define a right group action of G on X as an operation X × GX mapping (x, g) to x.g and satisfying the two axioms:

Compatibility
x.(gh) = (x.g).h for all g, h in G and all x in X;
Identity
x.e = x for all x in X.

The difference between left and right actions is in the order in which a product like gh acts on x. For a left action h acts first and is followed by g, while for a right action g acts first and is followed by h. Because of the formula (gh)−1 = h−1g−1, one can construct a left action from a right action by composing with the inverse operation of the group. Also, a right action of a group G on X is the same thing as a left action of its opposite group Gop on X. It is thus sufficient to only consider left actions without any loss of generality.

## Types of actions

The action of G on X is called

• Template:Visible anchor if X is non-empty and if for any x, y in X there exists a g in G such that g.x = y.
• Template:Visible anchor (or Template:Visible anchor) if for any two distinct g, h in G there exists an x in X such that g.xh.x; or equivalently, if for any ge in G there exists an x in X such that g.xx. Intuitively, in a faithful group action, different elements of G induce different permutations of X.
• Template:Visible anchor (or semiregular or fixed point free) if, given g, h in G, the existence of an x in X with g.x = h.x implies g = h. Equivalently: if g is a group element and there exists an x in X with g.x = x (that is, if g has at least one fixed point), then g is the identity.
• Template:Visible anchor (or Template:Visible anchor or sharply transitive) if it is both transitive and free; this is equivalent to saying that for any two x, y in X there exists precisely one g in G such that g.x = y. In this case, X is known as a principal homogeneous space for G or as a G-torsor.
• Template:Visible anchor if X has at least n elements and for any pairwise distinct x1, …, xn and pairwise distinct y1, …, yn there is a g in G such that g·xk = yk for 1 ≤ kn. A 2-transitive action is also called Template:Visible anchor, a 3-transitive action is also called triply transitive, and so on. Such actions define 2-transitive groups, 3-transitive groups, and multiply transitive groups.
• Template:Visible anchor if it is transitive and preserves no non-trivial partition of X. See primitive permutation group for details.
• Locally free if G is a topological group, and there is a neighbourhood U of e in G such that the restriction of the action to U is free; that is, if g.x = x for some x and some g in U then g = e.
• Irreducible if X is a non-zero module over a ring R, the action of G is R-linear, and there is no nonzero proper invariant submodule.

Every free action on a non-empty set is faithful. A group G acts faithfully on X if and only if the corresponding homomorphism G → Sym(X) has a trivial kernel. Thus, for a faithful action, G embeds into to a permutation group on X; specifically, G is isomorphic to its image in Sym(X).

The action of any group G on itself by left multiplication is regular, and thus faithful as well. Every group can, therefore, be embedded in the symmetric group on its own elements, Sym(G). This result is known as Cayley's theorem.

If G does not act faithfully on X, one can easily modify the group to obtain a faithful action. If we define N = {g in G : g.x = x for all x in X}, then N is a normal subgroup of G; indeed, it is the kernel of the homomorphism G → Sym(X). The factor group G/N acts faithfully on X by setting (gN).x = g.x. The original action of G on X is faithful if and only if N = {e}.

## Orbits and stabilizers In the compound of five tetrahedra, the symmetry group is the (rotational) icosahedral group I of order 60, while the stabilizer of a single chosen tetrahedron is the (rotational) tetrahedral group T of order 12, and the orbit space I/T (of order 60/12 = 5) is naturally identified with the 5 tetrahedra – the coset gT corresponds to which tetrahedron g sends the chosen tetrahedron to.

Consider a group G acting on a set X. The orbit of a point x in X is the set of elements of X to which x can be moved by the elements of G. The orbit of x is denoted by G.x:

$G.x=\left\{g.x\mid g\in G\right\}.$ The defining properties of a group guarantee that the set of orbits of (points x in) X under the action of G form a partition of X. The associated equivalence relation is defined by saying x ~ y if and only if there exists a g in G with g.x = y. The orbits are then the equivalence classes under this relation; two elements x and y are equivalent if and only if their orbits are the same; i.e., G.x = G.y.

The group action is transitive if and only if it has only one orbit, i.e. if there exists x in X with G.x = X. This is the case if and only if G.x = X for all x in X.

The set of all orbits of X under the action of G is written as X/G (or, less frequently: G\X), and is called the quotient of the action. In geometric situations it may be called the Template:Visible anchor, while in algebraic situations it may be called the space of Template:Visible anchor, and written XG, by contrast with the invariants (fixed points), denoted XG: the coinvariants are a quotient while the invariants are a subset. The coinvariant terminology and notation are used particularly in group cohomology and group homology, which use the same superscript/subscript convention.

### Invariant subsets

If Y is a subset of X, we write GY for the set { g.y : yY and gG}. We call the subset Y invariant under G if G.Y = Y (which is equivalent to G.YY). In that case, G also operates on Y by restricting the action to Y. The subset Y is called fixed under G if g.y = y for all g in G and all y in Y. Every subset that is fixed under G is also invariant under G, but not vice versa.

Every orbit is an invariant subset of X on which G acts transitively. The action of G on X is transitive if and only if all elements are equivalent, meaning that there is only one orbit.

A G-invariant element of X is xX such that g.x = x for all gG. The set of all such x is denoted XG and called the G-invariants of X. When X is a G-module, XG is the zeroth group cohomology group of G with coefficients in X, and the higher cohomology groups are the derived functors of the functor of G-invariants.

### Fixed points and stabilizer subgroups

Given g in G and x in X with g.x = x, we say x is a fixed point of g and g fixes x.

For every x in X, we define the stabilizer subgroup of x (also called the isotropy group) as the set of all elements in G that fix x:

$G_{x}=\{g\in G\mid g.x=x\}.$ This is a subgroup of G, though typically not a normal one. The action of G on X is free if and only if all stabilizers are trivial. The kernel N of the homomorphism G → Sym(X) is given by the intersection of the stabilizers Gx for all x in X. If N is trivial, the action is said to be faithful (or effective).

Let x and y be two elements in X, and let g be a group element such that y = g.x. Then the two stabilizer groups Gx and Gy are related by Gy = g Gx g−1. Proof: by definition, hGy if and only if h.(g.x) = g.x. Applying g−1 to both sides of this equality yields (g−1hg).x = (g−1g).x = x; that is, g−1hgGx.

The above says that the stabilizers of elements in the same orbit are conjugate to each other. Thus, to each orbit, one can associate a conjugacy class of a subgroup of G (i.e., the set of all conjugates of the subgroup). Let $(H)$ denote the conjugacy class of H. Then one says that the orbit O has type $(H)$ if the stabilizer $G_{x}$ of some/any x in O belongs to $(H)$ .

### Orbit-stabilizer theorem and Burnside's lemma

Orbits and stabilizers are closely related. For a fixed x in X, consider the map from G to X given by gg.x for all gG. The image of this map is the orbit of x and the coimage is the set of all left cosets of Gx. The standard quotient theorem of set theory then gives a natural bijection between G/Gx and G.x. Specifically, the bijection is given by hGxh.x. This result is known as the orbit-stabilizer theorem. From a more categorical perspective, the orbit-stabilizer theorem comes from the fact that every G-set is a sum of quotients of the G-set G.

If G and X are finite then the orbit-stabilizer theorem, together with Lagrange's theorem, gives

$|G.x|=[G\,:\,G_{x}]=|G|/|G_{x}|.$ This result is especially useful since it can be employed for counting arguments.

A result closely related to the orbit-stabilizer theorem is Burnside's lemma:

$\left|X/G\right|={\frac {1}{\left|G\right|}}\sum _{g\in G}\left|X^{g}\right|$ where Xg is the set of points fixed by g. This result is mainly of use when G and X are finite, when it can be interpreted as follows: the number of orbits is equal to the average number of points fixed per group element.

Fixing a group G, the set of formal differences of finite G-sets forms a ring called the Burnside ring of G, where addition corresponds to disjoint union, and multiplication to Cartesian product.

## Group actions and groupoids

The notion of group action can be put in a broader context by using the action groupoid $G'\;=\;G\,\ltimes \,X$ associated to the group action, thus allowing techniques from groupoid theory such as presentations and fibrations. Further the stabilisers of the action are the vertex groups, and the orbits of the action are the components, of the action groupoid. For more details, see the book Topology and groupoids referenced below.

This action groupoid comes with a morphism $p:\;G'\,\rightarrow \,G$ which is a covering morphism of groupoids. This allows a relation between such morphisms and covering maps in topology.

## Morphisms and isomorphisms between G-sets

If X and Y are two G-sets, we define a morphism from X to Y to be a function f : XY such that f(g.x) = g.f(x) for all g in G and all x in X. Morphisms of G-sets are also called equivariant maps or G-maps.

The composition of two morphisms is again a morphism.

If a morphism f is bijective, then its inverse is also a morphism, and we call f an isomorphism and the two G-sets X and Y are called isomorphic; for all practical purposes, they are indistinguishable in this case.

Some example isomorphisms:

• Every regular G action is isomorphic to the action of G on G given by left multiplication.
• Every free G action is isomorphic to G × S, where S is some set and G acts on G × S by left multiplication on the first coordinate. (S can be taken to be the set of orbits X/G.)
• Every transitive G action is isomorphic to left multiplication by G on the set of left cosets of some subgroup H of G. (H can be taken to be the stabilizer group of any element of the original G-set.the original action.)

With this notion of morphism, the collection of all G-sets forms a category; this category is a Grothendieck topos (in fact, assuming a classical metalogic, this topos will even be Boolean).

## Continuous group actions

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One often considers continuous group actions: the group G is a topological group, X is a topological space, and the map G × XX is continuous with respect to the product topology of G × X. The space X is also called a G-space in this case. This is indeed a generalization, since every group can be considered a topological group by using the discrete topology. All the concepts introduced above still work in this context, however we define morphisms between G-spaces to be continuous maps compatible with the action of G. The quotient X/G inherits the quotient topology from X, and is called the quotient space of the action. The above statements about isomorphisms for regular, free and transitive actions are no longer valid for continuous group actions.

If G is a discrete group acting on a topological space X, the action is properly discontinuous if for any point x in X there is an open neighborhood U of x in X, such that the set of all g in G for which $g(U)\,\cap \,U\;\neq \;\emptyset$ consists of the identity only. If X is a regular covering space of another topological space Y, then the action of the deck transformation group on X is properly discontinuous as well as being free. Every free, properly discontinuous action of a group G on a path-connected topological space X arises in this manner: the quotient map XX/G is a regular covering map, and the deck transformation group is the given action of G on X. Furthermore, if X is simply connected, the fundamental group of X/G will be isomorphic to G.

These results have been generalised in the book Topology and Groupoids referenced below to obtain the fundamental groupoid of the orbit space of a discontinuous action of a discrete group on a Hausdorff space, as, under reasonable local conditions, the orbit groupoid of the fundamental groupoid of the space. This allows calculations such as the fundamental group of the symmetric square of a space X, namely the orbit space of the product of X with itself under the twist action of the cyclic group of order 2 sending (x, y) to (y, x).

An action of a group G on a locally compact space X is cocompact if there exists a compact subset A of X such that GA = X. For a properly discontinuous action, cocompactness is equivalent to compactness of the quotient space X/G.

The action of G on X is said to be proper if the mapping G × XX × X that sends (g, x) ↦ (g.x, x) is a proper map.

### Strongly continuous group action and smooth points

A group action of a topological group G on a topological space X is said to be strongly continuous if for all x in X, the map gg.x is continuous with respect to the respective topologies. Such an action induces an action on the space of continuous functions on X by defining (g.f)(x) = f(g−1.x) for every g in G, f a continuous function on X, and x in X. Note that, while every continuous group action is strongly continuous, the converse is not in general true.

The subspace of smooth points for the action is the subspace of X of points x such that gg.x is smooth; i.e., it is continuous and all derivativesTemplate:Where are continuous.

## Variants and generalizations

One can also consider actions of monoids on sets, by using the same two axioms as above. This does not define bijective maps and equivalence relations however. See semigroup action.

Instead of actions on sets, one can define actions of groups and monoids on objects of an arbitrary category: start with an object X of some category, and then define an action on X as a monoid homomorphism into the monoid of endomorphisms of X. If X has an underlying set, then all definitions and facts stated above can be carried over. For example, if we take the category of vector spaces, we obtain group representations in this fashion.

One can view a group G as a category with a single object in which every morphism is invertible. A group action is then nothing but a functor from G to the category of sets, and a group representation is a functor from G to the category of vector spaces. A morphism between G-sets is then a natural transformation between the group action functors. In analogy, an action of a groupoid is a functor from the groupoid to the category of sets or to some other category.

In addition to continuous actions of topological groups on topological spaces, one also often considers smooth actions of Lie groups on smooth manifolds, regular actions of algebraic groups on algebraic varieties, and actions of group schemes on schemes. All of these are examples of group objects acting on objects of their respective category.