In algebra and geometry, a group action is a way of describing 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 a flexible generalization of the notion 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, 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 from G to the symmetric group of X. The action assigns a permutation of X to each element of the group in such a way that
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 widereaching theorems, such as the orbit stabilizer theorem, which can be used to prove deep results in several fields.
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If G is a group and X is a set, then a (left) group action of G on X is a binary function
denoted
which satisfies the following two axioms:
The set X is called a (left) Gset. 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 a function X × G → X by the two axioms:
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. From a right action a left action can be constructed by composing with the inverse operation on the group. If r is a right action, then
is a left action, since
and
Similarly, any left action can be converted into a right action. Therefore in the sequel we consider only left group actions, since right actions add nothing new.
The action of G on X is called
Every free action on a nonempty set is faithful. A group G acts faithfully on X if and only if the homomorphism G → Sym(X) has a trivial kernel. Thus, for a faithful action, G is isomorphic 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) — a result 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}.
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 Gx:
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. Gx = Gy.
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 orbit space.
If Y is a subset of X, we write GY for the set { g·y : y Y and g G}. We call the subset Y invariant under G if GY = Y (which is equivalent to GY ⊆ Y). In that case, G also operates on 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's 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.
For every x in X, we define the stabilizer subgroup of x (also called the isotropy group or little group) as the set of all elements in G that fix 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 G_{x} for all x in X.
Orbits and stabilizers are closely related. For a fixed x in X, consider the map from G to X given by g ↦ g·x. The image of this map is the orbit of x and the coimage is the set of all left cosets of G_{x}. The standard quotient theorem of set theory then gives a natural bijection between G /G_{x} and Gx. Specifically, the bijection is given by hG_{x} ↦ h·x. This result is known as the orbitstabilizer theorem.
If G and X are finite then the orbitstabilizer theorem, together with Lagrange's theorem, gives
This result is especially useful since it can be employed for counting arguments.
Note that if two elements x and y belong to the same orbit, then their stabilizer subgroups, G_{x} and G_{y}, are isomorphic (or conjugate). More precisely: if y = g·x, then G_{y} = gG_{x} g^{−1}. Points with conjugate stabilizer subgroups are said to have the same orbittype.
A result closely related to the orbitstabilizer theorem is Burnside's lemma:
where X^{g} 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.
The set of formal differences of finite Gsets forms a ring called the Burnside ring, where addition corresponds to disjoint union, and multiplication to Cartesian product.
The notion of group action can be put in a broader context by using the associated `action groupoid' 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 which is a `covering morphism of groupoids'. This allows a relation between such morphisms and covering maps in topology.
If X and Y are two Gsets, we define a morphism from X to Y to be a function f : X → Y such that f(g·x) = g·f(x) for all g in G and all x in X. Morphisms of Gsets are also called equivariant maps or Gmaps.
If such a function f is bijective, then its inverse is also a morphism, and we call f an isomorphism and the two Gsets X and Y are called isomorphic; for all practical purposes, they are indistinguishable in this case.
Some example isomorphisms:
With this notion of morphism, the collection of all Gsets forms a category; this category is a Grothendieck topos (in fact, assuming a classical metalogic, this topos will even be Boolean).
One often considers continuous group actions: the group G is a topological group, X is a topological space, and the map G × X → X is continuous with respect to the product topology of G × X. The space X is also called a Gspace 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 Gspaces 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 for which 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 pathconnected topological space X arises in this manner: the quotient map X ↦ X/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 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 a symmetric square.
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×X → X×X that sends (g,x)↦(gx,x) is a proper map.
If is an action of a topological group G on another topological space X, one says that it is strongly continuous if for all , the map g ↦ α_{g}x is continuous with respect to the respective topologies. Such an action induce an action on the space of continuous function on X by .
The subspace of smooth points for the action α is the subspace of X of points x such that g ↦ α_{g}(x) is smooth, i.e. it is continuous and all derivatives are continuous.
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.
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 Gsets 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.
Without using the language of categories, one can extend the notion of a group action on a set X by studying as well its induced action on the power set of X. This is useful, for instance, in studying the action of the large Mathieu group on a 24set and in studying symmetry in certain models of finite geometries.
