Group action (mathematics)

TheInfoList

In
mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no general consensus abo ...
, a group action on a
space Space is the boundless extent in which and events have relative and . In , physical space is often conceived in three s, although modern s usually consider it, with , to be part of a boundless known as . The concept of space is considere ...
is a
group homomorphism Image:Group homomorphism ver.2.svg, 250px, Image of a group homomorphism (h) from G (left) to H (right). The smaller oval inside H is the image of h. N is the Kernel_(algebra)#Group_homomorphisms, kernel of h and aN is a coset of N. In mathematics ...

of a given
group A group is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal number, label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with ...
into the group of
transformations Transformation may refer to: Science and mathematics In biology and medicine * Metamorphosis, the biological process of changing physical form after birth or hatching * Malignant transformation, the process of cells becoming cancerous * Transf ...
of the space. Similarly, a group action on a
mathematical structure In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
is a group homomorphism of a group into the
automorphism group In mathematics, the automorphism group of an object ''X'' is the group (mathematics), group consisting of automorphisms of ''X''. For example, if ''X'' is a Dimension (vector space), finite-dimensional vector space, then the automorphism group of ' ...
of the structure. It is said that the group ''acts'' on the space or structure. If a group acts on a structure, it will usually also act on objects built from that structure. For example, the group of Euclidean isometries acts on
Euclidean space Euclidean space is the fundamental space of . Originally, it was the of , but in modern there are Euclidean spaces of any nonnegative integer , including the three-dimensional space and the ''Euclidean plane'' (dimension two). It was introduce ...
and also on the figures drawn in it. In particular, it acts on the set of all
triangle A triangle is a polygon In geometry, a polygon () is a plane (mathematics), plane Shape, figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The b ...

s. Similarly, the group of
symmetries Symmetry (from Ancient Greek, Greek συμμετρία ''symmetria'' "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" ...
of a
polyhedron In geometry, a polyhedron (plural polyhedra or polyhedrons) is a Three-dimensional space, three-dimensional shape with flat polygonal Face (geometry), faces, straight Edge (geometry), edges and sharp corners or Vertex (geometry), vertices. The wo ...
acts on the vertices, the edges, and the
faces The face is the front of an animal's head that features three of the head's Sense, sense organs, the eyes, nose, and mouth, and through which animals express many of their Emotion, emotions. The face is crucial for human Personal identity, ident ...
of the polyhedron. A group action on a (finite-dimensional)
vector space In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
is called a
representation Representation may refer to: Law and politics *Representation (politics) Political representation is the activity of making citizens "present" in public policy making processes when political actors act in the best interest of citizens. This defin ...
of the group. It allows one to identify many groups with subgroups of , the group of the
invertible matrices In linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and t ...
of dimension over a Field (mathematics), field . The symmetric group acts on any set (mathematics), set with elements by permuting the elements of the set. Although the group of all permutations of a set depends formally on the set, the concept of group action allows one to consider a single group for studying the permutations of all sets with the same cardinality.

# Definition

## Left group action

If is a group with identity element , and is a set, then a (''left'') ''group action'' of on is a function :$\alpha\colon G \times X \to X,$ that satisfies the following two axioms: : (with often shortened to or when the action being considered is clear from context): : for all and in and all in . The group is said to act on (from the left). A set together with an action of is called a (''left'') -''set''. From these two axioms, it follows that for any fixed in , the function from to itself which maps to is a bijection, with inverse bijection the corresponding map for . Therefore, one may equivalently define a group action of on as a group homomorphism from into the symmetric group of all bijections from to itself.

## Right group action

Likewise, a ''right group action'' of on is a function :$\alpha\colon X \times G \to X,$ that satisfies the analogous axioms: : (with often shortened to or when the action being considered is clear from context) : for all and in and all in . The difference between left and right actions is in the order in which a product acts on . For a left action, acts first, followed by second. For a right action, acts first, followed by second. Because of the formula , a left action can be constructed from a right action by composing with the inverse operation of the group. Also, a right action of a group on can be considered as a left action of its opposite group on . Thus, for establishing general properties of group actions, it suffices to consider only left actions. However, there are cases where this is not possible. For example, the multiplication of a group induces both a left action and a right action on the group itself—multiplication on the left and on the right, respectively.

# Types of actions

The action of ''G'' on ''X'' is called: * ' if ''X'' is Empty set, non-empty and if for each pair ''x'', ''y'' in ''X'' there exists a ''g'' in ''G'' such that . For example, the action of the symmetric group of ''X'' is transitive, the action of the general linear group or the special linear group of a vector space ''V'' on is transitive, but the action of the orthogonal group of a Euclidean space ''E'' is not transitive on (it is transitive on the unit sphere of ''E'', though). * ' (or ') if for every two distinct ''g'', ''h'' in ''G'' there exists an ''x'' in ''X'' such that ; or equivalently, if for each in ''G'' there exists an ''x'' in ''X'' such that . In other words, in a faithful group action, different elements of ''G'' induce different permutations of ''X''. In algebraic terms, a group ''G'' acts faithfully on ''X'' if and only if the corresponding homomorphism to the symmetric group, , has a trivial kernel (algebra), kernel. Thus, for a faithful action, ''G'' Embedding, embeds into a permutation group on ''X''; specifically, ''G'' is isomorphic to its image in Sym(''X''). If ''G'' does not act faithfully on ''X'', we can easily modify the group to obtain a faithful action. If we define , then ''N'' is a normal subgroup of ''G''; indeed, it is the kernel of the homomorphism . The factor group ''G''/''N'' acts faithfully on ''X'' by setting . The original action of ''G'' on ''X'' is faithful if and only if . The smallest set on which a faithful action can be defined can vary greatly for groups of the same size. For example: ** Three groups of size 120 are the symmetric group ''S''5, the icosahedral group, and the cyclic group $\mathbb/120\mathbb$. The smallest sets on which faithful actions can be defined are of size 5, 12, and 16 respectively. ** The abelian groups of size 2''n'' include a cyclic group $\mathbb/2^n\mathbb$ as well as $\left(\mathbb/2\mathbb\right)^n$ (the direct product of ''n'' copies of $\mathbb/2\mathbb$), but the latter acts faithfully on a set of size 2''n'', whereas the former cannot act faithfully on a set smaller than itself. * ' (or ''semiregular'' or ''fixed-point free'') if, given ''g'', ''h'' in ''G'', the existence of an ''x'' in ''X'' with implies . Equivalently: if ''g'' is a group element and there exists an ''x'' in ''X'' with (that is, if ''g'' has at least one fixed point), then ''g'' is the identity. Note that a free action on a non-empty set is faithful. * ' (or ' or ''sharply transitive'') if it is both transitive and free; this is equivalent to saying that for every two ''x'', ''y'' in ''X'' there exists precisely one ''g'' in ''G'' such that . In this case, ''X'' is called a principal homogeneous space for ''G'' or a ''G''-torsor. 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 ''X'' has at least ''n'' elements, and for all distinct ''x''1, ..., ''xn'' and all distinct ''y''1, ..., ''yn'', there is a ''g'' in ''G'' such that for . A 2-transitive action is also called ', a 3-transitive action is also called ''triply transitive'', and so on. Such actions define interesting classes of subgroups in the symmetric groups: 2-transitive groups and more generally multiply transitive groups. The action of the symmetric group on a set with ''n'' elements is always ''n''-transitive; the action of the alternating group is (''n'' − 2)-transitive. * if there is exactly one such ''g''. * ' 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 neighborhood (mathematics), neighborhood ''U'' of ''e'' in ''G'' such that the restriction of the action to ''U'' is free; that is, if for some ''x'' and some ''g'' in ''U'' then . Furthermore, if ''G'' acts on a topological space ''X'', then the action is: *''Wandering set, Wandering'' if every point ''x'' in ''X'' has a neighborhood ''U'' such that $\$ is finite. For example, the action of $\mathbb Z^n$ on $\mathbb R^n$ by translations is wandering. The action of the modular group on the Poincaré half-plane is also wandering. *''Properly discontinuous'' if ''X'' is a locally compact space and for every compact subset ''K'' ⊂ ''X'' the set $\$ is finite. The wandering actions given above are also properly discontinuous. On the other hand, the action of $\mathbb Z$ on $\mathbb^2 \setminus \$ given by $n\cdot \left(x, y\right) = \left(2^n x, 2^ y\right)$ is wandering and free but not properly discontinuous. *' if ''G'' is a topological group and the map from $G \times X \rightarrow X \times X : \left(g,x\right) \mapsto \left(g \cdot x,x\right)$ is Proper map, proper. If ''G'' is Discrete group, discrete then properness is equivalent to proper discontinuity for ''G''-actions. * Said to have ''discrete orbits'' if the orbit of each ''x'' in ''X'' under the action of ''G'' is discrete in ''X''. *A ''covering space action'' if every point ''x'' in ''X'' has a neighborhood ''U'' such that $\ = \$. If ''X'' is a Zero element#Zero module, non-zero module (mathematics), module over a Ring (mathematics), ring ''R'' and the action of ''G'' is ''R''-linear then it is said to be * ''Irreducible'' if there is no nonzero proper invariant submodule.

# Orbits and stabilizers

Consider a group ''G'' acting on a set ''X''. The ' of an element ''x'' in ''X'' is the set of elements in ''X'' to which ''x'' can be moved by the elements of ''G''. The orbit of ''x'' is denoted by $G \cdot x$: $G \cdot x = \.$ 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 a set, partition of ''X''. The associated equivalence relation is defined by saying $x \sim y$ if and only if there exists a ''g'' in ''G'' with $g \cdot x = y.$ 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, that is, $G \cdot x = G \cdot y.$ The group action is Group action (mathematics)#Types of actions, transitive if and only if it has exactly one orbit, that is, if there exists ''x'' in ''X'' with $G \cdot x = X.$ This is the case if and only if $G \cdot x = X$ for ''x'' in ''X'' (given that ''X'' is non-empty). 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 ' of the action. In geometric situations it may be called the ', while in algebraic situations it may be called the space of ', and written $X_G,$ by contrast with the invariants (fixed points), denoted ''XG'': the coinvariants are a while the invariants are a . 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'', then $G \cdot Y$ denotes the set $\.$ The subset ''Y'' is said to be ''invariant under G'' if $G \cdot Y = Y$ (which is equivalent to $G \cdot Y \subseteq Y$). In that case, ''G'' also operates on ''Y'' by restricting the action to ''Y''. The subset ''Y'' is called ''fixed under G'' if $g \cdot y = y$ for all ''g'' in ''G'' and all ''y'' in ''Y''. Every subset that is fixed under ''G'' is also invariant under ''G'', but not conversely. Every orbit is an invariant subset of ''X'' on which ''G'' acts Group action (mathematics)#Types of actions, transitively. Conversely, any invariant subset of ''X'' is a union of orbits. 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 $x \in X$ such that $g \cdot x = x$ for all $g \in G.$ The set of all such ''x'' is denoted $X_G$ and called the ''G-invariants'' of ''X''. When ''X'' is a G-module, ''G''-module, ''XG'' is the zeroth Group cohomology, 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 $g \cdot x = x,$ it is said that "''x'' is a fixed point of ''g''" or that "''g'' fixes ''x''". For every ''x'' in ''X'', the of ''G'' with respect to ''x'' (also called the ''isotropy group'' or ''little group'') is the set of all elements in ''G'' that fix ''x'': $G_x = \.$ This is a subgroup of ''G'', though typically not a normal one. The action of ''G'' on ''X'' is Group action (mathematics)#Types of actions, free if and only if all stabilizers are trivial. The kernel ''N'' of the homomorphism with the symmetric group, $G \to \operatorname\left(X\right),$ is given by the Intersection (set theory), 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 $y = g \cdot x.$ Then the two stabilizer groups $G_x$ and $G_y$ are related by $G_y = g G_x g^.$ Proof: by definition, $h \in G_y$ if and only if $h \cdot \left(g \cdot x\right) = g \cdot x.$ Applying $g^$ to both sides of this equality yields $\left\left(g^ hg\right\right) \cdot x = x;$ that is, $g^ h g \in G_x.$ An opposite inclusion follows similarly by taking $h \in G_x$ and supposing $x = g^ \cdot y.$ The above says that the stabilizers of elements in the same orbit are Conjugacy class, conjugate to each other. Thus, to each orbit, we can associate a conjugacy class of a subgroup of ''G'' (that is, the set of all conjugates of the subgroup). Let $\left(H\right)$ denote the conjugacy class of ''H''. Then the orbit ''O'' has type $\left(H\right)$ if the stabilizer $G_x$ of some/any ''x'' in ''O'' belongs to $\left(H\right)$. A maximal orbit type is often called a principal orbit type.

## and Burnside's lemma

Orbits and stabilizers are closely related. For a fixed ''x'' in ''X'', consider the map $f : G \to X$ given by $g \mapsto g \cdot x.$ By definition the image $f\left(G\right)$ of this map is the orbit $G \cdot x.$ The condition for two elements to have the same image is $f(g)=f(h) \iff g\cdot x=h \cdot x \iff g^h \cdot x=x \iff g^h \in G_x \iff h \in gG_x.$ In other words, $f\left(g\right) = f\left(h\right)$ ''if and only if'' $g$ and $h$ lie in the same coset for the stabilizer subgroup $G_x$. Thus, the Fiber (mathematics), fiber $f^\left(\\right)$ of ''f'' over any ''y'' in ''G''·''x'' is contained in such a coset, and every such coset also occurs as a fiber. Therefore ''f'' defines a between the set $G/G_x$ of cosets for the stabilizer subgroup and the orbit $G \cdot x,$ which sends $gG_x \mapsto g \cdot x$. This result is known as the ''orbit-stabilizer theorem''. If ''G'' is finite then the orbit-stabilizer theorem, together with Lagrange's theorem (group theory), Lagrange's theorem, gives $, G \cdot x, = [G\,:\,G_x] = , G, / , G_x, ,$ in other words the length of the orbit of ''x'' times the order of its stabilizer is the order of the group. In particular that implies that the orbit length is a divisor of the group order. : Example: Let ''G'' be a group of prime order ''p'' acting on a set ''X'' with ''k'' elements. Since each orbit has either 1 or ''p'' elements, there are at least $k \bmod p$ orbits of length 1 which are ''G''-invariant elements. This result is especially useful since it can be employed for counting arguments (typically in situations where ''X'' is finite as well). : Example: We can use the orbit-stabilizer theorem to count the automorphisms of a Graph (discrete mathematics), graph. Consider the cubical graph as pictured, and let ''G'' denote its Graph automorphism, automorphism group. Then ''G'' acts on the set of vertices , and this action is transitive as can be seen by composing rotations about the center of the cube. Thus, by the orbit-stabilizer theorem, $, G, = , G\cdot1, , G_1, = 8, G_1, .$ Applying the theorem now to the stabilizer $G_1,$ we can obtain $, G_1, = , \left(G_1\right)\cdot2, , \left(G_1\right)_2, .$ Any element of ''G'' that fixes 1 must send 2 to either 2, 4, or 5. As an example of such automorphisms consider the rotation around the diagonal axis through 1 and 7 by $2\pi/3$ which permutes 2,4,5 and 3,6,8, and fixes 1 and 7. Thus, $\left, \left(G_1\right)\cdot2\ = 3.$ Applying the theorem a third time gives $, \left\left(G_1\right\right)_2, = , \left\left(\left\left(G_1\right\right)_2\right\right)\cdot3, , \left\left(\left\left(G_1\right\right)_2\right\right)_3, .$ Any element of ''G'' that fixes 1 and 2 must send 3 to either 3 or 6. Reflecting the cube at the plane through 1,2,7 and 8 is such an automorphism sending 3 to 6, thus $\left, \left\left(\left\left(G_1\right\right)_2\right\right)\cdot3\ = 2$. One also sees that $\left\left(\left\left(G_1\right\right)_2\right\right)_3$ consists only of the identity automorphism, as any element of ''G'' fixing 1, 2 and 3 must also fix all other vertices, since they are determined by their adjacency to 1, 2 and 3. Combining the preceding calculations, we can now obtain $, G, = 8\cdot3\cdot2\cdot1 = 48.$ A result closely related to the orbit-stabilizer theorem is Burnside's lemma: $, X/G, =\frac\sum_ , X^g, ,$ 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. 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.

# Examples

* The ' action of any group ''G'' on any set ''X'' is defined by for all ''g'' in ''G'' and all ''x'' in ''X''; that is, every group element induces the identity function, identity permutation on ''X''. * In every group ''G'', left multiplication is an action of ''G'' on ''G'': for all ''g'', ''x'' in ''G''. This action is free and transitive (regular), and forms the basis of a rapid proof of Cayley's theorem - that every group is isomorphic to a subgroup of the symmetric group of permutations of the set ''G''. * In every group ''G'' with subgroup ''H'', left multiplication is an action of ''G'' on the set of cosets ''G/H'': for all ''g'',''a'' in ''G''. In particular if H contains no nontrivial normal subgroups of ''G'' this induces an isomorphism from ''G'' to a subgroup of the permutation group of degree ''[G : H]''. * In every group ''G'', inner automorphism, conjugation is an action of ''G'' on ''G'': . An exponential notation is commonly used for the right-action variant: ; it satisfies (. * In every group ''G'' with subgroup ''H'', conjugation is an action of ''G'' on conjugates of ''H'': for all ''g'' in ''G'' and ''K'' conjugates of ''H''. * The symmetric group S''n'' and its subgroups act on the set by permuting its elements * The symmetry group of a polyhedron acts on the set of vertices of that polyhedron. It also acts on the set of faces or the set of edges of the polyhedron. * The symmetry group of any geometrical object acts on the set of points of that object. * The automorphism group of a vector space (or graph theory, graph, or group, or ring . . .) acts on the vector space (or set of vertices of the graph, or group, or ring . . .). * The general linear group and its subgroups, particularly its Lie subgroups (including the special linear group , orthogonal group , special orthogonal group , and symplectic group ) are Lie groups that act on the vector space ''K''''n''. The group operations are given by multiplying the matrices from the groups with the vectors from ''K''''n''. * The general linear group acts on Z''n'' by natural matrix action. The orbits of its action are classified by the greatest common divisor of coordinates of the vector in Z''n''. * The affine group acts #Types of actions, transitively on the points of an affine space, and the subgroup V of the affine group (that is, a vector space) has transitive and free (that is, ''regular'') action on these points; indeed this can be used to give a definition of an Affine space#Definition, affine space. * The projective linear group and its subgroups, particularly its Lie subgroups, which are Lie groups that act on the projective space Pn(''K''). This is a quotient of the action of the general linear group on projective space. Particularly notable is , the symmetries of the projective line, which is sharply 3-transitive, preserving the cross ratio; the Möbius group is of particular interest. *The Isometry, isometries of the plane act on the set of 2D images and patterns, such as wallpaper group, wallpaper patterns. The definition can be made more precise by specifying what is meant by image or pattern, for example, a function of position with values in a set of colors. Isometries are in fact one example of affine group (action). *The sets acted on by a group ''G'' comprise the Category (mathematics), category of ''G''-sets in which the objects are ''G''-sets and the morphisms are ''G''-set homomorphisms: functions such that for every ''g'' in ''G''. * The Galois group of a field extension ''L''/''K'' acts on the field L but has only a trivial action on elements of the subfield K. Subgroups of Gal(L/K) correspond to subfields of L that contain K, that is, intermediate field extensions between L and K. * The additive group of the real numbers acts on the phase space of "well-behaved" systems in classical mechanics (and in more general dynamical systems) by time translation: if ''t'' is in R and ''x'' is in the phase space, then ''x'' describes a state of the system, and is defined to be the state of the system ''t'' seconds later if ''t'' is positive or −''t'' seconds ago if ''t'' is negative. *The additive group of the real numbers acts on the set of real functions of a real variable in various ways, with (''t''⋅''f'')(''x'') equal to, for example, , , , , , or , but not . *Given a group action of ''G'' on ''X'', we can define an induced action of ''G'' on the power set of ''X'', by setting for every subset ''U'' of ''X'' and every ''g'' in ''G''. This is useful, for instance, in studying the action of the large Mathieu group on a 24-set and in studying symmetry in certain models of finite geometry, finite geometries. * The quaternions with Norm of a quaternion, norm 1 (the versors), as a multiplicative group, act on R3: for any such quaternion , the mapping is a counterclockwise rotation through an angle ''α'' about an axis given by a unit vector v; ''z'' is the same rotation; see quaternions and spatial rotation. Note that this is not a faithful action because the quaternion −1 leaves all points where they were, as does the quaternion 1. * Given left ''G''-sets $X,Y$, there is a left ''G''-set $Y^X$ whose elements are ''G''-equivariant maps $\alpha:X\times G\to Y$, and with left ''G''-action given by $g\cdot\alpha=\alpha\circ \left(id_X\times-g\right)$ (where "$-g$" indicates right multiplication by $g$). This ''G''-set has the property that its fixed points correspond to equivariant maps $X\to Y$; more generally, it is an exponential object in the category of ''G''-sets.

# Group actions and groupoids

The notion of group action can be put in a broader context by using the ''action groupoid'' $G\text{'}=G \ltimes X$ associated to the group action, thus allowing techniques from groupoid theory such as presentations and fibrations. Further the stabilizers 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′'' → ''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, a ''morphism'' from ''X'' to ''Y'' is a function such that 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. In this case ''f'' is called an ''isomorphism'', and the two ''G''-sets ''X'' and ''Y'' are called ''isomorphic''; for all practical purposes, isomorphic ''G''-sets are indistinguishable. 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 , where ''S'' is some set and ''G'' acts on 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.) With this notion of morphism, the collection of all ''G''-sets forms a category theory, category; this category is a Grothendieck topos (in fact, assuming a classical metalogic, this topos will even be Boolean).

# Continuous group actions

One often considers ''continuous group actions'': the group ''G'' is a topological group, ''X'' is a topological space, and the map is continuous function (topology), continuous with respect to the product topology of . 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 space, 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 ''X'' is a Covering map#Deck transformation group, regular covers, regular covering space of another topological space ''Y'', then the action of the Covering map#Deck transformation group, regular covers, 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 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 generalized 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 to . 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 . 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 that sends 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 is continuous with respect to the respective topologies. Such an action induces an action on the space of continuous functions on ''X'' by defining 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 is smooth, that is, it is continuous and all derivatives are continuous.

# Variants and generalizations

We 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, we 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. We can view a group ''G'' as a category with a single object in which every morphism is invertible. A (left) group action is then nothing but a (covariant) 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 group action, continuous actions of topological groups on topological spaces, one also often considers Lie group action, smooth actions of Lie groups on manifold, smooth manifolds, regular actions of algebraic groups on algebraic variety, algebraic varieties, and group-scheme action, actions of group schemes on scheme (mathematics), schemes. All of these are examples of group objects acting on objects of their respective category.

# Gallery

File:Octahedral-group-action.png, Orbit of a fundamental spherical triangle (marked in red) under action of the full octahedral group. File:Icosahedral-group-action.png, Orbit of a fundamental spherical triangle (marked in red) under action of the full icosahedral group.

* Gain graph * Group with operators * Measurable group action * Monoid action

# References

* * Brown, Ronald (2006)
''Topology and groupoids''
Booksurge PLC, .

downloadable reprint of van Nostrand Notes in Mathematics, 1971, which deal with applications of groupoids in group theory and topology. * * * *