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 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

• the permutation of X assigned to the identity element of G is the identity transformation of X;
• the permutation of X assigned to a product gh of two elements of the group is the composite of the permutations assigned to g and h.

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 of G on X is a binary function

$G\; \backslash times\; X\; \backslash to\; X\backslash ,$

denoted

$(g,x)\backslash mapsto\; g\backslash cdot\; x\backslash ,$

which satisfies the following two axioms:

1. (gh)·x = g·(h·x) for all g, h in G and x in X;
2. e·x = x for every x in X (where e denotes the identity element of G).

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