In group theory, Cayley's theorem, named in honor of Arthur Cayley, states that every group G is isomorphic to a subgroup of the symmetric group on G. This can be understood as an example of the group action of G on the elements of G.

A permutation of a set G is any bijective function taking G onto G; and the set of all such functions forms a group under function composition, called the symmetric group on G, and written as Sym(G).

Cayley's theorem puts all groups on the same footing, by considering any group (including infinite groups such as (R,+)) as a permutation group of some underlying set. Thus, theorems which are true for permutation groups are true for groups in general.

Proof of the theorem

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From elementary group theory, we can see that for any element g in G, we must have g*G = G; and by cancellation rules, that g*x = g*y if and only if x = y. So multiplication by g acts as a bijective function f_g : G → G, by defining f_g(x) = g*x. Thus, f_g is a permutation of G, and so is a member of Sym(G).

The subset K of Sym(G) defined as K = {f_g : g in G and f_g(x) = g*x for all x in G} is a subgroup of Sym(G) which is isomorphic to G. The fastest way to establish this is to consider the function T : G → Sym(G) with T(g) = f_g for every g in G. T is a group homomorphism because (using "•" for composition in Sym(G)):(f_g • f_h)(x) = f_g(f_h(x)) = f_g(h*x) = g*(h*x) = (g*h)*x = f_(g*h)(x), for all x in G, and hence: T(g) • T(h) = f_g • f_h = f_(g*h) = T(g*h). The homomorphism T is also injective since T(g) = id_G (the identity element of Sym(G)) implies that g*x = x for all x in G, and taking x to be the identity element e of G yields g = g*e = e.

Thus G is isomorphic to the image of T, which is the subgroup K considered earlier.

T is sometimes called the regular representation of G.

Alternate setting of proof

An alternate setting uses the language of group actions. We consider the group $G$ as a G-set, which can be shown to have permutation representation, say $\backslash phi$.

Firstly, suppose $G=G/H$ with $H=\backslash \{e\backslash \}$. Then the group action is $g.e$ by classification of G-orbits (also known as the orbit-stabilizer theorem).

Now, the representation is faithful if $\backslash phi$ is injective, that is, if the kernel of $\backslash phi$ is trivial. Suppose $g$ ∈ ker $\backslash phi$ Then, $g=g.e=\backslash phi(g).e$ by the equivalence of the permutation representation and the group action. But since $g$ ∈ ker $\backslash phi$, $\backslash phi(g)=e$ and thus ker $\backslash phi$ is trivial. Then im and thus the result follows by use of the first isomorphism theorem.

Remarks on the regular group representation

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The identity group element corresponds to the identity permutation. All other group elements correspond to a permutation that does not leave any element unchanged. Since this also applies for powers of a group element, lower than the order of that element, each element corresponds to a permutation which consists of cycles which are of the same length: this length is the order of that element. The elements in each cycle form a left coset of the subgroup generated by the element.

Examples of the regular group representation

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Z₂ = {0,1} with addition modulo 2; group element 0 corresponds to the identity permutation e, group element 1 to permutation (12).

Z₃ = {0,1,2} with addition modulo 3; group element 0 corresponds to the identity permutation e, group element 1 to permutation (123), and group element 2 to permutation (132). E.g. 1 + 1 = 2 corresponds to (123)(123)=(132).

Z₄ = {0,1,2,3} with addition modulo 4; the elements correspond to e, (1234), (13)(24), (1432).

The elements of Klein four-group {e, a, b, c} correspond to e, (12)(34), (13)(24), and (14)(23).

S₃ (dihedral group of order 6) is the group of all permutations of 3 objects, but also a permutation group of the 6 group elements:

*	e	a	b	c	d	f	permutation
e	e	a	b	c	d	f	e
a	a	e	d	f	b	c	(12)(35)(46)
b	b	f	e	d	c	a	(13)(26)(45)
c	c	d	f	e	a	b	(14)(25)(36)
d	d	c	a	b	f	e	(156)(243)
f	f	b	c	a	e	d	(165)(234)

Group theory | Permutations | Mathematical theorems | Proofs

Satz von Cayley | Théorème de Cayley | משפט קיילי | Twierdzenie Cayley'a | Cayleyn lause