Cayley's theorem

In group theory, Cayley's theorem, named in honour of Arthur Cayley, states that every group G is isomorphic to a subgroup of the symmetric group acting on G.[1] This can be understood as an example of the group action of G on the elements of G.[2] The theorem can be obtained by explicitly constructing the representation within the representation of the symmetric group of permutation matrices, which is sometimes known as the regular representation.

A permutation of a set G is any bijective function taking G onto G. The set of all permutations of G forms a group under function composition, called the symmetric group on G, and written as Sym(G).[3]

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 that are true for subgroups of permutation groups are true for groups in general. Nevertheless, Alperin and Bell note that "in general the fact that finite groups are imbedded in symmetric groups has not influenced the methods used to study finite groups".[4]

The regular action used in the standard proof of Cayley's theorem does not produce the representation of G in a minimal-order permutation group. For example, , itself already a symmetric group of order 6, would be represented by the regular action as a subgroup of (a group of order 720).[5] Indeed, the complex regular representation of finite groups is actually formed by the direct sum of all the irreducible representations with multiplicity equal to their dimension. The problem of finding an embedding of a group in a minimal-order symmetric group is rather difficult.[6][7]

HistoryEdit

While it seems elementary enough, at the time the modern definitions didn't exist, and when Cayley introduced what are now called groups it wasn't immediately clear that this was equivalent to the previously known groups, which are now called permutation groups. Cayley's theorem unifies the two.

Although Burnside[8] attributes the theorem to Jordan,[9] Eric Nummela[10] nonetheless argues that the standard name—"Cayley's Theorem"—is in fact appropriate. Cayley, in his original 1854 paper,[11] showed that the correspondence in the theorem is one-to-one, but he failed to explicitly show it was a homomorphism (and thus an embedding). However, Nummela notes that Cayley made this result known to the mathematical community at the time, thus predating Jordan by 16 years or so.

The theorem was later published by Walther Dyck in 1882[12] and is attributed to Dyck in the first edition of Burnside's book.[13]

Proof of the theoremEdit

If g is any element of a group G with operation ∗, consider the function fg : GG, defined by fg(x) = gx. By the existence of inverses, this function has a two-sided inverse,  . So multiplication by g acts as a bijective function. Thus, fg is a permutation of G, and so is a member of Sym(G).

The set K = {fg : gG} is a subgroup of Sym(G) that is isomorphic to G. The fastest way to establish this is to consider the function T : G → Sym(G) with T(g) = fg for every g in G. T is a group homomorphism because (using · to denote composition in Sym(G)):

 

for all x in G, and hence:

 

The homomorphism T is injective since T(g) = idG (the identity element of Sym(G)) implies that gx = x for all x in G, and taking x to be the identity element e of G yields g = ge = e, i.e. the kernel is trivial. Alternatively, T is also injective since gx = g′ ∗ x implies that g = g (because every group is cancellative).

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

T is sometimes called the regular representation of G.

Alternative setting of proofEdit

An alternative setting uses the language of group actions. We consider the group   as acting on itself by left multiplication, i.e.  , which has a permutation representation, say  .

The representation is faithful if   is injective, that is, if the kernel of   is trivial. Suppose  . Then,  . Thus,   is trivial. The result follows by use of the first isomorphism theorem, from which we get  .

Remarks on the regular group representationEdit

The identity element of the group corresponds to the identity permutation. All other group elements correspond to derangements: permutations that do 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 that consists of cycles all of the same length: this length is the order of that element. The elements in each cycle form a right coset of the subgroup generated by the element.

Examples of the regular group representationEdit

Z2 = {0,1} with addition modulo 2; group element 0 corresponds to the identity permutation e, group element 1 to permutation (12). E.g. 0 +1 = 1 and 1+1 = 0, so 1 -> 0 and 0 -> 1, as they would under a permutation.

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, and the latter is how it is realized by its regular representation.

* 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)

More general statement of the theoremEdit

A more general statement of Cayley's theorem consist of considering the core of an arbitrary group  . In general if   is a group and   is a subgroup with  , then   is isomorphic to a subgroup of  . In particular if   is a finite group and we set   then we get the classic result.

See alsoEdit

NotesEdit

  1. ^ Jacobson (2009, p. 38)
  2. ^ Jacobson (2009, p. 72, ex. 1)
  3. ^ Jacobson (2009, p. 31)
  4. ^ J. L. Alperin; Rowen B. Bell (1995). Groups and representations. Springer. p. 29. ISBN 978-0-387-94525-5.
  5. ^ Peter J. Cameron (2008). Introduction to Algebra, Second Edition. Oxford University Press. p. 134. ISBN 978-0-19-852793-0.
  6. ^ Johnson, D. L. (1971). "Minimal Permutation Representations of Finite Groups". American Journal of Mathematics. 93 (4): 857. doi:10.2307/2373739. JSTOR 2373739.
  7. ^ Grechkoseeva, M. A. (2003). "On Minimal Permutation Representations of Classical Simple Groups". Siberian Mathematical Journal. 44 (3): 443–462. doi:10.1023/A:1023860730624.
  8. ^ Burnside, William (1911), Theory of Groups of Finite Order (2 ed.), Cambridge, p. 22, ISBN 0-486-49575-2
  9. ^ Jordan, Camille (1870), Traite des substitutions et des equations algebriques, Paris: Gauther-Villars
  10. ^ Nummela, Eric (1980), "Cayley's Theorem for Topological Groups", American Mathematical Monthly, Mathematical Association of America, 87 (3): 202–203, doi:10.2307/2321608, JSTOR 2321608
  11. ^ Cayley, Arthur (1854), "On the theory of groups as depending on the symbolic equation θn=1", Philosophical Magazine, 7 (42): 40–47
  12. ^ von Dyck, Walther (1882), "Gruppentheoretische Studien" [Group-theoretical Studies], Mathematische Annalen, 20 (1): 30, doi:10.1007/BF01443322, hdl:2027/njp.32101075301422, ISSN 0025-5831. (in German)
  13. ^ Burnside, William (1897), Theory of Groups of Finite Order (1 ed.), Cambridge, p. 22

ReferencesEdit