View source for Order of a group

{{basicdef}}

{{arithmetic function on groups}}
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==Definition==
{{quick phrase|size of a group, cardinality of a group, size of the underlying set, number of elements}}
===Symbol-free definition===

The '''order''' of a [[group]] is the cardinality (i.e., size, or number of elements) of its underlying set.

===Definition with symbols===

The '''order''' of a [[group]] <math>G</math> is the cardinality (i.e., size, or number of elements) of <math>G</math> as a set. it is denoted as <math>\left| G \right|</math>.

Note that a [[finite group]] is a group whose underlying set is finite, i.e., the size of the underlying set is finite. The order of a finite group is thus a [[natural number]] (note that the order cannot be zero because every group contains the identity element and is hence nonempty). For an infinite group, the order is an infinite cardinal.

==Examples==

* The [[trivial group]], which is the group with only the identity element, has order <math>1</math>. In fact, it is the ''only'' group (up to isomorphism}) that has order 1.

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(see the next section for more examples).

==Operations on groups and effects on the order==

===Operations that admit a clear formula===

{| class="sortable" border="1"
! Operation !! Input groups and their orders !! Output group and its order !! Proof and comment
|-
| [[external direct product]] of two groups || <math>G</math> has order <math>a</math>, <math>H</math> has order <math>b</math> || <math>G \times H</math> has order <math>ab</math> || [[order of direct product is product of orders]]; the same formula works for [[internal direct product]], which is equivalent to external direct product.
|-
| [[external direct product]] of finitely many groups || <math>G_1, G_2, \dots, G_n</math> with orders <math>a_1, a_2, \dots, a_n</math> respectively || <math>G_1 \times G_2 \times \dots \times G_n</math> has order <math>\prod_{i=1}^n a_i = a_1a_2 \dots a_n</math> || [[order of direct product is product of orders]]; same formula works for internal direct product
|-
| [[external semidirect product]] of two groups || <math>G</math>, order <math>a</math>, <math>H</math>, order <math>b</math>, acting on it || <math>G \rtimes H</math> has order <math>ab</math> || [[order of semidirect product is product of orders]]; same formula works for [[internal semidirect product]]
|-
| group extension || normal subgroup <math>N</math>, order <math>a</math>, quotient group <math>G/N</math>, order <math>b</math> || <math>ab</math> || follows from [[Lagrange's theorem]]
|-
| product of two subgroups || <math>H</math>, order <math>a</math>, <math>K</math> order <math>b</math>, <math>H \cap K</math> has order <math>c</math> || <math>ab/c</math> || see [[product formula]], also [[second isomorphism theorem]]
|-
| [[external wreath product]] || <math>H</math> order <math>a</math>, <math>K</math> with wreathing action, order <math>b</math>, acting on a set of size <math>n</math> || <math>H \wr K</math> has order <math>a^nb</math> || Note that when we take the regular action, <math>n = b</math> and we get <math>a^bb</math>. When <math>K</math> is the symmetric group of degree <math>n</math> acting naturally on a set of size <math>n</math>, we get <math>a^nn!</math>.
|}

===Operations that do not admit a clear formula===

These include, for instance, measuring the sizes of homomorphism sets between two groups.

==Particular cases and general patterns==

See also [[number of groups of given order]] for more information on how the number of groups of order <math>n</math> depends on <math>n</math>.

===Small orders===

{| class="wikitable" border="1"
! Number !! Groups of that order !! More collected information on the list
|-
| 1 || [[trivial group]] || --
|-
| 2 || [[cyclic group:Z2]] || --
|-
| 3 || [[cyclic group:Z3]] || --
|-
| 4 || [[cyclic group:Z4]], [[Klein four-group]] ||
|-
| 5 || [[cyclic group:Z5]] || --
|-
| 6 || [[symmetric group:S3]], [[cyclic group:Z6]] || 
|-
| 7 || [[cyclic group:Z7]] || --
|-
| 8 || [[cyclic group:Z8]], [[direct product of Z4 and Z2]], [[dihedral group:D8]], [[quaternion group]], [[elementary abelian group:E8]] || [[groups of order 8]]
|-
| 16 || (14 groups, too long to list) || [[groups of order 16]]
|}
 
===General facts===

{| class="wikitable" border="1"
! Nature of order !! What we can say about groups of that order !! Proof
|-
| 1 || unique group: [[trivial group]] ||
|-
| prime <math>p</math> || unique up to isomorphism: [[group of prime order]], which is cyclic and is the additive group of a [[prime field]] || [[equivalence of definitions of group of prime order]]
|-
| <math>p^2</math>, <math>p</math> prime || [[cyclic group of prime-square order]], [[elementary abelian group of prime-square order]] || ?
|-
| <math>p^3</math>, <math>p</math> prime || 5 groups -- 3 abelian and 2 non-abelian ||
|-
| [[cyclicity-forcing number]] -- product of distinct primes such that no prime divides any other prime minus one || unique [[cyclic group]] of that order || [[classification of cyclicity-forcing numbers]]
|}

==Divisibility relations==

===Divisibility relations for constructed objects===

{| class="wikitable" border="1"
! Construct !! Divisibility statement related to order !! Proof !! Comments
|-
| [[subgroup]] || order of subgroup divides order of group || [[Lagrange's theorem]] || Not every order dividing the order of the group is realized as the order of a subgroup, see [[group having subgroups of all orders dividing the group order]]
|-
| [[quotient group]] || order of quotient group divides order of group || [[order of quotient group divides order of group]] -- corollary to Lagrange's theorem ||
|-
| [[order of an element|order of element in the group]] || order of element divides order of group || [[order of element divides order of group]] || Follows from Lagrange's theorem because the order of an element is the order of the cyclic subgroup it generates.
|-
| set on which the group acts transitively || size of such a set divides the order of the group || [[fundamental theorem of group actions]] identifies any set with a transitive group action with the left coset space of a stabilizer, then we use [[Lagrange's theorem]] ||
|-
| [[conjugacy class]] || size of conjugacy class divides order of group || [[size of conjugacy class divides order of group]] || in fact, it divides order of [[inner automorphism group]] and is strictly smaller for a non-abelian group
|-
| [[degree of irreducible representation]] || degree of irreducible representation over a [[splitting field]] divides order of group || [[degree of irreducible representation divides order of group]] || in fact, [[degree of irreducible representation divides index of abelian normal subgroup]]
|}

===Divisibility relations for other arithmetic functions===

Any arithmetic function that arises as the maximum or lcm of a bunch of numbers each of which divides the order of the group, must again divide the order of the group. Some examples are given below:

{| class="wikitable" border="1"
! Arithmetic function !! Description !! Proof that it divides order
|-
| [[exponent of a group|exponent]] || lcm of orders of all elements || [[exponent divides order]]
|}

==Analogues==

An order is a size measure that works for finite groups. For infinite groups, the order, viewed as an infinite cardinal, is a very crude size measure since it is unable to differentiate between the group and subgroups of finite index. Some analogues that work are described below.

* Measure is an analogue that is used for [[amenable group]]s, and works particularly well for compact groups with a Haar measure.
* Dimension plays a role analogous to the ''logarithm'' of the order. In those situations where the orders multiply, ''dimensions'' tend to add. There are many different notions of dimension, including algebraic, analytic, and topological ones.
* For [[profinite group]]s, we can view the orders as [[supernatural number]]s, i.e., the orders take values that are products of powers of possibly infinitely many primes. Analogues of Lagrange's and Sylow's theorems hold in these contexts.

===Order from subgroup and quotient perspectives===

An infinite cyclic group (i.e., a group isomorphic to the [[group of integers]]) has no proper nontrivial finite subgroups, so from the perspective of [[Lagrange's theorem]] for subgroups, its order can be thought of as having no prime divisors. However, it has finite quotients of every order, so from the perspective of the fact that [[order of quotient group divides order of group]], every prime divides its order.

More generally, for [[periodic group]]s and [[locally finite group]]s, the ''order'' notion must capture the possible finite ''subgroups'' that arise, whereas for [[residually finite group]]s, the ''order'' notion must capture the possible finite ''quotients'' that arise.
==Computation==

{{GAP command for function|Order}}

The GAP command to compute the order of a group is: 

<pre>Order (group);</pre>

where <pre>group</pre> may either be an on-the-spot definition of a group or a name for something defined earlier.

==References==

===Textbook references===
* {{booklink-defined|Artin}}, Page 47, Point (2.10)