Editing Index of a subgroup

{{Short description|Mathematics group theory concept}}
In [[mathematics]], specifically [[group theory]], the '''index''' of a [[subgroup]] ''H'' in a group ''G'' is the 
number of left [[Coset|cosets]] of ''H'' in ''G'', or equivalently, the number of right cosets of ''H'' in ''G''.
The index is denoted <math>|G:H|</math> or <math>[G:H]</math> or <math>(G:H)</math>.
Because ''G'' is the disjoint union of the left cosets and because each left coset has the same [[cardinality|size]] as ''H'', the index is related to the [[order (group theory)|orders]] of the two groups by the formula
:<math>|G| = |G:H| |H|</math>
(interpret the quantities as [[cardinal numbers]] if some of them are infinite).
Thus the index <math>|G:H|</math> measures the "relative sizes" of ''G'' and ''H''.

For example, let <math>G = \Z</math> be the group of integers under [[addition]], and let <math>H = 2\Z</math> be the subgroup consisting of the [[Parity (mathematics)|even integers]].  Then <math>2\Z</math>  has two cosets in <math>\Z</math>, namely the set of even integers and the set of odd integers, so the index <math>|\Z:2\Z|</math> is 2.  More generally, <math>|\Z:n\Z| = n</math> for any positive integer ''n''.

When ''G'' is [[finite group|finite]], the formula may be written as <math>|G:H| = |G|/|H|</math>, and it implies
[[Lagrange's theorem (group theory)|Lagrange's theorem]] that <math>|H|</math> divides <math>|G|</math>.

When ''G'' is infinite, <math>|G:H|</math> is a nonzero [[cardinal number]] that may be finite or infinite.
For example, <math>|\Z:2\Z| = 2</math>, but <math>|\R:\Z|</math> is infinite.

If ''N'' is a [[normal subgroup]] of ''G'', then <math>|G:N|</math> is equal to the order of the [[quotient group]] <math>G/N</math>, since the underlying set of <math>G/N</math> is the set of cosets of ''N'' in ''G''.

==Properties==
* If ''H'' is a subgroup of ''G'' and ''K'' is a subgroup of ''H'', then
::<math>|G:K| = |G:H|\,|H:K|.</math>
* If ''H'' and ''K'' are subgroups of ''G'', then
::<math>|G:H\cap K| \le |G : H|\,|G : K|,</math>
:with equality if <math>HK=G</math>.  (If <math>|G:H\cap K|</math> is finite, then equality holds if and only if <math>HK=G</math>.)
* Equivalently, if ''H'' and ''K'' are subgroups of ''G'', then
::<math>|H:H\cap K| \le |G:K|,</math>
:with equality if <math>HK=G</math>.  (If <math>|H:H\cap K|</math> is finite, then equality holds if and only if <math>HK=G</math>.)
* If ''G'' and ''H'' are groups and <math>\varphi \colon G\to H</math> is a [[homomorphism]], then the index of the [[kernel (algebra)|kernel]] of <math>\varphi</math> in ''G'' is equal to the order of the image:
::<math>|G:\operatorname{ker}\;\varphi|=|\operatorname{im}\;\varphi|.</math>
* Let ''G'' be a group [[Group action (mathematics)|acting]] on a [[set (mathematics)|set]] ''X'', and let ''x''&nbsp;∈&nbsp;''X''.  Then the [[cardinality]] of the [[orbit (group theory)|orbit]] of ''x'' under ''G'' is equal to the index of the [[stabilizer subgroup|stabilizer]] of ''x'':
::<math>|Gx| = |G:G_x|.\!</math>
:This is known as the [[orbit-stabilizer theorem]].
* As a special case of the orbit-stabilizer theorem, the number of [[conjugacy class|conjugates]] <math>gxg^{-1}</math> of an element <math>x \in G</math> is equal to the index of the [[centralizer]] of ''x'' in ''G''.
* Similarly, the number of conjugates <math>gHg^{-1}</math> of a subgroup ''H'' in ''G'' is equal to the index of the [[normalizer]] of ''H'' in ''G''.
* If ''H'' is a subgroup of ''G'', the index of the [[core (group)|normal core]] of ''H'' satisfies the following inequality:
::<math>|G:\operatorname{Core}(H)| \le |G:H|!</math>
:where ! denotes the [[factorial]] function; this is discussed further [[#Finite index|below]].
:* As a corollary, if the index of ''H'' in ''G'' is 2, or for a finite group the lowest prime ''p'' that divides the order of ''G,'' then ''H'' is normal, as the index of its core must also be ''p,'' and thus ''H'' equals its core, i.e., it is normal.
:* Note that a subgroup of lowest prime index may not exist, such as in any [[simple group]] of non-prime order, or more generally any [[perfect group]].

==Examples==
* The [[alternating group]] <math>A_n</math> has index 2 in the [[symmetric group]] <math>S_n,</math> and thus is normal.
* The [[special orthogonal group]] <math>\operatorname{SO}(n)</math> has index 2 in the [[orthogonal group]] <math>\operatorname{O}(n)</math>, and thus is normal.
* The [[free abelian group]] <math>\Z\oplus \Z</math> has three subgroups of index 2, namely
::<math>\{(x,y) \mid x\text{ is even}\},\quad \{(x,y) \mid y\text{ is even}\},\quad\text{and}\quad
\{(x,y) \mid x+y\text{ is even}\}</math>.
* More generally, if ''p'' is [[prime number|prime]] then <math>\Z^n</math> has <math>(p^n-1)/(p-1)</math> subgroups of index ''p'', corresponding to the <math>(p^n-1)</math> nontrivial [[homomorphism]]s <math>\Z^n \to \Z/p\Z</math>.{{Citation needed|date=January 2010}}
* Similarly, the [[free group]] <math>F_n</math> has <math>(p^n-1)/(p - 1)</math>  subgroups of index ''p''.
* The [[infinite dihedral group]] has a [[cyclic group|cyclic subgroup]] of index 2, which is necessarily normal.

==Infinite index==
If ''H'' has an infinite number of cosets in ''G'', then the index of ''H'' in ''G'' is said to be infinite.  In this case, the index <math>|G:H|</math> is actually a [[cardinal number]].  For example, the index of ''H'' in ''G'' may be [[countable set|countable]] or [[Uncountable set|uncountable]], depending on whether ''H'' has a countable number of cosets in ''G''. Note that the index of ''H'' is at most the order of ''G,'' which is realized for the trivial subgroup, or in fact any subgroup ''H'' of infinite cardinality less than that of ''G.''

==Finite index==
A subgroup ''H'' of finite index in a group ''G'' (finite or infinite) always contains a [[normal subgroup]] ''N'' (of ''G''), also of finite index. In fact, if ''H'' has index ''n'', then the index of ''N'' will be some divisor of ''n''! and a multiple of ''n''; indeed, ''N'' can be taken to be the kernel of the natural homomorphism from ''G'' to the permutation group of the left (or right) cosets of ''H''.
Let us explain this in more detail, using right cosets:

The elements of ''G'' that leave all cosets the same form a group.
{{collapse top|Proof}}
If ''Hca'' ⊂ ''Hc'' ∀ ''c'' ∈ ''G'' and likewise ''Hcb'' ⊂ ''Hc'' ∀ ''c'' ∈ ''G'', then ''Hcab'' ⊂ ''Hc'' ∀ ''c'' ∈ ''G''. If ''h''<sub>1</sub>''ca'' = ''h''<sub>2</sub>''c'' for all ''c'' ∈ ''G'' (with ''h''<sub>1</sub>, ''h''<sub>2</sub> ∈ H) then ''h''<sub>2</sub>''ca''<sup>−1</sup> = ''h''<sub>1</sub>''c'', so ''Hca''<sup>−1</sup> ⊂ ''Hc''.
{{collapse bottom|Proof}}

Let us call this group ''A''. Note that ''A'' is a subgroup of ''H'', since ''Ha'' ⊂ ''H'' by the definition of ''A''. Let ''B'' be the set of elements of ''G'' which perform a given permutation on the cosets of ''H''. Then ''B'' is a right coset of ''A''.

{{collapse top|Proof}}
First let us show that if ''b''{{sub|1}}∈''B'', then any other element ''b''{{sub|2}} of ''B'' equals ''ab''{{sub|1}} for some ''a''∈''A''. Assume that multiplying the coset ''Hc'' on the right by elements of ''B'' gives elements of the coset ''Hd''. If ''cb''<sub>1</sub> = ''d'' and ''cb''<sub>2</sub> = ''hd'', then ''cb''<sub>2</sub>''b''<sub>1</sub><sup>−1</sup> = ''hc'' ∈ ''Hc'', or in other words ''b''{{sub|2}}=''ab''{{sub|1}} for some ''a''∈''A'', as desired. Now we show that for any ''b''∈''B'' and ''a''∈''A'', ''ab'' will be an element of ''B''. This is because the coset ''Hc'' is the same as ''Hca'', so ''Hcb'' = ''Hcab''. Since this is true for any ''c'' (that is, for any coset), it shows that multiplying on the right by ''ab'' makes the same permutation of cosets as multiplying by ''b'', and therefore ''ab''∈''B''.
{{collapse bottom|Proof}}

What we have said so far applies whether the index of ''H'' is finite or infinte. Now assume that it is the finite number ''n''. Since the number of possible permutations of cosets is finite, namely ''n''!, then there can only be a finite number of sets like ''B''. (If ''G'' is infinite, then all such sets are therefore infinite.) The set of these sets forms a group isomorphic to a subset of the group of permutations, so the number of these sets must divide ''n''!. Furthermore, it must be a multiple of ''n'' because each coset of ''H'' contains the same number of cosets of ''A''. Finally, if for some ''c'' ∈ ''G'' and ''a'' ∈ ''A'' we have ''ca = xc'', then for any ''d'' ∈ ''G dca = dxc'', but also ''dca = hdc'' for some ''h'' ∈ ''H'' (by the definition of ''A''), so ''hd = dx''. Since this is true for any ''d'', ''x'' must be a member of A, so ''ca = xc'' implies that ''cac{{sup|−1}}'' ∈ ''A'' and therefore ''A'' is a normal subgroup.

The index of the normal subgroup not only has to be a divisor of ''n''!, but must satisfy other criteria as well. Since the normal subgroup is a subgroup of ''H'', its index in ''G'' must be ''n'' times its index inside ''H''. Its index in ''G'' must also correspond to a subgroup of the [[symmetric group]] S{{sub|''n''}}, the group of permutations of ''n'' objects. So for example if ''n'' is 5, the index cannot be 15 even though this divides 5!, because there is no subgroup of order 15 in S{{sub|5}}.

In the case of ''n'' = 2 this gives the rather obvious result that a subgroup ''H'' of index 2 is a normal subgroup, because the normal subgroup of ''H'' must have index 2 in ''G'' and therefore be identical to ''H''. (We can arrive at this fact also by noting that all the elements of ''G'' that are not in ''H'' constitute the right coset of ''H'' and also the left coset, so the two are identical.) More generally, a subgroup of index ''p'' where ''p'' is the smallest prime factor of the order of ''G'' (if ''G'' is finite) is necessarily normal, as the index of ''N'' divides ''p''! and thus must equal ''p,'' having no other prime factors. For example, the subgroup ''Z''{{sub|7}} of the non-abelian group of order 21 is normal (see [[List of small groups#List of small non-abelian groups|List of small non-abelian groups]] and [[Frobenius group#Examples]]).

An alternative proof of the result that a subgroup of index lowest prime ''p'' is normal, and other properties of subgroups of prime index are given in {{Harv|Lam|2004}}.

=== Examples ===
The group '''O''' of chiral [[octahedral symmetry]] has 24 elements. It has a [[dihedral symmetry|dihedral]] D<sub>4</sub> subgroup (in fact it has three such) of order 8, and thus of index 3 in '''O''', which we shall call ''H''. This dihedral group has a 4-member D<sub>2</sub> subgroup, which we may call ''A''. Multiplying on the right any element of a right coset of ''H'' by an element of ''A'' gives a member of the same coset of ''H'' (''Hca = Hc''). ''A'' is normal in '''O'''. There are six cosets of ''A'', corresponding to the six elements of the [[symmetric group]] S<sub>3</sub>. All elements from any particular coset of ''A'' perform the same permutation of the cosets of ''H''.

On the other hand, the group T<sub>h</sub> of [[pyritohedral symmetry]] also has 24 members and a subgroup of index 3 (this time it is a D<sub>2h</sub> [[prismatic symmetry]] group, see [[point groups in three dimensions]]), but in this case the whole subgroup is a normal subgroup. All members of a particular coset carry out the same permutation of these cosets, but in this case they represent only the 3-element [[alternating group]] in the 6-member S<sub>3</sub> symmetric group.

==Normal subgroups of prime power index==
Normal subgroups of [[prime power]] index are kernels of surjective maps to [[p-group|''p''-groups]] and have interesting structure, as described at [[Focal subgroup theorem#Subgroups|Focal subgroup theorem: Subgroups]] and elaborated at [[focal subgroup theorem]].

There are three important normal subgroups of prime power index, each being the smallest normal subgroup in a certain class:
* '''E'''<sup>''p''</sup>(''G'') is the intersection of all index ''p'' normal subgroups; ''G''/'''E'''<sup>''p''</sup>(''G'') is an [[elementary abelian group]], and is the largest elementary abelian ''p''-group onto which ''G'' surjects.
* '''A'''<sup>''p''</sup>(''G'') is the intersection of all normal subgroups ''K'' such that ''G''/''K'' is an abelian ''p''-group (i.e., ''K'' is an index <math>p^k</math> normal subgroup that contains the derived group <math>[G,G]</math>): ''G''/'''A'''<sup>''p''</sup>(''G'') is the largest abelian ''p''-group (not necessarily elementary) onto which ''G'' surjects.
* '''O'''<sup>''p''</sup>(''G'') is the intersection of all normal subgroups ''K'' of ''G'' such that ''G''/''K'' is a (possibly non-abelian) ''p''-group (i.e., ''K'' is an index <math>p^k</math> normal subgroup): ''G''/'''O'''<sup>''p''</sup>(''G'') is the largest ''p''-group (not necessarily abelian) onto which ''G'' surjects. '''O'''<sup>''p''</sup>(''G'') is also known as the {{anchor|p-residual subgroup}}'''''p''-residual subgroup'''.
As these are weaker conditions on the groups ''K,'' one obtains the containments 
:<math>\mathbf{E}^p(G) \supseteq \mathbf{A}^p(G) \supseteq \mathbf{O}^p(G).</math>

These groups have important connections to the [[Sylow subgroup]]s and the transfer homomorphism, as discussed there.

=== Geometric structure ===
An elementary observation is that one cannot have exactly 2 subgroups of index 2, as the [[Complement (set theory)|complement]] of their [[symmetric difference]] yields a third. This is a simple corollary of the above discussion (namely the projectivization of the vector space structure of the elementary abelian group 
:<math>G/\mathbf{E}^p(G) \cong (\mathbf{Z}/p)^k</math>,

and further, ''G'' does not act on this geometry, nor does it reflect any of the non-abelian structure (in both cases because the quotient is abelian).

However, it is an elementary result, which can be seen concretely as follows: the set of normal subgroups of a given index ''p'' form a [[projective space]], namely the projective space 
:<math>\mathbf{P}(\operatorname{Hom}(G,\mathbf{Z}/p)).</math>

In detail, the space of homomorphisms from ''G'' to the (cyclic) group of order ''p,'' <math>\operatorname{Hom}(G,\mathbf{Z}/p),</math> is a vector space over the [[finite field]] <math>\mathbf{F}_p = \mathbf{Z}/p.</math> A non-trivial such map has as kernel a normal subgroup of index ''p,'' and multiplying the map by an element of <math>(\mathbf{Z}/p)^\times</math> (a non-zero number mod ''p'') does not change the kernel; thus one obtains a map from 
:<math>\mathbf{P}(\operatorname{Hom}(G,\mathbf{Z}/p)) :=  (\operatorname{Hom}(G,\mathbf{Z}/p))\setminus\{0\})/(\mathbf{Z}/p)^\times</math>

to normal index ''p'' subgroups. Conversely, a normal subgroup of index ''p'' determines a non-trivial map to <math>\mathbf{Z}/p</math> up to a choice of "which coset maps to <math>1 \in \mathbf{Z}/p,</math> which shows that this map is a bijection.

As a consequence, the number of normal subgroups of index ''p'' is 
:<math>(p^{k+1}-1)/(p-1)=1+p+\cdots+p^k</math>

for some ''k;'' <math>k=-1</math> corresponds to no normal subgroups of index ''p''. Further, given two distinct normal subgroups of index ''p,'' one obtains a [[projective line]] consisting of <math>p+1</math> such subgroups.

For <math>p=2,</math> the [[symmetric difference]] of two distinct index 2 subgroups (which are necessarily normal) gives the third point on the projective line containing these subgroups, and a group must contain <math>0,1,3,7,15,\ldots</math> index 2 subgroups – it cannot contain exactly 2 or 4 index 2 subgroups, for instance.

==See also==
* [[Virtually]]
* [[Codimension]]

==References==
{{Refimprove|date=January 2010}}
{{reflist}}
{{Refbegin}}
* {{ Citation | title = On Subgroups of Prime Index | first = T. Y. | last = Lam | journal = [[The American Mathematical Monthly]] | volume = 111 | number = 3 |date=March 2004 | pages = 256–258 | jstor = 4145135 | url=http://math.berkeley.edu/~lam/html/index-p.ps}}
{{Refend}}

== External links ==
* {{PlanetMath | urlname = NormalityOfSubgroupsOfPrimeIndex | title = Normality of subgroups of prime index }}
* "[http://groupprops.subwiki.org/wiki/Subgroup_of_least_prime_index_is_normal Subgroup of least prime index is normal]" at [http://groupprops.subwiki.org/wiki/Main_Page Groupprops, The Group Properties Wiki]

{{DEFAULTSORT:Index Of A Subgroup}}
[[Category:Group theory]]