Difference between revisions of "Abelian group"
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===Origin of the term=== | ===Origin of the term=== | ||
− | The term ''' | + | The term '''abelian group''' comes from Niels Henrick Abel, a mathematician who worked with groups even before the formal theory was laid down, in order to prove unsolvability of the quintic. |
+ | |||
+ | The word ''abelian'' is usually begun with a small ''a''. | ||
+ | |||
+ | {{quotation|'''wikinote''': Some older content on the wiki uses capital A for Abelian. We're trying to update this content.}} | ||
<section begin=beginner/> | <section begin=beginner/> | ||
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===Symbol-free definition=== | ===Symbol-free definition=== | ||
− | An ''' | + | An '''abelian group''' is a [[group]] where any two elements commute. |
===Definition with symbols=== | ===Definition with symbols=== | ||
− | A [[group]] <math>G</math> is termed ''' | + | A [[group]] <math>G</math> is termed '''abelian''' if for any elements <math>x</math> and <math>y</math> in <math>G</math>, <math>xy = yx</math> (here <math>xy</math> denotes the product of <math>x</math> and <math>y</math> in <math>G</math>). |
<section end=beginner/> | <section end=beginner/> | ||
===Equivalent formulations=== | ===Equivalent formulations=== | ||
− | * A group is | + | * A group is abelian if its [[defining ingredient::center]] is the whole group. |
− | * A group is | + | * A group is abelian if its [[defining ingredient::commutator subgroup]] is trivial. |
<section begin=beginner/> | <section begin=beginner/> | ||
==Notation== | ==Notation== | ||
− | When <math>G</math> is an | + | When <math>G</math> is an abelian group, we typically use ''additive'' notation and terminology. Thus, the group multiplication is termed ''addition'' and the product of two elements is termed the ''sum''. |
# The infix operator <math>+</math> is used for the group multiplication, so the sum of two elements <math>a</math> and <math>b</math> is denoted by <math>a + b</math>. The group multiplication is termed ''addition'' and the product of two elements is termed the ''sum''. | # The infix operator <math>+</math> is used for the group multiplication, so the sum of two elements <math>a</math> and <math>b</math> is denoted by <math>a + b</math>. The group multiplication is termed ''addition'' and the product of two elements is termed the ''sum''. | ||
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# <math>a + a + \ldots + a</math> done <math>n</math> times is denoted <math>na</math>, (where <math>n \in \mathbb{N}</math>) while <math>(-a) + (-a) + (-a) + \ldots + (-a)</math> done <math>n</math> times is denoted <math>(-n)a</math>. | # <math>a + a + \ldots + a</math> done <math>n</math> times is denoted <math>na</math>, (where <math>n \in \mathbb{N}</math>) while <math>(-a) + (-a) + (-a) + \ldots + (-a)</math> done <math>n</math> times is denoted <math>(-n)a</math>. | ||
− | This convention is typically followed in a situation where we are dealing with the | + | This convention is typically followed in a situation where we are dealing with the abelian group <math>G</math> in isolation, rather than as a subgroup of a possibly non-abelian group. If we are working with subgroups in a non-abelian group, we typically use multiplicative notation even if the subgroup happens to be abelian. |
==Examples== | ==Examples== | ||
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<section begin=revisit/> | <section begin=revisit/> | ||
===Finite examples=== | ===Finite examples=== | ||
− | [[Cyclic group]]s are good examples of | + | [[Cyclic group]]s are good examples of abelian groups, where the cyclic group of order <math>n</math> is the group of integers modulo <math>n</math>. |
− | Further, any direct product of cyclic groups is also an | + | Further, any direct product of cyclic groups is also an abelian group. Further, every [[finitely generated group|finitely generated]] Abelian group is obtained this way. This is the famous [[structure theorem for finitely generated abelian groups]]. |
− | The structure theorem can be used to generate a complete listing of finite | + | The structure theorem can be used to generate a complete listing of finite abelian groups, as described here: [[classification of finite Abelian groups]]. |
===Non-examples=== | ===Non-examples=== | ||
− | Not every group is | + | Not every group is abelian. The smallest non-abelian group is [[symmetric group:S3|the symmetric group on three letters]]: the group of all permutations on three letters, under composition. Its being non-abelian hinges on the fact that the order in which permutations are performed, matters. |
<section end=revisit/> | <section end=revisit/> | ||
==Facts== | ==Facts== | ||
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===Occurrence as subgroups=== | ===Occurrence as subgroups=== | ||
− | Every [[cyclic group]] is | + | Every [[cyclic group]] is abelian. Since each group is generated by its cyclic subgroups, every group is generated by a family of abelian subgroups. A trickier question is: do there exist abelian [[normal subgroup]]s? A good candidate for an abelian normal subgroup is the [[center]], which is the collection of elements of the group that commute with ''every'' element of the group. |
===Occurrence as quotients=== | ===Occurrence as quotients=== | ||
− | The maximal | + | The maximal abelian quotient of any group is termed its [[abelianization]], and this is the quotient by the [[commutator subgroup]]. A subgroup is normal with abelian quotient group if and only if the subgroup contains the commutator subgroup. |
==Metaproperties== | ==Metaproperties== | ||
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{{S-closed}} | {{S-closed}} | ||
− | Any [[subgroup]] of an | + | Any [[subgroup]] of an abelian group is abelian -- viz., the property of being abelian is [[subgroup-closed group property|subgroup-closed]]. This follows as a direct consequence of abelianness being varietal. {{proofat|[[Abelianness is subgroup-closed]]}} |
{{Q-closed}} | {{Q-closed}} | ||
− | Any [[quotient]] of an | + | Any [[quotient]] of an abelian group is abelian -- viz the property of being abelian is [[quotient-closed group property|quotient-closed]]. This again follows as a direct consequence of abelianness being varietal. {{proofat|[[Abelianness is quotient-closed]]}} |
{{DP-closed}} | {{DP-closed}} | ||
− | A [[direct product]] of | + | A [[direct product]] of abelian groups is abelian -- viz the property of being abelian is [[direct product-closed group property|direct product-closed]]. This again follows as a direct consequence of abelianness being varietal. {{proofat|[[Abelianness is direct product-closed]]}} |
==Testing== | ==Testing== | ||
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class = AbelianGroups}} | class = AbelianGroups}} | ||
− | To test whether a group is | + | To test whether a group is abelian, the GAP syntax is: |
<pre>IsAbelian (group)</pre> | <pre>IsAbelian (group)</pre> |
Revision as of 23:23, 21 January 2009
This article is about a basic definition in group theory. The article text may, however, contain advanced material.
VIEW: Definitions built on this | Facts about this: (facts closely related to Abelian group, all facts related to Abelian group) |Survey articles about this | Survey articles about definitions built on this
VIEW RELATED: Analogues of this | Variations of this | Opposites of this |[SHOW MORE]
This article defines a group property that is pivotal (i.e., important) among existing group properties
View a list of pivotal group properties | View a complete list of group properties [SHOW MORE]
History
Origin of the term
The term abelian group comes from Niels Henrick Abel, a mathematician who worked with groups even before the formal theory was laid down, in order to prove unsolvability of the quintic.
The word abelian is usually begun with a small a.
wikinote: Some older content on the wiki uses capital A for Abelian. We're trying to update this content.
Definition
Symbol-free definition
An abelian group is a group where any two elements commute.
Definition with symbols
A group is termed abelian if for any elements and in , (here denotes the product of and in ).
Equivalent formulations
- A group is abelian if its center is the whole group.
- A group is abelian if its commutator subgroup is trivial.
Notation
When is an abelian group, we typically use additive notation and terminology. Thus, the group multiplication is termed addition and the product of two elements is termed the sum.
- The infix operator is used for the group multiplication, so the sum of two elements and is denoted by . The group multiplication is termed addition and the product of two elements is termed the sum.
- The identity element is typically denoted as and termed zero
- The inverse of an element is termed its negative or additive inverse. The inverse of is denoted
- done times is denoted , (where ) while done times is denoted .
This convention is typically followed in a situation where we are dealing with the abelian group in isolation, rather than as a subgroup of a possibly non-abelian group. If we are working with subgroups in a non-abelian group, we typically use multiplicative notation even if the subgroup happens to be abelian.
Examples
Some infinite examples
The additive group of integers , the additive group of rational numbers , the additive group of real numbers , the multiplicative group of nonzero rationals , and the multiplicative group of nonzero real numbers are some examples of Abelian groups.
(More generally, for any field, the additive group, and the multiplicative group of nonzero elements, are Abelian groups).
Finite examples
Cyclic groups are good examples of abelian groups, where the cyclic group of order is the group of integers modulo .
Further, any direct product of cyclic groups is also an abelian group. Further, every finitely generated Abelian group is obtained this way. This is the famous structure theorem for finitely generated abelian groups.
The structure theorem can be used to generate a complete listing of finite abelian groups, as described here: classification of finite Abelian groups.
Non-examples
Not every group is abelian. The smallest non-abelian group is the symmetric group on three letters: the group of all permutations on three letters, under composition. Its being non-abelian hinges on the fact that the order in which permutations are performed, matters.
Facts
Occurrence as subgroups
Every cyclic group is abelian. Since each group is generated by its cyclic subgroups, every group is generated by a family of abelian subgroups. A trickier question is: do there exist abelian normal subgroups? A good candidate for an abelian normal subgroup is the center, which is the collection of elements of the group that commute with every element of the group.
Occurrence as quotients
The maximal abelian quotient of any group is termed its abelianization, and this is the quotient by the commutator subgroup. A subgroup is normal with abelian quotient group if and only if the subgroup contains the commutator subgroup.
Metaproperties
Varietal group property
This group property is a varietal group property, in the sense that the collection of groups satisfying this property forms a variety of algebras. In other words, the collection of groups satisfying this property is closed under taking subgroups, taking quotients and taking arbitrary direct products.
Abelian groups form a variety of algebras. The defining equations for this variety are the equations for a group along with the commutativity equation.
Subgroups
This group property is subgroup-closed, viz., any subgroup of a group satisfying the property also satisfies the property
View a complete list of subgroup-closed group properties
Any subgroup of an abelian group is abelian -- viz., the property of being abelian is subgroup-closed. This follows as a direct consequence of abelianness being varietal. For full proof, refer: Abelianness is subgroup-closed
Quotients
This group property is quotient-closed, viz., any quotient of a group satisfying the property also has the property
View a complete list of quotient-closed group properties
Any quotient of an abelian group is abelian -- viz the property of being abelian is quotient-closed. This again follows as a direct consequence of abelianness being varietal. For full proof, refer: Abelianness is quotient-closed
Direct products
This group property is direct product-closed, viz., the direct product of an arbitrary (possibly infinite) family of groups each having the property, also has the property
View other direct product-closed group properties
A direct product of abelian groups is abelian -- viz the property of being abelian is direct product-closed. This again follows as a direct consequence of abelianness being varietal. For full proof, refer: Abelianness is direct product-closed
Testing
The testing problem
Further information: Abelianness testing problem
GAP command
This group property can be tested using built-in functionality of Groups, Algorithms, Programming (GAP).
The GAP command for this group property is:IsAbelian
The class of all groups with this property can be referred to with the built-in command: AbelianGroups
View GAP-testable group properties
To test whether a group is abelian, the GAP syntax is:
IsAbelian (group)where
groupeither defines the group or gives the name to a group previously defined.
Study of this notion
Mathematical subject classification
Under the Mathematical subject classification, the study of this notion comes under the class: 20K
References
Textbook references
- Abstract Algebra by David S. Dummit and Richard M. Foote, 10-digit ISBN 0471433349, 13-digit ISBN 978-0471433347, ^{More info}, Page 17 (definition as Point (2) in general definition of a group)
- Groups and representations by Jonathan Lazare Alperin and Rowen B. Bell, ISBN 0387945261, ^{More info}, Page 2 (definition introduced in paragraph)
- Algebra by Michael Artin, ISBN 0130047635, 13-digit ISBN 978-0130047632, ^{More info}, Page 42 (defined immediately after the definition of group, as a group where the composition is commutative)
- Topics in Algebra by I. N. Herstein, ^{More info}, Page 28 (formal definition)
- A Course in the Theory of Groups by Derek J. S. Robinson, ISBN 0387944613, ^{More info}, Page 2 (formal definition)
- Finite Group Theory (Cambridge Studies in Advanced Mathematics) by Michael Aschbacher, ISBN 0521786754, ^{More info}, Page 1 (definition introduced in paragraph)