# Difference between revisions of "Unitriangular matrix group:UT(3,p)"

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In this case, for instance, we can take the subgroup with <math>a{12} = 0</math> as the elementary Abelian subgroup of order <math>p^2</math> and the subgroup with <math>a_{23} = a_{13} = 0</math> as the cyclic subgroup of order <math>p</math>. | In this case, for instance, we can take the subgroup with <math>a{12} = 0</math> as the elementary Abelian subgroup of order <math>p^2</math> and the subgroup with <math>a_{23} = a_{13} = 0</math> as the cyclic subgroup of order <math>p</math>. | ||

+ | |||

+ | ==Families== | ||

+ | |||

+ | # These groups fall in the more general family <math>U(n,p)</math> of unipotent upper-triangular matrices, which is the <math>p</math>-Sylow subgroup of the [[general linear group]] <math>GL(n,p)</math>. This further can be generalized to <math>U(n,q)</math> where <math>q</math> is the power of a prime <math>p</math>, which is the <math>p</math>-Sylow subgroup of <math>GL(n,q)</math>. | ||

+ | # These groups also fall into the general family of [[extraspecial group]]s. | ||

==Elements== | ==Elements== | ||

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==Subgroups== | ==Subgroups== | ||

− | + | {{further|[[Subgroup structure of prime-cube order group:U3p]]}} | |

Here is the complete list of subgroups: | Here is the complete list of subgroups: | ||

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* The characteristic subgroups are precisely the subgroups that occur in the [[derived series]], [[upper central series]] and [[lower central series]]. | * The characteristic subgroups are precisely the subgroups that occur in the [[derived series]], [[upper central series]] and [[lower central series]]. | ||

− | + | ==Subgroup-defining functions== | |

− | |||

− | |||

− | |||

− | == | ||

− | The | + | {| class="wikitable" border="1" |

+ | ! Subgroup-defining function !! Subgroup type in list !! Isomorphism class !! Comment | ||

+ | |- | ||

+ | | [[Center]] || (2) || [[Center::Group of prime order]] || | ||

+ | |- | ||

+ | | [[Commutator subgroup]] || (2) || [[Commutator subgroup::Group of prime order]] || | ||

+ | |- | ||

+ | | [[Frattini subgroup]] || (2) || [[Frattini subgroup::Group of prime order]] || The <math>p + 1</math> maximal subgroups of order <math>p^2</math> intersect here. | ||

+ | |- | ||

+ | | [[Socle]] || (2) || [[Socle::Group of prime order]] || This subgroup is the unique [[minimal normal subgroup]], i.e.,the [[monolith]], and the group is [[monolithic group|monolithic]]. Also, [[minimal normal implies central in nilpotent]]. | ||

+ | |} | ||

− | === | + | ===Quotient-defining function=== |

− | + | {| class="wikitable" border="1" | |

+ | ! Quotient-defining function !! Isomorphism class !! Comment | ||

+ | |- | ||

+ | | [[Inner automorphism group]] || [[Inner automorphism group::Elementary abelian group of prime-square order]] || It is the quotient by the center, which is of prime order. | ||

+ | |- | ||

+ | | [[Abelianization]] || [[Abelianization::Elementary abelian group of prime-square order]] || It is the quotient by the commutator subgroup, which is of prime order. | ||

+ | |- | ||

+ | | [[Frattini quotient]] || [[Frattini quotient::Elementary abelian group of prime-square order]] || It is the quotient by the Frattini subgroup, which is of prime order. | ||

+ | |} | ||

==Implementation in GAP== | ==Implementation in GAP== | ||

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==Related groups== | ==Related groups== | ||

− | For any prime <math>p</math>, there are two non- | + | For any prime <math>p</math>, there are (up to isomorphism) two non-abelian groups of order <math>p^3</math>. One of them is this, and the [[prime-cube order group:p2byp|other]] is the semidirect product of the cyclic group of order <math>p^2</math> by a group of order <math>p</math> acting by power maps (with the generator corresponding to multiplication by <math>p+1</math>). |

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## Revision as of 22:26, 13 May 2009

This article is about a family of groups with a parameter that is prime. For any fixed value of the prime, we get a particular group.

View other such prime-parametrized groups

## Contents

## Definition

### As a group of matrices

Given a prime , the group is defined as follows: it is the group of upper triangular matrices with 1s on the diagonal, and entries over (with the group operation being matrix multiplication).

Each such matrix can be described by the three entries . The matrix looks like:

The multiplication of matrices and gives the matrix where:

The analysis given below does not apply to the case . For , we get the dihedral group:D8, which is studied separately. For further information on the contrast between the case of 2 and of odd primes, refer U3p:odd prime versus two.

### As a semidirect product

This group of order can also be described as a semidirect product of the elementary Abelian group of order by the cyclic group of order , where the generator of the cyclic group of order acts via the automorphism:

In this case, for instance, we can take the subgroup with as the elementary Abelian subgroup of order and the subgroup with as the cyclic subgroup of order .

## Families

- These groups fall in the more general family of unipotent upper-triangular matrices, which is the -Sylow subgroup of the general linear group . This further can be generalized to where is the power of a prime , which is the -Sylow subgroup of .
- These groups also fall into the general family of extraspecial groups.

## Elements

### Upto conjugacy

Every element has order .

The conjugacy classes are as follows:

- The center has order precisely , so there are elements that form conjugacy classes of size 1. These are, specifically, the elements with , and they're thus parametrized by their entry.
- For every element outside the center, the centralizer of that element is the subgroup generated by that element and the center, and is hence of order . Thus, the conjugacy class of the element is of size . Thus, there is a total of conjugacy classes of size .

### Upto automorphism

There are only three classes of elements upto automorphism:

- The identity element, which forms a class of size 1
- The non-identity elements in the center, which form a class of size
- The non-central elements, which form a class of size

## Subgroups

`Further information: Subgroup structure of prime-cube order group:U3p`
Here is the complete list of subgroups:

- The trivial subgroup (1)
- The center, which is a group of order . In matrix terms, this is the subgroup comprising matrices with . (1)
- Subgroups of order generated by non-central elements. These are not normal, and occur in conjugacy classes of size . ()
- Subgroups of order containing the center. These are the inverse images via the quotient map by the center, of subgroups of order in the inner automorphism group. ()
- The whole group. (1)

### Normal subgroups

The subgroups in (1), (2), (4) and (5) above are normal.

### Characteristic subgroups

The subgroups in (1), (2) and (5) above are normal. In other words, there are only three characteristic subgroups. Some notable facts:

- The group is characteristic-comparable: any two characteristic subgroups can be compared
- More generally, any characteristic subgroup and any normal subgroup can be compared.
- The characteristic subgroups are precisely the subgroups that occur in the derived series, upper central series and lower central series.

## Subgroup-defining functions

Subgroup-defining function | Subgroup type in list | Isomorphism class | Comment |
---|---|---|---|

Center | (2) | Group of prime order | |

Commutator subgroup | (2) | Group of prime order | |

Frattini subgroup | (2) | Group of prime order | The maximal subgroups of order intersect here. |

Socle | (2) | Group of prime order | This subgroup is the unique minimal normal subgroup, i.e.,the monolith, and the group is monolithic. Also, minimal normal implies central in nilpotent. |

### Quotient-defining function

Quotient-defining function | Isomorphism class | Comment |
---|---|---|

Inner automorphism group | Elementary abelian group of prime-square order | It is the quotient by the center, which is of prime order. |

Abelianization | Elementary abelian group of prime-square order | It is the quotient by the commutator subgroup, which is of prime order. |

Frattini quotient | Elementary abelian group of prime-square order | It is the quotient by the Frattini subgroup, which is of prime order. |

## Implementation in GAP

### Group ID

For any prime , this group is the *third* group amoung the groups of order . Thus, for instance, if , the group is described as:

SmallGroup(343,3)

## Endomorphisms

### Automorphisms

The automorphisms essentially permute the subgroups of order containing the center, while leaving the center itself unmoved.

## Related groups

For any prime , there are (up to isomorphism) two non-abelian groups of order . One of them is this, and the other is the semidirect product of the cyclic group of order by a group of order acting by power maps (with the generator corresponding to multiplication by ).