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

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===As a group of matrices=== | ===As a group of matrices=== | ||

− | Given a prime <math>p</math>, the group <math> | + | Given a prime <math>p</math>, the group <math>U(3,p)</math> is defined as follows: it is the group of upper triangular matrices with 1s on the diagonal, and entries over <math>F_p</math> (with the group operation being matrix multiplication). |

Each such matrix <math>(a_{ij})</math> can be described by the three entries <math>a_{12}, a_{13}, a_{23}</math>. The matrix looks like: | Each such matrix <math>(a_{ij})</math> can be described by the three entries <math>a_{12}, a_{13}, a_{23}</math>. The matrix looks like: | ||

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* <math>c_{23} = a_{23} + b_{23}</math> | * <math>c_{23} = a_{23} + b_{23}</math> | ||

− | The analysis given below does not apply to the case <math>p = 2</math>. For <math>p = 2</math>, we get the [[dihedral group:D8]], which is studied separately | + | The analysis given below does not apply to the case <math>p = 2</math>. For <math>p = 2</math>, we get the [[dihedral group:D8]], which is studied separately. |

===As a semidirect product=== | ===As a semidirect product=== |

## Revision as of 15:39, 22 June 2011

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.

### 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

## Arithmetic functions

Compare and contrast arithmetic function values with other groups of prime-cube order at Groups of prime-cube order#Arithmetic functions

For some of these, the function values are different when and/or when . These are clearly indicated below.

### Arithmetic functions taking values between 0 and 3

Function | Value | Explanation |
---|---|---|

prime-base logarithm of order | 3 | the order is |

prime-base logarithm of exponent | 1 | the exponent is . Exception when , where the exponent is . |

nilpotency class | 2 | |

derived length | 2 | |

Frattini length | 2 | |

minimum size of generating set | 2 | |

subgroup rank | 2 | |

rank as p-group | 2 | |

normal rank as p-group | 2 | |

characteristic rank as p-group | 1 |

### Arithmetic functions of a counting nature

Function | Value | Explanation |
---|---|---|

number of conjugacy classes | elements in the center, and each other conjugacy class has size | |

number of subgroups | when , when | |

number of normal subgroups | ||

number of conjugacy classes of subgroups | for , for |

## 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. |

## GAP implementation

### GAP ID

For any prime , this group is the *third* group among the groups of order . Thus, for instance, if , the group is described using GAP's SmallGroup function as:

`SmallGroup(343,3)`

Note that we don't need to compute ; we can also write this as:

`SmallGroup(7^3,3)`

### As an extraspecial group

For any prime , we can define this group using GAP's ExtraspecialGroup function as:

`ExtraspecialGroup(p^3,'+')`

For , it can also be constructed as:

`ExtraspecialGroup(p^3,p)`

where the argument indicates that it is the extraspecial group of exponent . For instance, for :

`ExtraspecialGroup(5^3,5)`

## 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 ).