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

(→Subgroups) |
|||

Line 2: | Line 2: | ||

==Definition== | ==Definition== | ||

+ | |||

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

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). | 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). | ||

Line 19: | Line 21: | ||

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. For further information on the contrast between the case of 2 and of odd primes, refer [[U3p:odd prime versus two]]. | 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. 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 <math>p^3</math> can also be described as a semidirect product of the elementary Abelian group of order <math>p^2</math> by the cyclic group of order <math>p</math>, where the generator of the cyclic group of order <math>p</math> acts via the automorphism: | ||

+ | |||

+ | <math>(a,b) \mapsto (a,a+b)</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>. | ||

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

Line 65: | Line 75: | ||

The fully characteristic subgroups are precisely the same as the characteristic ones. | The fully characteristic subgroups are precisely the same as the characteristic ones. | ||

− | ===[[Central | + | ===[[Central factor]]s=== |

The central factors are precisely the same as the characteristic subgroups. | The central factors are precisely the same as the characteristic subgroups. | ||

− | ===[[Direct | + | ===[[Direct factor]]s=== |

There are no proper nontrivial direct factors. In other words, the group is [[directly indecomposable group|directly indecomposable]]. | There are no proper nontrivial direct factors. In other words, the group is [[directly indecomposable group|directly indecomposable]]. |

## Revision as of 17:51, 22 August 2008

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 .

## 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

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.

### Fully characteristic subgroups

The fully characteristic subgroups are precisely the same as the characteristic ones.

### Central factors

The central factors are precisely the same as the characteristic subgroups.

### Direct factors

There are no proper nontrivial direct factors. In other words, the group is directly indecomposable.

## 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 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 ).

## Subgroup-defining functions=

### Center

*The center of this group is abstractly isomorphic to*: group of prime order

### Commutator subgroup

*The commutator subgroup of this group is abstractly isomorphic to:* group of prime order

### Frattini subgroup

*The Frattini subgroup of this group is abstractly isomorphic to:* group of prime order

### Socle

*The socle of this group is abstractly isomorphic to:* group of prime order