# Difference between revisions of "Subgroup structure of dihedral group:D8"

(→The center (type (2))) |
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# The trivial subgroup (1) | # The trivial subgroup (1) | ||

# The [[center]], which is the unique minimal normal subgroup, and is a two-element subgroup generated by <math>a^2</math>. Isomorphic to [[cyclic group:Z2]]. {{further|[[center of dihedral group:D8]]}}(1) | # The [[center]], which is the unique minimal normal subgroup, and is a two-element subgroup generated by <math>a^2</math>. Isomorphic to [[cyclic group:Z2]]. {{further|[[center of dihedral group:D8]]}}(1) | ||

− | # The two-element subgroups generated by <math>x</math>, <math>ax</math>, <math>a^2x</math> and <math>a^3x</math>. Isomorphic to [[cyclic group:Z2]]. These come in two conjugacy classes: the subgroups generated by <math>x</math> and by <math>a^2x</math> are conjugate, and the subgroups generated by <math>ax</math> and by <math>a^3x</math> are conjugate. (4) | + | # The two-element subgroups generated by <math>x</math>, <math>ax</math>, <math>a^2x</math> and <math>a^3x</math>. Isomorphic to [[cyclic group:Z2]]. These come in two conjugacy classes: the subgroups generated by <math>x</math> and by <math>a^2x</math> are conjugate, and the subgroups generated by <math>ax</math> and by <math>a^3x</math> are conjugate. (4) {{further|[[non-normal subgroups of dihedral group:D8]]}} |

# The four-element subgroup generated by <math>a^2</math> and <math>x</math>. This comprises elements <math>e,a^2,x,a^2x</math>. It is isomorphic to the [[Klein four-group]]. A similar four-element subgroup is obtained as that generated by <math>a^2</math> and <math>ax</math>. These are both normal. (2) {{further|[[Klein four-subgroups of dihedral group:D8]]}} | # The four-element subgroup generated by <math>a^2</math> and <math>x</math>. This comprises elements <math>e,a^2,x,a^2x</math>. It is isomorphic to the [[Klein four-group]]. A similar four-element subgroup is obtained as that generated by <math>a^2</math> and <math>ax</math>. These are both normal. (2) {{further|[[Klein four-subgroups of dihedral group:D8]]}} | ||

# The four-element subgroup generated by <math>a</math>. Isomorphic to [[cyclic group:Z4]]. (1) {{further|[[Cyclic maximal subgroup of dihedral group:D8]]}} | # The four-element subgroup generated by <math>a</math>. Isomorphic to [[cyclic group:Z4]]. (1) {{further|[[Cyclic maximal subgroup of dihedral group:D8]]}} | ||

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==The non-characteristic four-element subgroups (type (4))== | ==The non-characteristic four-element subgroups (type (4))== | ||

+ | |||

+ | {{further|[[Klein four-subgroups of dihedral group:D8]]}} | ||

These two subgroups are related by an outer automorphism, but are ''not'' conjugate (in fact, both are normal subgroups). Since they're [[automorph]]s, they in particular satisfy and dissatisfy the same subgroup properties. | These two subgroups are related by an outer automorphism, but are ''not'' conjugate (in fact, both are normal subgroups). Since they're [[automorph]]s, they in particular satisfy and dissatisfy the same subgroup properties. | ||

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==The two-element non-normal subgroups (type (3))== | ==The two-element non-normal subgroups (type (3))== | ||

+ | |||

+ | {{further|[[non-normal subgroups of dihedral group:D8]]}} | ||

There are four of these: <math>\{ e, x \}</math>, <math>\{ e, ax \}</math>, <math>\{ e,a^2x \}</math>, and <math>\{ e,a^3x \}</math>. In terms of permutations, these are the subgroups <math>\{ (), (1,3) \}</math>, <math>\{ (), (1,2)(3,4) \}</math>, <math>\{ (), (2,4) \}</math> and <math>\{ (), (1,4)(2,3)</math>. | There are four of these: <math>\{ e, x \}</math>, <math>\{ e, ax \}</math>, <math>\{ e,a^2x \}</math>, and <math>\{ e,a^3x \}</math>. In terms of permutations, these are the subgroups <math>\{ (), (1,3) \}</math>, <math>\{ (), (1,2)(3,4) \}</math>, <math>\{ (), (2,4) \}</math> and <math>\{ (), (1,4)(2,3)</math>. |

## Revision as of 23:22, 4 January 2010

This article gives specific information, namely, subgroup structure, about a particular group, namely: dihedral group:D8.

View subgroup structure of particular groups | View other specific information about dihedral group:D8

The **dihedral group** , sometimes called , also called the dihedral group of order eight or the dihedral group acting on four elements, is defined by the following presentation:

In the permutation representation, we can think of the dihedral group as a subgroup of the symmetric group on the four-element set , and write:

.

The dihedral group has ten subgroups:

- The trivial subgroup (1)
- The center, which is the unique minimal normal subgroup, and is a two-element subgroup generated by . Isomorphic to cyclic group:Z2.
`Further information: center of dihedral group:D8`(1) - The two-element subgroups generated by , , and . Isomorphic to cyclic group:Z2. These come in two conjugacy classes: the subgroups generated by and by are conjugate, and the subgroups generated by and by are conjugate. (4)
`Further information: non-normal subgroups of dihedral group:D8` - The four-element subgroup generated by and . This comprises elements . It is isomorphic to the Klein four-group. A similar four-element subgroup is obtained as that generated by and . These are both normal. (2)
`Further information: Klein four-subgroups of dihedral group:D8` - The four-element subgroup generated by . Isomorphic to cyclic group:Z4. (1)
`Further information: Cyclic maximal subgroup of dihedral group:D8` - The whole group. (1)

We study here the properties of each of these subgroups (except the trivial subgroup and the whole group). We denote the whole group by .

First, a quick summary:

- Except the subgroups in (3), all subgroups are normal. Of the subgroups listed in (3), there are two conjugacy classes of subgroups, each comprising two subgroups. Both conjugacy classes are related by an outer automorphism.
- The subgroups listed in (1), (2), (5) and (6) are characteristic. The two subgroups listed in (4) are normal, but are automorphs of each other.

## Tables for quick information

### Table classifying subgroups up to automorphisms

Automorphism class of subgroups | Isomorphism class | Number of conjugacy classes | Size of each conjugacy class | Isomorphism class of quotient (if exists) | Subnormal depth | Nilpotency class |
---|---|---|---|---|---|---|

trivial subgroup | trivial group | 1 | 1 | dihedral group:D8 | 0 | 0 |

center | cyclic group:Z2 | 1 | 1 | Klein four-group | 1 | 1 |

other subgroups of order two | cyclic group:Z2 | 2 | 2 | -- | 1 | 2 |

Klein four-subgroups | Klein four-group | 2 | 1 | cyclic group:Z2 | 1 | 1 |

cyclic maximal subgroup | cyclic group:Z4 | 1 | 1 | cyclic group:Z2 | 1 | 1 |

whole group | dihedral group:D8 | 1 | 1 | trivial group | 0 | 2 |

### Table classifying isomorphism types of subgroups

Group name | GAP ID | Occurrences as subgroup | Conjugacy classes of occurrence as subgroup | Automorphism classes of occurrence as subgroup | Occurrences as normal subgroup | Occurrences as characteristic subgroup |
---|---|---|---|---|---|---|

Trivial group | 1 | 1 | 1 | 1 | 1 | |

Cyclic group:Z2 | 5 | 3 | 2 | 1 | 1 | |

Cyclic group:Z4 | 1 | 1 | 1 | 1 | 1 | |

Klein four-group | 2 | 2 | 1 | 2 | 0 | |

Dihedral group:D8 | 1 | 1 | 1 | 1 | 1 | |

Total | -- | 10 | 8 | 6 | 6 | 4 |

### Table listing number of subgroups by order

Group order | Occurrences as subgroup | Conjugacy classes of occurrence as subgroup | Automorphism classes of occurrences as subgroup | Occurrences as normal subgroup | Occurrences as characteristic subgroup |
---|---|---|---|---|---|

1 | 1 | 1 | 1 | 1 | |

5 | 3 | 2 | 1 | 1 | |

3 | 3 | 2 | 3 | 1 | |

1 | 1 | 1 | 1 | 1 | |

Total | 10 | 8 | 6 | 6 | 4 |

## The center (type (2))

`Further information: center of dihedral group:D8`

This is a two-element subgroup . It is a characteristic subgroup.

In the permutation representation, it is given by the set:

.

### Subgroup-defining functions yielding this subgroup

There are many subgroup-defining functions that yield this subgroup, for instance:

- The center: . These are the only two elements that commute with every element. It is also equal to , the subgroup generated by elements of order two in the center.
- The commutator subgroup: . The quotient group is isomorphic to the Klein four-group.
- The Frattini subgroup: It is the intersection of three maximal subgroups, each of order four. (these are covered in points (4) and (5) in the list).
- The socle: In fact, is the
*unique*minimal normal subgroup. - The first agemo subgroup: is the subgroup generated by all squares, and is hence .
- The ZJ-subgroup: It is the center of the join of abelian subgroups of maximum order.

### Subgroup properties satisfied by this subgroup

On account of being an agemo subgroup as well as on account of being the commutator subgroup, the center is a verbal subgroup -- it is a subgroup generated by words of a certain form (in the agemo description, these words are squares; in the commutator subgroup description, these words are commutators). Thus, it satisfies the following properties:

- Fully invariant subgroup: It is invariant under any endomorphism of the whole group.
`Further information: Verbal implies fully invariant` - Image-closed fully invariant subgroup: Its image under any surjective homomorphism is fully characteristic in the image.
`Further information: Verbal implies image-closed fully invariant` - Image-closed characteristic subgroup: Its image under any surjective homomorphism is characteristic in the image.
`Further information: Verbal implies image-closed characteristic` - Characteristic subgroup: It is characteristic in the whole group.

For obvious reasons, it satisfies the following properties:

- Central subgroup, Central factor: These follow from its being equal to the center.
- Simple normal subgroup, Transitively normal subgroup
- Base diagonal of a wreath product

### Subgroup properties not satisfied by this subgroup

- Homomorph-containing subgroup: There are homomorphic images of this subgroup that are not contained in it.
- Isomorph-free subgroup: There are other subgroups of the group isomorphic to it, namely, the subgroups of type (3) in the list.
- Intermediately characteristic subgroup: The subgroup is
*not*characteristic in every intermediate subgroup. In particular, it is not characteristic in subgroups of the type (4), such as .`Further information: Characteristicity does not satisfy intermediate subgroup condition, center not is intermediately characteristic, commutator subgroup not is intermediately characteristic, Frattini subgroup not is intermediately characteristic` - Complemented normal subgroup, Lattice-complemented subgroup, Permutably complemented subgroup: There is no complement to the center. This is a phenomenon for all nilpotent groups, and follows from the fact that nilpotent implies center is normality-large.

## The four-element characteristic subgroup (type (5))

`Further information: Cyclic maximal subgroup of dihedral group:D8`

This subgroup is the set . In the permutation representation, it is given by .

### Subgroup-defining functions yielding this subgroup

None of the standard choices of subgroup-defining functions yields this subgroup. It can be described using the following:

- It is the unique maximal among abelian characteristic subgroups. Note that for a general group, there need not exist a unique maximal among abelian characteristic subgroups.
`For full proof, refer: Maximal among abelian characteristic subgroups may be multiple and isomorphic` - It is the unique constructibly critical subgroup, i.e., the only subgroup that could arise through an application of the constructive procedure in Thompson's critical subgroup theorem. However, it is
*not*the only critical subgroup.

### Subgroup properties satisfied by this subgroup

The subgroup is a cyclic maximal subgroup. It satisfies the following properties:

- Isomorph-free subgroup, Isomorph-containing subgroup
- Prehomomorph-contained subgroup
- Maximal characteristic subgroup
- Intermediately characteristic subgroup: This subgroup is characteristic in every intermediate subgroup.
- Complemented normal subgroup
- Regular kernel
- Self-centralizing subgroup: It is not centralized by any outside element.
- Coprime automorphism-faithful subgroup
- Critical subgroup
- Constructibly critical subgroup

Some maximality properties of note:

- Abelian subgroup of maximum order
- Maximal among abelian subgroups
- Maximal among abelian normal subgroups
- Maximal among abelian characteristic subgroups
- Centrally large subgroup, centralizer-large subgroup, minimal CL-subgroup
- Subgroup with abelianization of maximum order

### Subgroup properties not satisfied by this subgroup

- Fully characteristic subgroup, retraction-invariant subgroup: There exists a retraction with kernel and image that does
*not*leave this subgroup invariant. Hence, it is not fully characteristic or retraction-invariant. - Verbal subgroup: Since the subgroup is not fully characteristic, it is not verbal.
- Image-closed characteristic subgroup: Quotienting out by the center of the whole group gives a subgroup that is not characteristic in the image.
`Further information: Characteristicity does not satisfy image condition`

## The non-characteristic four-element subgroups (type (4))

`Further information: Klein four-subgroups of dihedral group:D8`

These two subgroups are related by an outer automorphism, but are *not* conjugate (in fact, both are normal subgroups). Since they're automorphs, they in particular satisfy and dissatisfy the same subgroup properties.

The two subgroups are: and .

In terms of permutations, they are given by: and . Note that these two subgroups, while automorphs inside the dihedral group, are not automorphs inside the whole symmetric group.

### Subgroup properties satisfied by these subgroups

- Maximal normal subgroup: They are normal subgroups of index two.
- Isomorph-automorphic subgroup
- Self-centralizing subgroup

Properties related to maximality and abelianness:

- Maximal among abelian subgroups
- Abelian subgroup of maximum order
- Abelian subgroup of maximum rank
- Elementary abelian subgroup of maximum order
- Maximal among abelian normal subgroups
- Centrally large subgroup, centralizer-large subgroup, minimal CL-subgroup
- Subgroup with abelianization of maximum order

### Subgroup properties not satisfied by these subgroups

- Automorph-conjugate subgroup: Although the two subgroups are automorphs of each other, they are not conjugate. Hence, neither of them is an automorph-conjugate subgroup.
- p-normal-extensible automorphism-invariant subgroup: These subgroups are
*not*invariant under all the automorphisms of the whole group that are p-normal-extensible, i.e., those automorphisms that can be extended to automorphisms for any -group containing the dihedral group as a normal subgroup. This follows from the fact that*every*automorphism is p-normal-extensible.`Further information: Finite p-group with center of prime order and inner automorphism group maximal in p-Sylow-closure of automorphism group implies every p-automorphism is p-normal-extensible`

## The two-element non-normal subgroups (type (3))

`Further information: non-normal subgroups of dihedral group:D8`

There are four of these: , , , and . In terms of permutations, these are the subgroups , , and .

The subgroups and are conjugate to each other, and the subgroups and are conjugate to each other. These two pairs of subgroups are not conjugate, but they are related by an outer automorphism of the dihedral group -- one that does not extend to the symmetric group on four letters.

### Subgroup properties satisfied by these subgroups

- 2-subnormal subgroup
- 2-hypernormalized subgroup
- Conjugate-permutable subgroup
- Core-free subgroup
- Base of a wreath product

### Subgroup properties not satisfied by these subgroups

## Lattice of subgroups

### The entire lattice

The lattice of subgroups of the dihedral group has the following interesting features:

- Since the group has no nontrivial power automorphisms, all the automorphisms of act nontrivially on the lattice. The inner automorphism group, which has order four, contains the identity automorphism, an automorphism that flips the two left-most order two subgroup, an automorphism that flips the two right-most order two subgroups, and an automorphism that does both flips. Note that inner automorphisms preserve all the order four subgroups. The outer automorphisms (which form exactly one coset of the inner automorphism group) exchange the two normal Klein four-subgroups that are not characteristic, while preserving the cyclic characteristic order four subgroup.
- Every automorphism of the lattice
*does*arise from a group automorphism. Combined with the previous point, we get that the automorphism group of the subgroup lattice is isomorphic to the automorphism group of . - The lattice does
*not*enjoy reverse symmetry, i.e., it is not equal to its opposite lattice. This is because there are five subgroups of order two, as compared to three subgroups of order four.

### The lattice collapsed to conjugacy classes

If we collapse the lattice under the equivalence relation of conjugacy, we find that the 2-subnormal subgroups contained in the same normal subgroup of order four collapse into single entities. The new diagram obtained is considerably simpler, and the inner automorphism group acts trivially on it. The outer automorphism group simply flips the two conjugacy classes of 2-subnormal subgroups and the two normal non-characteristic subgroups. These are the only automorphisms of this lattice, so the automorphism group of this lattice is isomorphic to the outer automorphism group of .

### The sublattice of normal subgroups

For the sublattice of normal subgroups, we delete the four 2-subnormal subgroups of order two, leaving only the center. The center is the unique minimal normal subgroup (i.e., a monolith) and is contained in three maximal normal subgroups of order four. Note that this lattice is isomorphic to the lattice of normal subgroups of the quaternion group, but the quaternion group has no non-normal subgroups.

Some aspects of this generalize to arbitrary -groups: if the center is of order it is the unique minimal normal subgroup. In fact, more general results include: prime power order implies center is normality-large, minimal normal implies central in nilpotent, omega-1 of center is normality-large in nilpotent p-group. The upshot of all these results is that the minimal normal subgroups of a finite -group are *precisely* the subgroups of order in the center.

### The sublattice of characteristic subgroups

The sublattice of characteristic subgroups is totally ordered, comprising the identity, the center, the four-element cyclic subgroup, and the whole group. Note that there is one characteristic subgroup of every order dividing the group order. This differs from the quaternion group case, which has no characteristic subgroup of order four.

### The sublattice of fully characteristic subgroups

The sublattice of fully characteristic subgroups comprises the identity element, the center, and the whole group.

### Abelian subgroups and elementary abelian subgroups

The abelian subgroups are obtained simply by removing the whole group from the lattice, while the elementary abelian subgroups are obtained by removing the whole group *and* the cyclic subgroup of order four.

## Subgroups of subgroups

We look at the three main types of examples of subgroups , where is the dihedral group of order eight, is a subgroup of order four, and is a subgroup of order two.

### The center contained in the cyclic characteristic subgroup

In our notation, and .

Here, both and are characteristic in . Also, is characteristic in .

This gives examples for the following:

- Characteristicity does not satisfy image condition: Although is characteristic in , is not characteristic in .
- Intermediate characteristicity is not transitive
- Isomorph-freeness is not transitive

### The center contained in a Klein four-subgroup

There are two examples of this, equivalent via an outer automorphism. One is and . The other is , .

Here, is characteristic in . However, is not characteristic in and is not characteristic in .

This gives examples for the following:

- Characteristicity does not satisfy intermediate subgroup condition
- Full invariance does not satisfy intermediate subgroup condition
- Permutably complemented is not transitive
- Lattice-complemented is not transitive

### A 2-subnormal subgroup contained in a Klein four-subgroup

There are four examples of this, all related via automorphisms. One is , . This gives examples for the following:

## Intersecting pairs of subgroups

### A cyclic characteristic subgroup and Klein four-subgroup

Here, and . This gives examples of the following:

- Permutably complemented is not finite-intersection-closed
- Lattice-complemented is not finite-intersection-closed
- Isomorph-normality is not finite-intersection-closed

### Two Klein four-subgroups

Here and . This gives examples of the following:

- Permutably complemented is not finite-intersection-closed
- Lattice-complemented is not finite-intersection-closed
- Isomorph-normality is not finite-intersection-closed

## Aspects of subgroup structure relevant for embeddings in bigger groups

### 2-automorphism-invariance and 2-core-automorphism-invariance

A subgroup of a -group is termed a p-automorphism-invariant subgroup if it is invariant under all automorphisms of the whole group whose order is a power of , while it is termed a p-core-automorphism-invariant subgroup if it is invariant under all automorphisms in the -core of the automorphism group. We have:

Characteristic -automorphism-invariant -core-automorphism-invariant normal

In the case of the dihedral group, we have the following:

- The characteristic subgroups are the same as the -automorphism-invariant subgroups which in turn are the same as the -core-automorphism-invariant subgroups, namely: the whole group, the trivial subgroup, the cyclic subgroup of order four, and the center. Thus, the only subgroups of the dihedral group that are normal in every -group containing it are the whole group, the trivial subgroup, the cyclic subgroup of order four, and the center. In other words, for each of the elementary abelian subgroups of order four, we can find bigger -groups containing the dihedral group in which these are not normal.
- Also of note is the following: the dihedral group of order eight has an elementary abelian subgroup of order four but no elementary abelian -core-automorphism-invariant subgroup of order four. This gives, indirectly, an example of a group having an elementary abelian subgroup of order four but no elementary abelian normal subgroup of order four. Namely, we need to find a bigger group that contains the -core of the automorphism group of the dihedral group, acting on the dihedral group by inner automorphisms. The smallest such example is dihedral group:D16.

### 2-normal-extensible automorphism-invariance and normal-extensible automorphism-invariance

`Further information: Finite p-group with center of prime order and inner automorphism group maximal in p-Sylow-closure of automorphism group implies every p-automorphism is p-normal-extensible, Every automorphism is center-fixing and inner automorphism group is maximal in automorphism group implies every automorphism is normal-extensible`

It turns out that for the dihedral group of order eight, every -automorphism (and hence, every automorphism) is -normal extensible, and hence the -normal-extensible automorphism-invariant subgroups are precisely the same as the characteristic subgroups. In particular, the two non-characteristic normal subgroups of order four are not -normal-extensible automorphism-invariant.

More generally, it turns out that since the center of the dihedral group has order two, and the inner automorphism group is maximal in the automorphism group, every automorphism is a normal-extensible automorphism. In particular, the normal-extensible automorphism-invariant subgroups are the same as the characteristic subgroups, and thus, the two non-characteristic normal subgroups of order four are not normal-extensible automorphism-invariant.

It follows that there is no group containing the dihedral group of order eight as a normal subgroup, such that one of these subgroups becomes a characteristic subgroup.

### Coprime automorphism-invariance

`Further information: Coprime automorphism-invariant normal subgroup of Hall subgroup is normalizer-relatively normal, isomorph-normal coprime automorphism-invariant of Sylow implies weakly closed`

Since the automorphism group of this group is also a -group, every subgroup is coprime automorphism-invariant. In particular, every normal subgroup is a coprime automorphism-invariant subgroup, and every normal subgroup of order four is an isomorph-normal coprime automorphism-invariant subgroup. We can deduce the following:

- For any group containing the dihedral group of order eight as a -Sylow subgroup, all the subgroups of order are weakly closed subgroups.
- On the other hand, none of the subgroups of order two are weakly closed subgroups. Clearly, the non-normal subgroups are not weakly closed. Even the center, which is a normal subgroup, need not be weakly closed. The best example is the symmetric group of degree four, which has three conjugate -Sylow subgroups, all dihedral of order eight, intersecting in a normal Klein four-group comprising the double transpositions. The centers of the three subgroups are the two-element subgroups of this Klein four-group. There are inner automorphisms of the whole group that interchange these groups, and hence their centers, which are all inside this normal Klein four-group. However, there is no inner automorphism in the normalizer of the dihedral Sylow subgroup that sends its center to any of the other centers.

### Abelian subgroups of maximum order

There are three abelian subgroups of maximum order: a cyclic characteristic subgroup of order four (type (5)), and the two elementary abelian subgroups of order four (type (4)). These are the only subgroups that are maximal among abelian subgroups.

The join of abelian subgroups of maximum order, sometimes called the Thompson subgroup and denoted by , is thus the whole group. In particular, the ZJ-subgroup, which is the center of this Thompson subgroup, is simply the center of the whole group, i.e., the subgroup of type (2) in the listing.

### Abelian subgroups of maximum rank

The rank of the dihedral group is two, and there are two abelian subgroups of maximum rank. These are the two elementary abelian subgroups of order four (type (4)) and they are automorphic subgroups. The join of abelian subgroups of maximum rank is the whole group.

### Elementary abelian subgroups of maximum order

These coincide with the abelian subgroups of maximum rank.

### Centrally large subgroups

`Further information: Centralizer-large subgroup, centrally large subgroup, minimal CL-subgroup, Centralizer-large subgroups permute and their product and intersection are centralizer-large, all minimal CL-subgroups have the same commutator subgroup`

It turns out that all nontrivial normal subgroups of the group are centralizer-large subgroups, and the three subgroups of order four as well as the whole group are centrally large subgroups. The minimal CL-subgroups are thus the subgroups of order four, and their common commutator subgroup is the center (type (2)). The unique largest centrally large subgroup is the whole group.

### Subgroups with abelianization of maximum order

`Further information: Subgroup with abelianization of maximum order`

These are again the three subgroups of order four and the whole group. All of these have abelianizations of order four.