# Q8 in SD16

From Groupprops

This article is about a particular subgroup in a group, up to equivalence of subgroups (i.e., an isomorphism of groups that induces the corresponding isomorphism of subgroups). The subgroup is (up to isomorphism) quaternion group and the group is (up to isomorphism) semidihedral group:SD16 (see subgroup structure of semidihedral group:SD16).

The subgroup is a normal subgroup and the quotient group is isomorphic to cyclic group:Z2.VIEW: Group-subgroup pairs with the same subgroup part | Group-subgroup pairs with the same group part| Group-subgroup pairs with the same quotient part | All pages on particular subgroups in groups

## Contents

## Definition

Here, is the semidihedral group:SD16, the semidihedral group of order sixteen (and hence, degree eight). We use here the presentation:

has 16 elements:

The subgroup of interest is the subgroup . It is a quaternion group of order 8 and is given by:

## Arithmetic functions

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

order of whole group | 16 | |

order of subgroup | 8 | |

index of subgroup | 2 | |

size of conjugacy class (=index of normalizer) | 1 | |

number of conjugacy classes in automorphism class | 1 |

## Subgroup properties

### Invariance under automorphisms and endomorphisms

Property | Meaning | Satisfied? | Explanation |
---|---|---|---|

normal subgroup | invariant under inner automorphisms | Yes | index two implies normal |

characteristic subgroup | invariant under all automorphisms | Yes | On account of being an isomorph-free subgroup. |

fully invariant subgroup | invariant under all endomorphisms | No | The endomorphism (i.e., the retraction to ) does not preserve this subgroup. |

isomorph-free subgroup | no other isomorphic subgroup | Yes |

## GAP implementation

The group and subgroup pair can be constructed using GAP as follows:

`gap> G := SmallGroup(16,8); H := Filtered(NormalSubgroups(G), x -> IdGroup(x) = [8,4])[1];`

Here is the GAP display:

gap> G := SmallGroup(16,8); H := Filtered(NormalSubgroups(G), x -> IdGroup(x) = [8,4])[1]; <pc group of size 16 with 4 generators> Group([ f1, f3, f4 ])

Here is GAP code to verify various assertions made on this page:

gap> Order(G); 16 gap> Order(H); 8 gap> Index(G,H); 2 gap> StructureDescription(H); "Q8" gap> StructureDescription(G/H); "C2" gap> IsNormal(G,H); true gap> IsCharacteristicSubgroup(G,H); true gap> IsFullinvariant(G,H); false