# Extensions for nontrivial outer action of Z2 on D8

This article describes all the group extensions corresponding to a particular outer action with normal subgroup dihedral group:D8 and quotient group cyclic group:Z2.

We consider here the group extensions where the base normal subgroup is dihedral group:D8, the quotient group is cyclic group:Z2, and the induced outer action of the quotient group on the normal subgroup is the unique nontrivial map.

More explicitly, note that is isomorphic to cyclic group:Z2, and thus there is a unique nontrivial map from to it.

## Description in terms of cohomology groups

We have the induced outer action which is nontrivial:

Composing with the natural mapping , we get a trivial map:

Thus, the number of extensions for the trivial outer action of on equals the number of elements in the second cohomology group for trivial group action for the trivial group action. More explicitly, acts on the set of extensions (possibly with repetitions) in a manner that is equivalent to the regular group action. However, the extension set does *not* have a natural choice of extension corresponding to the identity element.

is the second cohomology group for trivial group action of Z2 on Z2, and is isomorphic to cyclic group:Z2. The extension set is thus a set of size two with this group acting on it.

## Extensions

Number of cohomology classes giving the extension | Corresponding group extension for on | Second part of GAP ID (order is 16) | Is the extension a semidirect product of by ? | Is the base characteristic in the whole group? | Nilpotency class of whole group | Derived length of whole group | Minimum size of generating set of whole group |
---|---|---|---|---|---|---|---|

1 | dihedral group:D16 | 7 | Yes | No | 3 | 2 | 2 |

1 | semidihedral group:SD16 | 8 | No | Yes | 3 | 2 | 2 |