# 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 $N$ is dihedral group:D8, the quotient group $Q$ 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 $\operatorname{Out}(N)$ is isomorphic to cyclic group:Z2, and thus there is a unique nontrivial map from $Q$ to it.

## Description in terms of cohomology groups

We have the induced outer action which is nontrivial:

$Q \to \operatorname{Out}(N)$

Composing with the natural mapping $\operatorname{Out}(N) \to \operatorname{Aut}(Z(N))$, we get a trivial map:

$Q \to \operatorname{Aut}(Z(N))$

Thus, the number of extensions for the trivial outer action of $Q$ on $N$ equals the number of elements in the second cohomology group for trivial group action $H^2(Q;Z(N))$ for the trivial group action. More explicitly, $H^2(Q;Z(N))$ 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.

$H^2(Q;Z(N))$ 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 $Q$ on $N$ Second part of GAP ID (order is 16) Is the extension a semidirect product of $N$ by $Q$? 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