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I have $G$ a finite group with a subgroup $H$ of index 7 and $C$ a cyclic group of order 2. I want to define in GAP the semidirect product $C^7 \rtimes G$ where every $g \in G$ acts on $C^7$ by permuting the coordinates in the same way $g$ permutes the cosets of $G/H$. How can I define this group? In particular I don't manage to find the elements of $Aut(C^7)$ corresponding to the action of $S_7$ that permutes the coordinates.

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What you want to construct is called a Wreath Product. You can construct it in GAP by specifying the group $C$ and the permutation action of $G$. For example:

gap> C:=CyclicGroup(2);
<pc group of size 2 with 1 generators>
gap> G:=SmallGroup(21,1); # some group with subgroup of index 7
<pc group of size 21 with 2 generators>
gap> H:=SylowSubgroup(G,3); # has index 7
Group([ f1 ])
gap> hom:=FactorCosetAction(G,H);
<action epimorphism>
gap> Image(hom); # indeed 7 points
Group([ (2,3,5)(4,7,6), (1,2,3,4,5,6,7) ])
gap> W:=WreathProduct(C,G,hom);
<group of size 2688 with 3 generators>

If you wanted to use the semi direct product construction you would need to construct an automorphism of $C^7$ that maps the components cyclically.

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  • $\begingroup$ Thank you, I didn't know FactorCosetAction existed, it's perfect! $\endgroup$ – M.Pintonello Apr 22 '18 at 17:18

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