I'm wondering how to specify to GAP which homomorphism to use when constructing a semidirect product. I'm trying to have it construct $\left(\mathbb{Z}_p\times\mathbb{Z}_p\right)\rtimes_\varphi S_3$. Since $\mathrm{Aut}\left(\mathbb{Z}_p\times\mathbb{Z}_p\right)\cong GL_2(\mathbb{F}_p)$, I want to specify $\varphi:S_3\rightarrow\mathrm{Aut}\left(\mathbb{Z}_p\times\mathbb{Z}_p\right)$ as $\varphi:S_3\rightarrow GL_2(\mathbb{F}_p)$ element by element (i.e. give a 2×2 matrix for each element of $S_3$).
I want to do this just to do group element multiplications quickly. I'll obviously specify $p$ when necessary.
I'm fairly new to GAP so a low-level answer (or reference) would be appreciated.
Best Answer
In case you still don't have an answer: You want to use the GAP command
SemidirectProduct( <g>, <hom>, <n> )
. Here,<hom>
is a group homomorphism from the group<g>
to the automorphism group of the group<n>
. In GAP this might look like this (for $p=7$):a
andb
are your favourite images of(1,2,3)
and(1,2)
, respectively. I choseOne(GL(2,7))
because I'm lazy. Note that you have to tell GAP that your matrices are over $\mathbb{F}_p$. Fortunately, that can be done like this:Now you can compose
hom
andiso
and define your semidirect product: