For a vector space $V$, I have constructed $AGL(V) = V \rtimes GL(V)$ as the elements $(v, A) \in V \times GL(V)$ (Cartesian product of sets, not a direct product) with multiplication $(v, A) (w, B) = (v + Aw, AB)$. This then has a faithful action on $V$ given by $$x^{(v,A)} = A^{-1}(x + v)$$ which is fairly ugly.
It was suggested to me that letting $GL(V)$ act on $V$ from the right (that is, write $V$ as row vectors rather than column vectors) would be nicer, but I'm struggling to make a construction that is actually nicer.
I'm theoretically trying to get to an action like $$x^{(v,A)} = xA + v, \tag{$\ast$}$$ (this is the action that e.g. [DM96, section 2.8] and [Neu87, section 2] use, but neither spell out the full group) but every multiplication I define is either invalid with the group actions, seems to not be a valid multiplication for a semidirect product, or is ugly.
E.g.
- Taking the obvious $(v,A)(w,B) = (v+wA, AB)$ does not work with $(\ast)$ (nor does it work with $(x + v)A^{\pm 1}$).
- The "multiplication" $(v,A)(w, B) = (Bv + w, AB)$ does work with $(\ast)$, $$x^{(v,A)(w,B)} = (xA + v)^{(w,B)} = (xA + v)B + w = x(AB) + (vB + w) = x^{(vB + w, AB)}.$$ However, AFAIK, a semidirect product should be adjusting the first element of second term by (some image in $\mathrm{Aut}(V)$ of) the second element of the first time, that is $(a,b)(c,d) = (ac^b, bd)$, and this is the reverse: $(a,b)(c,d) = (a^d c, bd)$. (Maybe I am being too inflexible with the definition of a semidirect product?)
- Defining $(v,A)(w,B) = (v + wA^{-1}, AB)$ works with $x^{(v, A)} = (x + v)A$, but this does not seem any clearer than my original one, and also doesn't match the natural action above.
I feel like I am missing something simple; could someone suggest a more natural representation & action of $AGL(V)$ (from either the left or right)? (I imagine there may be a canonical one that is so 'obvious' it is not worth mentioning in anything but the most basic text.)
[DM96] J. Dixon and B. Mortimer. Permutation Groups. Graduate Texts
in Mathematics. Springer New York, 1996.
[Neu87] P. Neumann. “Some algorithms for computing with finite permutation groups”. In: Proceedings of Groups St Andrews 1985.
Ed. by E. F. Robertson and C. M. Campbell. Cambridge Books
Online. Cambridge University Press, 1987, pp. 59–92.