[GAP Forum] Orbit of group action on matrices
Burkhard Höfling
burkhard at hoefling.name
Sat Sep 25 09:59:58 BST 2010
On 25/09/2010, at 8:36 , Benjamin Sambale wrote:
> Dear Forum,
>
> I've defined an action of a permutation group on a set of square matrices in the following way:
>
> act:=function(x,g)
> local i;
> for i in [1..Length(x[1])] do x[i]:=Permuted(x[i],g); od;
> return Permuted(x,g);
> end;
>
> This works as expected, for example:
>
> gap> act([[1,2],[3,4]],(1,2));
> [ [ 4, 3 ], [ 2, 1 ] ]
>
> However, using the Orbit command gives
>
> gap> Orbit(Group((1,2)),[[1,2],[3,4]],act);
> Lists Assignment: <list> must be a mutable list
>
> What is wrong here?
Your action tries to change the argument x (by assigning permuted rows). But this is probably not what you had intended anyway. Try the following.
act:=function(x,g)
local i, y;
y := [];
for i in [1..Length(x[1])] do y[i]:=Permuted(x[i],g); od;
return Permuted(y,g);
end;
Cheers,
Burkhard.
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