[GAP Forum] applying permutation to subset: strange problem?

[Burkhard Höfling]

On 2012-11-06, at 12:32 , Frederic Vanhove <fvanhove at cage.UGent.be> wrote:

> gap> orbits:=Orbits(h,[1..12]);
> [ [ 1, 2 ], [ 3, 4, 7, 8, 10, 12, 5, 6, 9, 11 ] ]
> gap> OnSets(orbits[2],w);
> OnSets: <set> must be a set (not a list (plain,cyc,nsort,imm))
> not in any function
> Entering break read-eval-print loop ...
> you can 'quit;' to quit to outer loop, or
> you can replace <set> via 'return <set>;' to continue

The error message tells you that the second orbit is not a set in the GAP sense (i.e., a strictly sorted list). As the documentation about Orbits states, the result contains lists of points, not necessarily sets. To do what you intended, just convert the list to a set first:

OnSets(Set(orbits[2]),w);

Regards,

Burkhard.