Re: idealadd() details

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

On Mon, Jun 15, 2009 at 04:46:21PM +0200, Jeroen Demeyer wrote:
> Hello list,
>
> I have a question about idealadd().  Either I'm misunderstanding  
> something, or the implementation is wrong, or the documentation is wrong.
>
> Consider the following:
> gp> idealadd(nfinit(t^2+1), [1,0;0,2], [1,0;0,2])
> %20 =
> [1 0]
>
> [0 1]
>
> Here I expected to get [1,0;0,2] as a result, since the documentation  
> states it just adds Z-modules.  Of course, as ideals, the answer is 
> correct.

The documentation agree with you (even if it is weird for idealadd to
do that).

The variant below works (which is equivalent as Z-modules)

? idealadd(nfinit(t^2+1), [2,0;0,1], [2,0;0,1])
%1 =
[2 0]

[0 1]

so this look like a bug caused by this shortcut (in idealadd)
  a = gcdii(gcoeff(x,1,1), gcoeff(y,1,1));
  if (is_pm1(a))
  {
    long N = lg(x)-1;
    if (!dz) { avma = av; return matid(N); }
    return gerepileupto(av, scalarmat(ginv(dz), N));
  }

Cheers,
Bill.