#13054: Pernicious bug for algebraic numbers
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Reporter: rbeezer | Owner:
davidloeffler
Type: defect | Status: new
Priority: major | Milestone:
sage-5.7
Component: number fields | Resolution:
Keywords: sd40.5 | Work issues:
Report Upstream: Reported upstream. No feedback yet. | Reviewers:
Authors: | Merged in:
Dependencies: | Stopgaps:
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Old description:
> PARI's `polred()` returns reducible polynomials. GP session:
> {{{
> gp> pol = x^4 - 4294967296*x^2 + 54265257667816538374400;
> gp> L = polred(pol);
> gp> factor(L[4])
> %14 =
> [x^2 + 211955648366398871041 2]
> }}}
>
> This bug causes problem with arithmetic over `QQbar`:
>
> Line 1563 (5.1.beta0) of sage/rings/qqbar.py:
>
> {{{
> rev = parent(best_elt.Mod(pari_poly).modreverse().lift())
> }}}
>
> Here, we are calling `modreverse()` on a element of a subfield. It would
> make a lot of exact linear algebra '''much''' more reliable if this was
> resolved.
New description:
PARI's `polred()` returns reducible polynomials. GP session:
{{{
gp> pol = x^4 - 4294967296*x^2 + 54265257667816538374400;
gp> L = polred(pol);
gp> factor(L[4])
%14 =
[x^2 + 211955648366398871041 2]
}}}
'''Upstream''': [http://pari.math.u-bordeaux.fr/cgi-
bin/bugreport.cgi?bug=1395]
This bug causes problem with arithmetic over `QQbar`:
Line 1563 (5.1.beta0) of sage/rings/qqbar.py:
{{{
rev = parent(best_elt.Mod(pari_poly).modreverse().lift())
}}}
Here, we are calling `modreverse()` on a element of a subfield. It would
make a lot of exact linear algebra '''much''' more reliable if this was
resolved.
--
Comment (by jdemeyer):
The workaround looks good to me. You should add a comment though saying
why we do this, refering to this ticket. And of course add the obligatory
doctest.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13054#comment:9>
Sage <http://www.sagemath.org>
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