#9524: Nasty bug with polynomial arithmetic and NTL contexts
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Reporter: craigcitro | Owner: AlexGhitza
Type: defect | Status: new
Priority: major | Milestone: sage-6.2
Component: basic arithmetic | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Old description:
> I just ran into the following nasty bug:
>
> {{{
> sage: polygen(GF(49, 'a')) ; polygen(GF(9, 'a'))
> x
> x
> sage: x = polygen(GF(49, 'a'))
> sage: -x
> 2*x
> sage: x + 0
> x
> sage: -x
> 6*x
> }}}
>
> This is still present in sage-4.5.
>
> I think I know what the problem is, but I haven't had time to sit down
> and fix it. It's an issue with the NTL context being correctly restored
> when switching from 7 to 3 and back. There's a very nice note (by David
> Roe, I think) in most of the `_p` files in NTL that describes a pattern
> one must avoid to get context restoration right; unfortunately, the code
> generated by `sage.rings.polynomial.polynomial_zz_pex.pyx` (via
> `polynomial_template.pxi`) exactly conforms to this anti-pattern.
New description:
I just ran into the following nasty bug:
{{{
sage: polygen(GF(49, 'a')) ; polygen(GF(9, 'a'))
x
x
sage: x = polygen(GF(49, 'a'))
sage: -x
2*x
sage: x + 0
x
sage: -x
6*x
}}}
This is still present in sage-4.5.
We have to ensure two things before playing with NTL finite field
elements:
* restore the ZZ_p context (characteristic);
* restore the ZZ_pE context (defining polynomial).
Restoring the latter does not automatically restore the former.
--
Comment (by jpflori):
The problem is actually different from what was stated in the original
description.
In fact, calling PY_NEw or whatever before restoring context was not the
(only) rpoblem.
The main problem is that restoring the ZZ_pE context does not restore the
"current" prime from the ZZ_p context.
--
Ticket URL: <http://trac.sagemath.org/ticket/9524#comment:5>
Sage <http://www.sagemath.org>
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