Alex, you have found it! The variable i in that code is undefined.
Replace it by 0 so it reads
{{{
n1 = naInit(GF2_conv_to_long(GF2X_coeff(rep,0)))
}}}
and then it works.
Must run to a seminar or I would make the patch...
John
2009/4/28 Alex Ghitza <[email protected]>:
>
> On Tue, Apr 28, 2009 at 12:33 AM, Martin Albrecht
> <[email protected]> wrote:
>>
>> On Monday 27 April 2009, John Cremona wrote:
>>> 2^16 is the smallest for which givaro is not used. In char.2, we switch to
>>> NTL.
>>
>> Yes, it is likely in the conversion routine from NTL to Singular.
>>
>
> I managed to track this all the way to line 379 in
> libs/singular/singular.pyx, which is
>
> n1 = naInit(GF2_conv_to_long(GF2X_coeff(rep,i)))
>
> In fact, the problem is not with naInit but with what goes into it (so
> it looks like an NTL issue). To see this, add the following two lines
> before 379:
>
> temp = GF2_conv_to_long(GF2X_coeff(rep,i))
> print "result of conversion: %s"%temp
>
> And now run the following in Sage:
>
> {{{
> sage: F.<a> = GF(2^16)
> sage: R.<x, y> = F[]
> sage: R({(1,2):1})
> }}}
>
> The correct response (which occurs on some computers) is:
>
> {{{
> result of conversion: 1
> x*y^2
> }}}
>
> The incorrect response (caught by John and also occuring on sage.math) is:
>
> {{{
> result of conversion: 0
> 0*x*y^2
> }}}
>
> Somehow NTL takes the coefficient 1 and messes it up to get 0, and
> this is machine-dependent. This is as far as my detective skills go.
> I don't even know how to check whether the problem is in GF2X_coeff or
> in GF2_conv_to_long, let alone how to fix this. Hopefully someone
> else can figure it out.
>
> Best,
> Alex
>
>
>
> --
> Alex Ghitza -- Lecturer in Mathematics -- The University of Melbourne
> -- Australia -- http://www.ms.unimelb.edu.au/~aghitza/
>
> >
>
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