thanks for both replies!  OK, so there are various fast algorithms and
Magma has implemented at least one of them.  Sage is using Singular.
I wonder why Singular has not got a fast algorithm for this?

For this application of mine, I need to do a number of one-off
factorizations like this in order to implement some cases of elliptic
curve isogenies (specifically in characteristics 2 and 3) which were
not already done.  So of course I can use Magma for those, and the
resulting factors are then hard-wired into the Sage code.  But it
would be more satisfying to have the whole thing done in Sage!

John

2010/1/12 YannLC <yannlaiglecha...@gmail.com>:
>
>
> On Jan 12, 2:46 pm, javier <vengor...@gmail.com> wrote:
>> There are indeed well known (sort of) fast algorithms for
>> factorization of multivariable polynomials over finite 
>> fields:http://portal.acm.org/citation.cfm?id=808748http://www.jstor.org/stable/2008063?seq=1
>>
>> In the second paper there is a particular (probabilistic) algorithm
>> for bivariate polynomials. Maybe magma has something like that?
>>
>
> Citing Magma's help (http://magma.maths.usyd.edu.au/magma/htmlhelp/
> text315.htm):
>
> For bivariate polynomials, a polynomial-time algorithm in the same
> spirit as van Hoeij's Knapsack factoring algorithm [vH02] is used.
>
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