Alexander Shumakovitch on Thu, 29 May 2003 23:41:37 +0200

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Re: matsnf segfaults

On Thu, May 29, 2003 at 11:06:41PM +0200, Karim BELABAS wrote:
> Having fiddled some more with the routine, I committed another patch to
> matsnf to allow arbitrary rectangular matrices [ for integer matrices only ],
> as a preliminary cleanup for the introduction a modular algorithm.
> When the matrix is singular, modular HNF reductions are done to reduce to
> that case. This already reduces your example to a trivial form (dealt with
> in half a second).
> It's quite straightforward to make the resulting algorithm modular
> [ provided one doesn't ask for transformation matrices ], but I'd like to
> know first whether the above works as expected !
Yes it does. Partially ;-) SNF works perfectly now and going through HNF
requires twice as little time. The reason could be that, since I'm
interested in the torsion only, I remove all columns on the right side
of the matrix brought to HNF that have pivot 1.

I used to pad the matrix with zeros to make it square. Now if I remove
this padding, matsnf doesn't complain anymore and produces the correct
result 10% faster than before. But if I go through HNF again, Pari
immediately complains about low memory (200MB stack not enough!). The
padding is obviously done _after_ mathnf is completed, so it's not to

I can send you the original matrix, if you like to test it.

Thanks for your quick response!

  --- Alexander.