Karim BELABAS on Thu, 29 May 2003 23:11:02 +0200 (MEST)

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Re: matsnf segfaults

On Thu, 29 May 2003, Alexander Shumakovitch wrote:

> On Thu, May 29, 2003 at 08:53:31PM +0200, Karim BELABAS wrote:
>> P.S: Note that the algorithm used is spectacularly inefficient in medium to
>> large dimensions (it was written to be used for 10 x 10 matrices...), so I
>> doubt the computation will ever finish.
> That's weird! In fact, this matrix was produced by mathnf to speed up
> the calculations. It was always the case for me that matsnf performs
> faster on matrices prepared by mathnf first.

Shouldn't be the case if you add up the HNF precomputation time to the
matsnf timings: the first (non-trivial) instruction in matsnf is to replace
the matrix by its HNF...  [ that is, unless you know a good multiple of the
determinant to speed up the modular hnf computation ]

> Now in this case, it takes less than a second to get the SNF of the
> original matrix directly. But way too much after HNF (it complained about
> low memory after a couple of minutes). Is it expected?

Well, my followup patch fixed that :-)

Karim Belabas                     Tel: (+33) (0)1 69 15 57 48
Dép. de Mathématiques, Bât. 425   Fax: (+33) (0)1 69 15 60 19
Université Paris-Sud              http://www.math.u-psud.fr/~belabas/
F-91405 Orsay (France)            http://www.parigp-home.de/  [PARI/GP]