Re: Bug in bnfcertify?

[Stephan Ehlen <ehlen@mathematik.tu-darmstadt.de>]

Great!

Thanks a lot for your reply.
(Of course, I should have mentioned the version number... it is 2.5.1.).

Best,
Stephan


On Mon, Jul 22, 2013 at 8:45 PM, Karim Belabas
<Karim.Belabas@math.u-bordeaux1.fr> wrote:
> Hallo,
>
> * Stephan Ehlen [2013-07-22 19:49]:
>> I tried to compute class numbers of Hilbert class fields of several
>> imaginary quadratic fields, originally using sage but sage basically
>> interfaces pari/gp for this purpose. In one particular example, the
>> computation took very long, so I tried to find out what was going on.
>> If I perform bnfinit() in pari/gp with the polynomial that defines the
>> Hilbert class field in this case, the process terminates after less
>> than a minute or so on my laptop. It is the bnfcertify that takes so
>> long. Well, in order to see if we end up in an infinite loop or so, I
>> turned on debugging and was a bit surprised to see that pari continued
>> with the function testprimes beyond the Minkowski bound in this case.
>
> (Actually, Zimmert's twin class bound)
>
>> Looking at the source code only briefly, it seems that the second for
>> loop in 'testprimes' in the file buch2.c does never terminate.
>
> In 'master' (2.6.1, development git-6446ba6), the computation stops for
> p = 1015919 as expected (with default primelimit).
>
> In 'stable' (2.5.4), the second loop in testprimes() indeed does not
> terminate :-(. The attached patch fixes this.
>
>
> 2.6.0 and later are not affected because all loops over consecutive primes
> have been rewritten (and simplified) using the new forprime_next() iterator.
>
> 2.3.* and earlier are not affected because a loop over primes larger than
> primelimit in this function simply raised an exception.
>
> Cheers,
>
>     K.B.
> --
> Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
> Universite Bordeaux 1          Fax: (+33) (0)5 40 00 69 50
> 351, cours de la Liberation    http://www.math.u-bordeaux1.fr/~kbelabas/
> F-33405 Talence (France)       http://pari.math.u-bordeaux1.fr/  [PARI/GP]
> `