Stephan Ehlen on Mon, 22 Jul 2013 21:23:01 +0200 |
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Re: Bug in bnfcertify? |
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] > `