Re: bnrgaloismatrix: precision too low in makematal

[Karim Belabas <Karim.Belabas@math.u-bordeaux.fr>]

* Karim Belabas [2018-03-06 18:56]:
> * Melanie Matchett Wood [2018-03-06 17:22]:
> > I am trying to use bnrgaloismatrix to compute the action of the Galois
> > group on the class group of a (Galois) sextic field.
> > 
> > I get an error:
> > *** bnrgaloismatrix: precision too low in makematal.
> > 
> > Here is an example that gives this:
> > 
> > f= x^6 + 2854*x^4 + 2036329*x^2 + 513996528;
> > K=bnrinit(bnfinit(f,1),1,1);
> > G=galoisinit(K);
> > M=bnrgaloismatrix(K,G);
> > 
> > I cannot find any documentation on "makematal."
> > 
> > Can someone explain what this error means, and in particular, am I just
> > running up against the limits of computing power (or built in pari limits)
> > or I can do something like increase a precision level or stacksize to try
> > to complete this computation?
> 
> 0) In current 'master' branch, the computation no longer triggers the error:
[...]
I can reproduce the problem in 'stable' (pari-2.9.5)

> 2) You can probably avoid the problem by computing the initial K at 
> a larger accuracy. Just start your computation with \p200 or so
> [ for instance ]. Or more verbosely default(realprecision,200)

Make that \p800. It works in 'stable', at least (while smaller values don't).

Cheers,

    K.B.
--
Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
Universite de Bordeaux         Fax: (+33) (0)5 40 00 21 23
351, cours de la Liberation    http://www.math.u-bordeaux.fr/~kbelabas/
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