Re: precp and valp for zero p-adics

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

* Jeroen Demeyer [2014-01-09 13:22]:
> Hello pari-dev,
> 
> it seems that p-adic numbers which equal zero (e.g. O(3^10)) are
> always created with precp=0. However, the manual doesn't really
> document the meaning of precp and valp for zero p-adic numbers.
> 
> A 3-adic zero with precp=10 and valp=0 is printed as O(3^10),
> exactly the same as precp=0 and valp=10 but the behaviour is
> different.
> 
> For example, trying to compute the poldisc() of
> (1 + O(3^10))*x^3 + O(3^10)*x^2 + (1 + O(3^10))*x + (1 + O(3^10))
> yields the wrong answer if the x^2 term has precp=10 and valp=0.
> 
> From this, I conclude that p-adic zero numbers should always have
> precp=0. Is this a correct conclusion?

Yes.

> If yes, it should be documented better in the libpari manual.

Done. :-)

The 'setxxx' macros should never be used outside of the PARI kernel:
there is no safety mechanism (you can trivially create invalid objects
or corrupt one of the universal constants such as gen_0, etc.)
and they usually provide negligible savings.

We still document them for the sake of backward compatibility, but the
preferred way is to use a higher lever routines, e.g. zeropadic(p, e) to
create O(p^e).

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]
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