Karim BELABAS on Sat, 7 Sep 2002 23:04:01 +0200 (MEST)

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Re: polcoeff

On Sat, 27 Jul 2002, Ilya Zakharevich wrote:

> It looks like polcoeff() is silently converting a rational function
> into a power series.  Is it a good thing to do?

No. It is rather obcure, and quite useless since can have Ser() for explicit
conversion. Since this was not documented, I call it a bug.

I have removed that conversion, as I was fixing the
polcoeff(1/x, -3) --> SEGV bug.

> The fact that it *can* operates on power series *is* mentioned in the
> docs, but not in the docs for the function.  The fact that it would
> coerce a "wrong" type is not mentioned at all...

I have modified the docs:

(22:04) gp > ??polcoeff

   coefficient  of  degree  s  of  the  polynomial x,  with respect to the main
variable if v is omitted,  with respect to v otherwise.   Also applies to power
series,  scalars  (polynomial of degree 0),  and to rational functions provided
the denominator is a monomial.

Karim Belabas                    Tel: (+33) (0)1 69 15 57 48
Dép. de Mathematiques, Bat. 425  Fax: (+33) (0)1 69 15 60 19
Université Paris-Sud             Email: Karim.Belabas@math.u-psud.fr
F-91405 Orsay (France)           http://www.math.u-psud.fr/~belabas/
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