Re: pari-2.2.1.alpha released

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

On Fri, 26 Oct 2001, Ilya Zakharevich wrote:
> If I use
>
>   { subst_poly(pol, from, to) =
>     local(t='subst_poly_t);
>     subst(lift(Mod(pol,from-t),t),t,to)
>   }
>
>   subst_poly(x^3+1, x^2+1,y)
>
> I get
>
>   Mod((y - 1)*x + 1, x^2 + (-y + 1))
>
> instead of (y - 1)*x + 1 which I want.  Probably I misunderstand the
> intent of the second argument to lift.
>
> I want to hint it to lift that it should get rid of Mod(*,from-t), and
> not of "something else".  How to do it?

{ subst_poly(pol, from, to) =
  local(t='subst_poly_t, M);
  M = from - t;
  subst(lift(Mod(pol,M), variable(M)),t,to)
}

> P.S.  BTW, would the subroutine above work OK if given polynomials
>       including t?

It should.

    Karim.
-- 
Karim Belabas                    email: Karim.Belabas@math.u-psud.fr
Dep. de Mathematiques, Bat. 425
Universite Paris-Sud             Tel: (00 33) 1 69 15 57 48
F-91405 Orsay (France)           Fax: (00 33) 1 69 15 60 19
--
PARI/GP Home Page: http://www.parigp-home.de/