Re: subst()

[Walter Neumann <neumann@cpw.math.columbia.edu>]

subst(p,x^2,x) may be useless, but subst(p,x^2,y) is not.

On Wed, 12 Mar 2003, Igor Schein wrote:

> On Wed, Mar 12, 2003 at 06:32:08PM +0100, Karim BELABAS wrote:
> > On Tue, 11 Mar 2003, Igor Schein wrote:
> > > I remember seeing a recent subst() discussion, which I wasn't paying
> > > attention to.  Now I see this behavior, which I don't like at all:
> > >
> > > ? p=x^2-x-1;
> > > ? \y
> > >    simplify = 0 (off)
> > > ? type(subst(subst(p,x^2,x),x,x^2))
> > > "t_INT"
> > >
> > > I don't see how a transformation from a polynomial to an intereger can
> > > be justified, but I'd be glad if someone could make sense out of it.
> >
> > (18:30) gp > p = x^2 - x - 1;
> > (18:30) gp > subst(p,x^2,x)
> > %2 = -1
> >
> > What's wrong with this ? [ x^2 - x - 1 --> (x - x) - 1 = -1 ]
>
> As far as I'm concerned, the above is a correct but also a useless
> operations.  I don't see where anyone would use it.  For me an error
> would be more useful, detecting that I'm doing something wrong by
> mistake.
>
> Do other CAS have anything similar to subst(,x^n,x)?
>
> Igor
>