Re: subst

[Markus Endres <me@mendres.org>]

On Wed, 2004-01-21 at 15:13, Bill Allombert wrote:
> On Wed, Jan 21, 2004 at 02:59:48PM +0100, Markus Endres wrote:
> > hallo at all,
> > 
> > I have the following situation:
> > 
> > I want to substitute the variable x in a polmod through y like this:
> > 
> > ? subst(Mod(1,z^2+19)*x, x, y)
> > %53 = Mod(y, z^2 + 19)
> > ? 
> > 
> > I expect Mod(1,z^2+19)*y (this I need!) but get something strange (for
> > me) like Mod(y, z^2+19).
> > 
> > Now, why do I get this "wrong" result and how do I get the right one?
> 
> Bad variable ordering again:
> In a new session:
> 
> parisize = 4000000, primelimit = 500000
> ? x;y;z;
> ? subst(Mod(1,z^2+19)*x, x, y)
> %2 = Mod(1, z^2 + 19)*y
> 
> parisize = 4000000, primelimit = 500000
> ? x;z;y;
> ? subst(Mod(1,z^2+19)*x, x, y)
> %2 = Mod(y, z^2 + 19)
> 
> Whether Mod(1,z^2+19) by y give Mod(1,z^2+19)*y or Mod(y,z^2+19)
> depend of the relative priority of y and z.
> 
> if z << y you work in Q[Z]/(Z^2+19)[Y] and if z >> y you work in
> Q(Y)[Z]/(Z^2+19).


Ok, now I know the reason for this behavior. But, is there any
possibility to change the variables without the knowledge of the
priority?

The thing I want to do is the following: I compute a special polynomial
for a class field with my function called ClassField(...).

At the moment the polynomial is initialized in x, but sometimes I want
it in y or z. So I want to specify the variable if I call the function,
i.e. like ClassField(....,y) to get the polyonmial in y.

thanks for every help

cheers

me