I think you mean `x = div*y` and it allows p(x) = 0 to be rewritten as m*q(x) where the coefficients of q are smaller than p:

```
>>> eq=x**5 + 512*x + 1024
>>> eq.subs(x,y/4)
y**5/1024 + 128*y + 1024
>>> factor(_)
(y**5 + 131072*y + 1048576)/1024
>>> eq.subs(x,y*4)
1024*y**5 + 2048*y + 1024
>>> factor(_)
1024*(y**5 + 2*y + 1)

```