On Thursday 03 September 2009, William Stein wrote:
> Hi Martin (and Sage-devel),
>
> I discovered that polynomial substitution is badly inconsistent between the
> symbolic ring and multivariate polynomials:
>
> sage: R.<x,y> = QQ[]
> sage: f = x + 2*y
> sage: f.subs(x=y,y=x)
> 3*y
> sage: var('x,y')
> sage: f = x + 2*y
> sage: f.subs(x=y,y=x)
> 2*x + y
>
> See http://trac.sagemath.org/sage_trac/ticket/6873 for more and a proposal
> about how to solve this.
FWIW this behaviour is documented:
.. note::
The evaluation is performed by evaluating every
``variable:value`` pair separately. This has side effects
if e.g. x=y, y=z is provided. If x=y is evaluated first,
all x variables will be replaced by z eventually.
Also, I think that ticket is dupe of
http://trac.sagemath.org/sage_trac/ticket/6482
Still, it needs to be addressed.
Cheers,
Martin
--
name: Martin Albrecht
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