On Thursday, January 14, 2016 at 12:36:32 PM UTC-8, Matthias Goerner wrote:
>
>
> # Lift polynomial back to Q[x][y]
> lifted = factor.map_coefficients(lambda c:Rx(c.lift()), Rxy)
>
> Are you sure this is what you want and what you did before? The
documentation of map_coefficients says that the second argument is to be
the new *base ring*. Indeed, with your code I get:
sage: parent(lifted)
Univariate Polynomial Ring in y over Univariate Polynomial Ring in y over
Univariate Polynomial Ring in x over Rational Field
i.e., an element in the polynomial ring QQ[x][y][Y], where "Y" is really
"y", (which is difficult to distinguish from the previous "y" visually ,
but sage does distinguish them).
Hence:
sage: parent(val)
Univariate Polynomial Ring in y over Univariate Polynomial Ring in x over
Real Interval Field with 53 bits of precision
(who knows which y was substituted or was used before ... You could figure
it out if you wanted).
If instead you do:
sage: lifted=factor.map_coefficients(lambda c:Rx(c.lift()), Rx)
sage: val = lifted.substitute(x = RIF(-2.67512520581, -2.67512520582), y =
RIF(.00001, 1.00002))
sage: type(val)
<type 'sage.rings.real_mpfi.RealIntervalFieldElement'>
I don't think there are any issues.
Incidentally, the whole "x=var('x')" thing is making things unnecessarily
complicated. You'd be better off setting them to the appropriate values
right away:
Rx.<x>=PolynomialRing(RationalField(),'x')
Rxy.<y>=PolynomialRing(Rx,'y')
Or you could use
x=Rx.0
y=Rxy.0
> Something has changed. Was I using it incorrectly and this improved or is
> this a regression?
>
Yes, according to the documentation (which presumably hasn't changed
between your old version an 6.10), you were using something that shouldn't
result in what you apparently were trying to obtain.
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