Fix conversion from PARI to multivariate polynomial rings

[pbruin]

Conversion of multivariate polynomials from PARI does not always work:

sage: A.<a> = PolynomialRing(QQ)
sage: B.<d,e> = PolynomialRing(A)
sage: f = pari(a*d)
sage: B(f)
Traceback (most recent call last):
...
TypeError: Unable to coerce PARI d to an Integer

A similar example:

sage: A.<a,b> = PolynomialRing(QQ)
sage: B.<d,e> = PolynomialRing(A)
sage: f = pari(a*d)
sage: B(f)
Traceback (most recent call last):
...
TypeError: Could not find a mapping of the passed element to this ring.

[vdelecroix]

Hello,

Do you know how pari chooses the order of the variables

sage: A.<a> = PolynomialRing(QQ)
sage: B.<d,e> = PolynomialRing(A)
sage: f = pari(a*d)
sage: f
d*a
sage: B(f)
a*d

On the other hand, what do you think of

sage: f.sage()
Traceback (most recent call last):
...
NameError: name 'd' is not defined

Vincent

[pbruin]

Hello Vincent,

Do you know how pari chooses the order of the variables

sage: A.<a> = PolynomialRing(QQ)
sage: B.<d,e> = PolynomialRing(A)
sage: f = pari(a*d)
sage: f
d*a
sage: B(f)
a*d

In PARI, the variables x and y are predefined (with x of higher priority) and subsequent variables get a successively lower priority in the order in which they are encountered. Polynomials are printed with the variable of highest priority on the right. In this example PARI sees a first, therefore it has higher priority than d. Compare

sage: gp('a*d')
d*a

On the other hand, what do you think of

sage: f.sage()
Traceback (most recent call last):
...
NameError: name 'd' is not defined

This is because user-defined names are not part of the default context in which the evaluation is done. You have to do

sage: f.sage({'a': a, 'd': d})
a*d

or

sage: f.sage(globals())
a*d

[vdelecroix]

Replying to pbruin:

On the other hand, what do you think of

sage: f.sage()
Traceback (most recent call last):
...
NameError: name 'd' is not defined

This is because user-defined names are not part of the default context in which the evaluation is done. You have to do

sage: f.sage({'a': a, 'd': d})
a*d

or

sage: f.sage(globals())
a*d

Ha nice! Could you add it to the documentation where you check pari conversion?

[pbruin]

Replying to vdelecroix:

Replying to pbruin:

You have to do

sage: f.sage({'a': a, 'd': d})
a*d

or

sage: f.sage(globals())
a*d

Ha nice! Could you add it to the documentation where you check pari conversion?

OK, I can add it for completeness (there are somewhat similar examples in the documentation of f.sage()).

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​7a7e1cd

Trac 17974: add example for f.sage()