problem with _mpoly_dict_recursive()

[sagetrac-mhampton]

The following produces errors:

sage: testR.<a,b,c> = PolynomialRing(QQ,3)
sage: id_ringA = ideal([a^2-b,b^2-c,c^2-a])
sage: id_ringB = ideal(id_ringA.gens()).change_ring(PolynomialRing(QQ,'c,b,a')) 

although this does not:

sage: testR.<a,b,c> = PolynomialRing(QQ,3)
sage: id_ringA = ideal([a^2-b,b^2-c,c^2-a])
sage: id_ringB = ideal(id_ringA.gens()).change_ring(PolynomialRing(QQ,'c,a,b')) 

Component: commutative algebra

Keywords: ideal, change_ring

Issue created by migration from https://trac.sagemath.org/ticket/2601

[aghitza]

comment:1

I don't know what's happening, but maybe the following simpler example will help someone figure it out:

sage: R.<a,b,c> = PolynomialRing(QQ, 3)
sage: P = PolynomialRing(QQ, 'c, b, a')
sage: Q = PolynomialRing(QQ, 'c, a, b')
sage: Q(a)
a
sage: P(a)
...
<type 'exceptions.IndexError'>: list assignment index out of range

Note also that out of the 6 permutations of 'a, b, c', only 'c, b, a' and 'b, c, a' throw this exception; the other 4 seem to work properly.

[aghitza]

comment:2

I believe the problem is in _mpoly_dict_recursive() in multi_polynomial.pyx:

sage: R.<a,b,c> = PolynomialRing(QQ, 3)
sage: a._mpoly_dict_recursive(['c', 'b', 'a'])
---------------------------------------------------------------------------
<type 'exceptions.IndexError'>            Traceback (most recent call last)

/root/<ipython console> in <module>()

/root/multi_polynomial.pyx in sage.rings.polynomial.multi_polynomial.MPolynomial._mpoly_dict_recursive()

<type 'exceptions.IndexError'>: list assignment index out of range

This should return {(0, 0, 1): 1}. I'm having trouble understanding exactly how the code works so I'm giving up on trying to fix this. Someone familiar with the code should be able to do this properly, and much faster than me.

[williamstein]

Attachment: trac_2601.patch.gz

[aghitza]

comment:5

Looks good to me.

[sagetrac-mabshoff]

comment:6

Merged in Sage 3.3.alpha3