#11474: Elliptic curves should be unique parent structures
-------------------------------+--------------------------------------------
Reporter: SimonKing | Owner: cremona
Type: defect | Status: needs_info
Priority: major | Milestone: sage-4.7.1
Component: elliptic curves | Keywords: unique parent
Work_issues: | Upstream: N/A
Reviewer: | Author: Simon King
Merged: | Dependencies:
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Changes (by newvalueoldvalue):
* status: new => needs_info
* author: => Simon King
Comment:
The discussion on [http://groups.google.com/group/sage-
nt/browse_thread/thread/ec8d0ad14a819082 sage-nt] shows that it is even
not clear whether we want elliptic curves to be unique parents (yet).
Anyway, I posted a preliminary patch. There are some doctest failures -- a
test log is attached as well.
Some of the errors are easy to understand: Previously, one had a "Generic
morphism", but with unique parents one has a "Generic endomorphism".
Others seem more tricky. For example one gets
{{{
sage -t devel/sage/sage/schemes/elliptic_curves/ell_field.py
// ** nInitExp failed: using Z/2^2
**********************************************************************
File "/mnt/local/king/SAGE/sage-4.7.rc2/devel/sage-
main/sage/schemes/elliptic_curves/ell_field.py", line 1038
:
sage: E.weierstrass_p(prec=8, algorithm='pari')
Exception raised:
Traceback (most recent call last):
File "/mnt/local/king/SAGE/sage-4.7.rc2/local/bin/ncadoctest.py",
line 1231, in run_one_test
self.run_one_example(test, example, filename, compileflags)
File "/mnt/local/king/SAGE/sage-4.7.rc2/local/bin/sagedoctest.py",
line 38, in run_one_example
OrigDocTestRunner.run_one_example(self, test, example, filename,
compileflags)
File "/mnt/local/king/SAGE/sage-4.7.rc2/local/bin/ncadoctest.py",
line 1172, in run_one_example
compileflags, 1) in test.globs
File "<doctest __main__.example_14[7]>", line 1, in <module>
E.weierstrass_p(prec=Integer(8), algorithm='pari')###line 1038:
sage: E.weierstrass_p(prec=8, algorithm='pari')
File "/mnt/local/king/SAGE/sage-4.7.rc2/local/lib/python/site-
packages/sage/schemes/elliptic_curves/ell
_field.py", line 1064, in weierstrass_p
return weierstrass_p(self, prec=prec, algorithm=algorithm)
File "/mnt/local/king/SAGE/sage-4.7.rc2/local/lib/python/site-
packages/sage/schemes/elliptic_curves/ell_wp.py", line 141, in
weierstrass_p
wp = compute_wp_pari(E, prec)
File "/mnt/local/king/SAGE/sage-4.7.rc2/local/lib/python/site-
packages/sage/schemes/elliptic_curves/ell_wp.py", line 168, in
compute_wp_pari
wpp = ep.ellwp(n=prec)
AttributeError: 'dict' object has no attribute 'ellwp'
}}}
Hence, first there is a (warning or error?) message "`// ** nInitExp
failed: using Z/2^2`", and then something is a dict that ought to be a
completely different object.
On the bright side, one has the following.
Elliptic curves are unique, even when they are created in different ways:
{{{
sage: E = EllipticCurve('5077a'); E
Elliptic Curve defined by y^2 + y = x^3 - 7*x + 6 over Rational Field
sage: E is EllipticCurve('5077a') is EllipticCurve(QQ, E.a_invariants())
is EllipticCurve(j = E.j_invariant())
True
}}}
If an elliptic curve is provided with some custom attribute and the "same"
curve is found in the database with different attributes, then one has no
uniqueness of parents; this is in order to prevent the database from
overriding stuff that the user has computed:
{{{
sage: E = EllipticCurve([0, 1, 1, -2, 0])
sage: E is EllipticCurve('389a')
True
sage: E._EllipticCurve_rational_field__cremona_label = 'bogus'
sage: E is EllipticCurve('389a')
False
sage: E.label()
'bogus'
sage: EllipticCurve('389a').label()
'389a1'
}}}
However, if the custom attribute is removed, then the data from the
database are used to provide a value for that attribute:
{{{
sage: del E._EllipticCurve_rational_field__cremona_label
sage: E is EllipticCurve('389a') # uniqueness of parents again
True
# The label has implicitly been updated
sage: E._EllipticCurve_rational_field__cremona_label
'389 a 1'
}}}
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11474#comment:4>
Sage <http://www.sagemath.org>
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