Okay, that’s reasonable. How about the attached, then? Robin
On Sat, 30 Jul 2016 at 10:43 John Cremona <john.crem...@gmail.com> wrote: > I think that an error should be raised if the projective point constructor > is called with (0,0,0). > On 29 Jul 2016 23:54, "Robin Houston" <robin.hous...@gmail.com> wrote: > > I think there’s another case that isn’t handled correctly, exemplified by: > > R.<x,y,z> = QQ[] > > T = EllipticCurve_from_cubic(y^2*z - x^3 - z^3, (0,1,0)) > > A simple change that fixes this problem is to make the projective_point() > method not blow up when given (0,0,0), e.g.: > > > *--- a/src/sage/schemes/elliptic_curves/constructor.py* > > *+++ b/src/sage/schemes/elliptic_curves/constructor.py* > > @@ -1127,6 +1127,8 @@ def projective_point(p): > > p_lcm = LCM_list([x.denominator() for x in p]) > > except AttributeError: > > return p > > + if p_gcd == 0: > > + return p > > scale = p_lcm / p_gcd > > return [scale * x for x in p] > > > > > This change may be undesirable for other reasons, though. > > > Robin > > > On Monday, 25 July 2016 11:25:00 UTC+1, Robin Houston wrote: >> >> I ran into an unexpected error in EllipticCurve_from_cubic, with the >> following cubic and rational point: >> >> R.<x,y,z> = QQ[] >> cubic = -3*x^2*y + 3*x*y^2 + 4*x^2*z + 4*y^2*z - 3*x*z^2 + 3*y*z^2 - 8*z^3 >> EllipticCurve_from_cubic(cubic, (-4/5, 4/5, 3/5)) >> >> Note that it works as expected using instead the different rational point >> (1, 1, 0). >> >> On investigation, I found there is a case that isn’t handled correctly. >> The code computes >> >> P2 = chord_and_tangent(F, P) >> >> and if P2 is projectively equivalent to P then it uses a different >> algorithm. If they’re different, it then computes >> >> P3 = chord_and_tangent(F, P2) >> >> and uses an algorithm that fails if P3 is equivalent to P2. >> >> I think the attached patch fixes this problem. At least, with this patch >> it now works for my examples. >> >> Best wishes, >> Robin >> >> -- > You received this message because you are subscribed to the Google Groups > "sage-devel" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sage-devel+unsubscr...@googlegroups.com. > > > To post to this group, send email to sage-devel@googlegroups.com. > Visit this group at https://groups.google.com/group/sage-devel. > For more options, visit https://groups.google.com/d/optout. > > -- > You received this message because you are subscribed to a topic in the > Google Groups "sage-devel" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/sage-devel/ywH31HkRo7g/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > sage-devel+unsubscr...@googlegroups.com. > To post to this group, send email to sage-devel@googlegroups.com. > Visit this group at https://groups.google.com/group/sage-devel. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at https://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
cubic2.patch
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