[GiNaC-list] Bug report

[Vladimir V. Kisil]

	Dear Mark,

>>>>> On Thu, 23 Aug 2018 23:32:29 -0400, Mark Rahner <rahner at alum.mit.edu> said:

    MR> Hi Vladimir, Yang's expectation, that applying numer() and
    MR> denom() to the same expression would produce expressions with
    MR> consistent signs, is reasonable.  Even if the potential
    MR> inconsistency of results is well documented, it will remain
    MR> unexpected behavior by many users, potentially leading to
    MR> errors.  The fact that consistent signs will sometimes be
    MR> produced may make it difficult to detect or debug those errors.
    MR> The potential inconsistency of sign results also suggests that
    MR> the concepts of numerator and denominator aren't well defined,
    MR> which is unsettling even if there is a good underlying reason.
    MR> The explanation given, because a/b = (-a)/(-b), seems
    MR> insufficient to justify an interface that can lead to errors.
    MR> Would enforcement of consistent signs result in a performance
    MR> penalty?

    I see your point. However, it is more then just performance penalty
  on the stake. The behaviours of these two functions depends on
  normal() method. And what shall be the normal form of, say,
  (1-x)/(y-1)? Why not (x-1)/(1-y)? Since there is no definite reason
  why one shall be preferred to another, each time GiNaC will run
  normal() method either of them may appear.

  To deal with a user's natural expectation we may rename numer() and
  denum() to, say,  numer_up_to_a_sign() and denumr_up_to_a_sign(). But
  this seems to be overdone. Alternatively, we may be add few more lines
  of warnings and explanation to the tutorial just before numer() and
  denum() are introduced.

  Best wishes,
  Vladimir
-- 
Vladimir V. Kisil                 http://www.maths.leeds.ac.uk/~kisilv/
  Book:     Geometry of Mobius Transformations     http://goo.gl/EaG2Vu
  Software: Geometry of cycles          http://moebinv.sourceforge.net/


    MR> Respectfully, -Mark

    MR> On Thu, Aug 23, 2018 at 6:31 PM Vladimir V. Kisil
    MR> <kisilv at maths.leeds.ac.uk> wrote:

    >> Hi,
    >> 
    >> >>>>> On Thu, 23 Aug 2018 13:22:06 +0800 (GMT+08:00), 杨强 <
    >> acrobat at mail.ustc.edu.cn> said:
    >> 
    >> > Dear all, I use GiNaC to perform analytic calculation, but when
    >> I > try to use .numer() and .denom() methods, I get a wrong
    >> result.
    >> 
    >> 
    >> > 1. expr.numer()/expr.denom() is not always equal to the
    >> original > expression!!!  2. Using .numer_denom() method could
    >> always get the > correct answer.
    >> 
    >> It is not actually a bug. Since a/b = (-a)/(-b), numer() and
    >> denom() are only defined up to a sign. Thus their separate
    >> evaluation may lead to expr.numer()/expr.denom() with the
    >> opposite sign to expr.
    >> 
    >> I am proposing a patch to tutorial reminding that,
    >> 
    >> Best wishes, Vladimir
    >> --
    >> Vladimir V. Kisil http://www.maths.leeds.ac.uk/~kisilv/ Book:
    >> Geometry of Mobius Transformations http://goo.gl/EaG2Vu Software:
    >> Geometry of cycles http://moebinv.sourceforge.net/