#2804 solve_rat_ineq includes non-solution

None
open
nobody
None
5
2014-09-19
2014-08-20
kcrisman
No

Should this solution include x = -3?
Maxima 5.33.0 http://maxima.sourceforge.net
using Lisp ECL 12.12.1
Distributed under the GNU Public License. See the file COPYING.
Dedicated to the memory of William Schelter.
The function bug_report() provides bug reporting information.
(%i1) load("solve_rat_ineq");
(%o1) /Users/.../sage/local/share/maxima/5.33.0/share/solve\ _rat_ineq/solve_rat_ineq.mac
(%i2) solve_rat_ineq((x^2-9)/(x-3) >= 0);
(%o2) [[x >= - 3]]
I understand that
(%i6) x^2/x;
(%o6) x
so maybe this is intended behavior, but maybe solve_rat_ineq shouldn't do any simplifications like this - I hope I haven't asked this question before, years ago...

Downstream at http://ask.sagemath.org/question/23837/

Discussion

  • kcrisman
    kcrisman
    2014-08-22

    That is, should it include x=3?

     
  • Volker van Nek
    Volker van Nek
    2014-09-19

    Actually, this is intended behaviour. Both solve and algsys behave the same:

    (%i1) algsys([x^2/x],[x]);
    (%o1) [[x = 0]]
    (%i2) solve(x^2/x);
    (%o2) [x = 0]

    The inequality is fullratsimplified just right in the beginning. So if we could identify the discontinuities before simplifying them away, we could use a mechanism which is already inherent in solve_rat_ineq to clean out these discontinuities.

    But I don't see a quick solution to that problem.