#35 Cannot solve some solveable expressions

open
nobody
None
5
2006-10-14
2006-10-09
Anonymous
No

Cannot solve some expressions in the form "x = f(x)"

Example:
x = sqrt(x) + a;
solve(%,x);

Still, we can find the answer:
x = sqrt(x) + a;
%-a;
%^2;
solve(%,x);

--Mario/Mexico

Discussion

  • Barton Willis

    Barton Willis - 2006-10-10

    Logged In: YES
    user_id=895922

    This isn't a bug, it's a deficiency. I suggest that
    you submit this as a feature request.

    There was a discussion about such equations on the
    mailing list not long ago. If you have not subscribed
    to the list, I suggest that you do so. And of course,
    we are always looking for new volunteers to help
    improve Maxima.

     
  • Barton Willis

    Barton Willis - 2006-10-14
    • labels: 887072 -->
     
  • Nobody/Anonymous

    Logged In: NO

    Workaround (Maxima 5.11.0):

    (%i1) load("topoly")$
    (%i2) eq: x = sqrt(x) + a;
    (%o2) x = sqrt(x)+a
    (%i3) solve(topoly(eq),x);
    (%o3) [x = -(sqrt(4*a+1)-2*a-1)/2,x = (sqrt(4*a+1)+2*a+1)/2]
    (%i4)

     
  • Barton Willis

    Barton Willis - 2007-01-29

    Logged In: YES
    user_id=895922
    Originator: NO

    Yon need to be careful with topoly--it can make the solution set bigger. So you need to filter
    the solutions and expunge the bad solutions.

     

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