## #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 - 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 - 2006-10-14
• labels: 887072 -->

• Nobody/Anonymous - 2007-01-29

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Workaround (Maxima 5.11.0):

(%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 - 2007-01-29

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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.