# [Maxima] Strengthening solve

Barton Willis willisb at unk.edu
Mon Sep 21 07:03:41 CDT 2009

```I fixed some bugs in the to_poly_solver. The new version should be
better at rejecting spurious solutions (using Maxima 5.19post + SBCL)

(%i13) to_poly_solve((x-%pi)/sin(x)=0, x);
(%o13) %union()

(%i14) to_poly_solve((sqrt(x)-1)/(x-1),x);
(%o14) %union()

(%i15) to_poly_solve((x^2 - 2) * (sqrt(x)-1)/(x-1),x);
(%o15) %union([x = -sqrt(2)],[x = sqrt(2)])

OK, Maxima doesn't know that 2 * integer + 1 # 0

(%i16) nicedummies(to_poly_solve(sin(x)/x,x));
(%o16) %union(%if(2*%z0+1 # 0,[x = 2*%pi*%z0+%pi],%union()), %if(%z1 #
0,[x = 2*%pi*%z1],%union()))

>From the Sage mailing list, circa 18 May 2009:

(%i22) to_poly_solve(Q*sqrt(Q^2+2)-1,Q);
(%o22) %union([Q = 1/sqrt(1-sqrt(2))],[Q = 1/sqrt(sqrt(2)+1)])

As always, if you all know of something I should read, think about, or
revise, let me know. There are plenty of equations to_poly_solve is
unable to solve:

http://www.maplesoft.com/support/help/view.aspx?path=examples/solve
x^(1/9) + (2^(8/9) - 2^(1/9))*(x - 1) - x^(8/9) = 0.

(Last example sent to me by an alert user.) So there is much room for
improvement.
If you all would like to try the new version, you'll need the CVS
versions of compar.lisp (patch for sign-log) and simp.lisp (patch for
simpsignum):

(%i21) signum(%i*sqrt(1-sqrt(2)));
(%o21) -1

Barton

-----maxima-bounces at math.utexas.edu wrote: -----

>To: Maxima List <maxima at math.utexas.edu>
>From: Richard Fateman <fateman at cs.berkeley.edu>
>Sent by: maxima-bounces at math.utexas.edu
>Date: 09/01/2009 12:53PM
>Subject: [Maxima] Strengthening solve
>
>looking at making Maxima's solve program more robust, maybe along the
>lines of Mathematica's Reduce, I came up with this simple problem,
>solve((x-%pi)/sin(x)=0, x)
>
>which Maxima says has the solution x=%pi,
>
>which is (a) false in that it reduces to 0/0=0 by substitution, and
>         (b) false in the limit as x->%pi, since it is then -1=0.
>
>
>Mathematica 6.0 gives a warning in that a potential solution was
>should be check by hand. May require use of limits.
>It also warns that inverse functions are being used by Solve, and
>suggests using Reduce.
>
>RJF
>
>
>_______________________________________________
>Maxima mailing list
>Maxima at math.utexas.edu
>http://www.math.utexas.edu/mailman/listinfo/maxima

```