Incorrect expansion of expression

[justinacolmena]

False coefficients are showing up on some terms, and other terms are omitted.

This is the equation that arises when solving for the leading coefficient of the resolvent quadratic equation for a quartic curve, when the resultant quartic equation is determined so as to have two distinct double roots.

The equation AFAICT is reported correctly when input, but the expand(%); step produces a false result.

wxWidgets: 3.2.5
Unicode support: yes
Maxima version: 5.47.0 (x86_64-redhat-linux-gnu)
Maxima compiled using: SBCL 2.3.11-3.fc41

[justinacolmena]

The unsimplified equation is solved in R, but I noticed that the expanded/simplified expression produced a large residual.

> (2*a*y1_ - 2*a^2*x1 - q3 - 2*q4*x1)^2 - 4*(a^2 - q4)*
+       (y1_^2 + a^2*x1^2 + 2*a*y1 - 2*a*x1*y1_ -
+               q2 - 2*q3*x1 - 3*q4*x1^2) 
[1] -0.003895801
> 
> -8*y1*a^3 + (24*q4*x1^2 + 12*q3*x1 + 4*q2)*a^2 +
+       ((-16*q4*x1 - 4*q3)*y1_ + 8*q4*y1)*a +
+       4*q4*y1_^2 - 8*q4^2*x1^2 - 4*q2*q4 + q3^2
[1] 0.1718608

The correct result

> (- 8*a^3*y1 + 4*a^2*y1_^2 + 12*a^2*q3*x1 + 24*a^2*q4*x1^2 - 4*a^2*y1_^2 + 4*a^2*q2
+       - 4*a*q3*y1_ - 8*a*q4*x1*y1_ + q3^2 + 4*q4*q3*x1 + 4*q4^2*x1^2 + 4*q4*y1_^2
+       + 8*a*q4*y1 - 8*a*q4*x1*y1_ - 4*q4*q2 - 8*q4*q3*x1 - 12*q4^2*x1^2)
[1] -0.003895801
> 

[daute]

This is clearly a Maxima (the backend, which does the mathematics in the background) issue and have nothing to do with wxMaxima (the graphical frontend), and might be reported/discussed there:
https://sourceforge.net/projects/maxima/

But I am not sure,what you want to do? Solve an equation (as you wrote)? Which one? Where do the R-results (e.g. -0.003895801) come from?
Best regards, Wolfgang