Hi Brian,
Here is a program that reproduces the results:
#include <stdio.h>
#include <gsl_poly.h>
int main(int argc, char **argv) {
int i;
double coeffs[4]={1, -5, -25, 125};
double roots[6];
gsl_poly_complex_workspace *u = gsl_poly_complex_workspace_alloc(4);
gsl_poly_complex_solve(coeffs, 4, u, roots);
gsl_poly_complex_workspace_free(u);
for(i=0; i<3; i++)
printf("\t %.5f, %.5f\n", roots[i], roots[2*i+1]);
printf("\t------------\n");
coeffs[0]=1; coeffs[1]=-1; coeffs[2]=-5; coeffs[3]=125;
gsl_poly_complex_workspace *w = gsl_poly_complex_workspace_alloc(4);
gsl_poly_complex_solve(coeffs, 4, w, roots);
gsl_poly_complex_workspace_free(w);
for(i=0; i<3; i++)
printf("\t %.5f, %.5f\n", roots[i], roots[2*i+1]);
}
The output of the program is:
0.20000, 0.00000
0.00000, -0.00000
0.20000, 0.00000
------------
-0.20000, 0.00000
0.00000, 0.16000
0.12000, -0.16000
But the first polynomial's roots are 0.2, 0.2, and -0.2. The second
polynomial has roots 0.2, 0.12+0.16*i, and 0.12-0.16*i.
I'm running Ubuntu with kernel 2.6.20-16-386 Version 2 and GSL 1.8-3. I
compiled using g++ 4.1.2-1. Is there any other info you need?
Brian Gough wrote:
At Mon, 10 Dec 2007 16:41:31 -0600 (CST),
salman wrote:
I am using gsl_poly_complex_solve() to determine the roots of some basic
polynomials. The results I am getting are fairly off from the correct
roots. Here is the relevant section of the code (deg_L is the degree of
the polynomial I am considering):
gsl_poly_complex_workspace *u=gsl_poly_complex_workspace_alloc(deg_L+1);
gsl_poly_complex_solve(coeffs, deg_L+1, u, roots);
gsl_poly_complex_workspace_free(u);
for(i=0; i<deg_L; i++)
printf("\t %.5f, %.5f\n", roots[i], roots[2*i+1]);
Thanks for your email. Please can you send a complete example program
so we can reproduce the problem. Also, details of which version of
GSL you are using and operating system etc.
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