#include <stdio.h>#include <gsl/gsl_multifit.h>intmain (int argc, char **argv){  int i, n;  double xi, yi, ei, chisq;  gsl_matrix *X, *cov;  gsl_vector *y, *w, *c;  if (argc != 2)    {      fprintf (stderr,"usage: fit n < data\n");      exit (-1);    }  n = atoi (argv[1]);  X = gsl_matrix_alloc (n, 3);  y = gsl_vector_alloc (n);  w = gsl_vector_alloc (n);  c = gsl_vector_alloc (3);  cov = gsl_matrix_alloc (3, 3);  for (i = 0; i < n; i++)    {      int count = fscanf (stdin, "%lg %lg %lg",                          &xi, &yi, &ei);      if (count != 3)        {          fprintf (stderr, "error reading file\n");          exit (-1);        }      printf ("%g %g +/- %g\n", xi, yi, ei);            gsl_matrix_set (X, i, 0, 1.0);      gsl_matrix_set (X, i, 1, xi);      gsl_matrix_set (X, i, 2, xi*xi);            gsl_vector_set (y, i, yi);      gsl_vector_set (w, i, 1.0/(ei*ei));    }  {    gsl_multifit_linear_workspace * work       = gsl_multifit_linear_alloc (n, 3);    gsl_multifit_wlinear (X, w, y, c, cov,                          &chisq, work);    gsl_multifit_linear_free (work);  }#define C(i) (gsl_vector_get(c,(i)))#define COV(i,j) (gsl_matrix_get(cov,(i),(j)))  {    printf ("# best fit: Y = %g + %g X + %g X^2\n",             C(0), C(1), C(2));    printf ("# covariance matrix:\n");    printf ("[ %+.5e, %+.5e, %+.5e  \n",               COV(0,0), COV(0,1), COV(0,2));    printf ("  %+.5e, %+.5e, %+.5e  \n",                COV(1,0), COV(1,1), COV(1,2));    printf ("  %+.5e, %+.5e, %+.5e ]\n",                COV(2,0), COV(2,1), COV(2,2));    printf ("# chisq = %g\n", chisq);  }  return 0;}