lubksb.c

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void lubksb(double **a, int n, int *indx, double *b) {
  int i, ii = 0, ip, j;
  double sum;
 
  for (i = 0; i < n; i++) {
    ip = indx[i];
    sum = b[ip];
    b[ip] = b[i];
    if (ii)
      for (j = ii - 1; j < i; j++) sum -= a[i][j] * b[j];
    else if (sum)
      ii = i + 1;
    b[i] = sum;
  }
  for (i = n - 1; i >= 0; i--) {
    sum = b[i];
    for (j = i + 1; j < n; j++) sum -= a[i][j] * b[j];
    b[i] = sum / a[i][i];
  }
}