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-1;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];
	}
}