function x = fpi-err(g, x1, tol, tol_0)
	x = fpi(g, x1, tol, tol_0);
	n = size(x)(1);
	m = size(x)(2);
	r = x(n,m);
	for k= 1:n,
		x(k,3) = abs(r-x(k,1));
		if k > 1
			x(k,4) = x(k,3)/x(k-1,3);
		end
	end