x=2;  n=4; h=0.5;

for i=1:n,
  A(i) = dx_central(x,h,'f1');
  h=h/2;
end

for k=2:n
  for j=n:-1:k
     A(j) = (4^(k-1)*A(j)-A(j-1))/(4^(k-1) - 1);
  end
end

e = exp(x) - A(n)