Next the approximation A ≈ ≈ A 0 + ε ε A 1 is inserted into ε ε D 1 . This results in an equation for A 1 , which, in the general case, can be written in closed form as a sum over integrals over A 0 . Thus, one has obtained the first-order correction A 1 and thus A ≈ ≈ A 0 + ε ε A 1 is a good approximation to A . It is a good approximation, precisely because the parts that were ignored were of size ε ε 2 . The process can then be repeated, to obtain corrections A 2 , and so on.