The process is generally mechanical, if laborious. One begins by writing the equations D so that they split into two parts: some collection of equations D 0 which can be solved exactly, and some additional remaining part ε ε D 1 for some small ε ε ≪ ≪ 1 . The solution A 0 (to D 0 ) is known, and one seeks the general solution A to D = D 0 + ε ε D 1 .