Keeping the above example in mind, one follows a general recipe to obtain the perturbation series. The perturbative expansion is created by adding successive corrections to the simplified problem. The corrections are obtained by forcing consistency between the unperturbed solution, and the equations describing the system in full. Write D for this collection of equations; that is, let the symbol D stand in for the problem to be solved. Quite often, these are differential equations, thus, the letter "D".