Perturbation Series

Abstract

Perturbation theory is a large collection of iterative methods for obtaining approximate solutions to problems involving a small parameter ε. These methods are so powerful that sometimes it is actually advisable to introduce a parameter e temporarily into a difficult problem having no small parameter, and then finally to set ε = 1 to recover the original problem. This apparently artificial conversion to a perturbation problem may be the only way to make progress.

You have erred perhaps in attempting to put colour and life into each of your statements instead of confining yourself to the task of placing upon record that severe reasoning from cause to effect which is really the only notable feature about the thing. You have degraded what should have been a course of lectures into a series of tales.

Sherlock Holmes, The Adventure of the Copper Beeches Sir Arthur Conan Doyle