**Perturbation theory** comprises mathematical methods that are used to find an approximate solution to a problem which cannot be solved exactly, by starting from the exact solution of a related problem. Perturbation theory is applicable if the problem at hand can be formulated by adding a "small" term to the mathematical description of the exactly solvable problem.

Perturbation theory leads to an expression for the desired solution in terms of a formal power series in some "small" parameter – known as a **perturbation series** – that quantifies the deviation from the exactly solvable problem. The leading term in this power series is the solution of the exactly solvable problem, while further terms describe the deviation in the solution, due to the deviation from the initial problem. Formally, we have for the approximation to the full solution A, a series in the small parameter (here called ), like the following:

In this example, would be the known solution to the exactly solvable initial problem and, ... represent the **higher-order terms** which may be found iteratively by some systematic procedure. For small these higher-order terms in the series become successively smaller. An approximate "perturbation solution" is obtained by truncating the series, usually by keeping only the first two terms, the initial solution and the "first-order" perturbation correction:

Read more about Perturbation Theory: General Description, Examples, History, Perturbation Orders, First-order Non-singular Perturbation Theory, Perturbation Theory of Degenerate States, Example of Second-order Singular Perturbation Theory, Example of Degenerate Perturbation Theory - Stark Effect in Resonant Rotating Wave, Commentary, Perturbation Theory in Chemistry

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### Famous quotes containing the word theory:

“There never comes a point where a *theory* can be said to be true. The most that one can claim for any *theory* is that it has shared the successes of all its rivals and that it has passed at least one test which they have failed.”

—A.J. (Alfred Jules)