Perturbation theory comprises mathematical methods for finding an approximate solution to a problem, by starting from the exact solution of a related problem. A critical feature of the technique is a middle step that breaks the problem into `solvable` and `perturbation` parts. Perturbation theory is applicable if the problem at hand cannot be ...
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http://en.wikipedia.org/wiki/Perturbation_theory
(from the article `quantum electrodynamics`) QED is often called a perturbation theory because of the smallness of the fine-structure constant and the resultant decreasing size of higher-order ...
Found on
http://www.britannica.com/eb/a-z/p/48
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