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### Perturbation theory

**Perturbation theory** is a set of approximation schemes for describing complicated quantum systems in terms of simpler systems.

##### Explanation

Perturbation methods start with a simplified form of the original problem, which is simple enough to be solved exactly. The perturbative expansion is created by adding successive corrections to the simplified problem.

##### Example 1

One of the fundamental breakthroughs for controlling the expansion are the Feynman diagrams, which allow perturbation series to be written down diagrammatically.

## HistoryThe Italian mathematician Joseph-Louis Lagrange (1736 – 1813) generalized the methods of perturbation theory. |