Theorems in quantum field theory proving that the correct result of a calculation is given by the first term in perturbation theory, with Feynman diagrams (see quantum electrodynamics) used to perform calculations involving renormalization of the theory, with no corrections due to higher-order terms in the perturbation series. Nonrenormalization theorems occur when perturbation theory is capable of giving a general nonperturbative result for the theory. Examples of nonrenormalization theorems occur in calculations involving anomalies and in certain theories involving supersymmetry.