Sum rule

From Wikipedia, the free encyclopedia

Sum rule may refer to:

• Sum rule in differentiation
• Sum rule in integration
• Rule of sum, a counting principle in combinatorics
• in Quantum Mechanics -- Formulas for transitions between energy levels, in which the sum of the transition strengths is expressed in a simple form. Sum rules are used to describe the properties of many physical systems, including solids, atoms, atomic nuclei, and nuclear constituents such as protons and neutrons. The sum rules are derived from quite general principles, and are useful in situations where the behavior of individual energy levels is too complex to describe by a precise quantum-mechanical theory. In general, sum rules are derived by using Heisenberg's quantum-mechanical algebra to construct operator equalities, which are then applied to particles or the energy levels of a system. See also selection rule.

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