Lerche–Newberger sum rule

The Lerche–Newberger sum rule is a principle in the field of statistical mechanics and thermodynamics, related to the behavior of systems in equilibrium. Specifically, it provides a relationship between correlation functions and the equilibrium properties of a system, particularly in contexts where random variables influence outcomes. The rule states that the sum of certain statistical correlators (usually related to physical observables) over all possible states of a system leads to significant simplifications.