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}}</ref> This is known as the ''[[Onsager Regression Hypothesis|Onsager regression hypothesis]]''. As the values of microscopic variables separated by large timescales, <math>\tau</math>, should be uncorrelated beyond what we would expect from thermodynamic equilibrium, the evolution in time of a correlation function can be viewed from a physical standpoint as the system gradually 'forgetting' the initial conditions placed upon it via the specification of some microscopic variable. There is actually an intuitive connection between the time evolution of correlation functions and the time evolution of macroscopic systems: on average, the correlation function evolves in time in the same manner as if a system was prepared in the conditions specified by the correlation function's initial value and allowed to evolve.<ref name=Sethna/>
Equilibrium fluctuations of the system can be related to its response to external perturbations via the [[Fluctuation-dissipation theorem]].
==The connection between phase transitions and correlation functions==
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