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{{short description|Expectation value of time-ordered quantum operators}}
In [[quantum field theory]], '''correlation functions''', often referred to as '''correlators''' or '''[[Greens function|Green's functions]]''', are [[vacuum expectation value|vacuum expectation values]] of [[time ordering|time-ordered]] products of field operators. They are a key object of study in quantum field theory where they can be used to calculate various [[observable|observables]] such as [[S-matrix]] elements. They are closely related to [[correlation function]]s between [[random variable]]s, although they are nonetheless different objects, being defined in [[Minkowski space]]time and on quantum operators.
== Definition ==
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