Correlation function (quantum field theory)
 Correlation function (quantum field theory)

In quantum field theory, the matrix element computed by inserting a product of operators between two states, usually the vacuum states, is called a correlation function.
Sometimes, the timeordering operator T is included. Time ordering appears in the path integral formulation and the SchwingerDyson equations.
Without time ordering, they are called Wightman functions/Wightman distributions.
Depending on n (the number of inserted operators), the correlation functions are called onepoint function (tadpole), twopoint function, and so on.
Correlation functions are also called simply correlators. Sometimes, the phrase Green's function is used not only for twopoint functions, but for any correlators.
See also connected correlation function, one particle irreducible correlation function, Green's function (manybody theory).
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