# Correlation function (quantum field theory)

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In quantum field theory, the (real space) *n*-point correlation function is defined as the functional average (functional expectation value) of a product of field operators at different positions

For time-dependent correlation functions, the time-ordering operator is included.

Correlation functions are also called simply **correlators**. Sometimes, the phrase Green's function is used not only for two-point functions, but for any correlators.

## See also

- Connected correlation function
- Correlation does not imply causation
- One particle irreducible correlation function
- Green's function (many-body theory)
- Partition function (mathematics)

## References

### Books

- Alexander Altland, Ben Simons (2006):
*Condensed Matter Field Theory*Cambridge University Press