Partition function (quantum field theory)
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In quantum field theory, we have a generating functional, Z[J] of correlation functions and this value, called the partition function is usually expressed by something like the following functional integral:

where S is the action functional. This is very analogous to statistical mechanics. In fact, a Wick rotation emphasizes the similarities.

The original article can be found at: http://en.wikipedia.org/wiki/Partition_function_(quantum_field_theory).
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