It is pointed out that a method recently described by Hillery and Zubairy can be generalized to yield the multitime correlation function in any order for an interacting quantum field in the form of a phase-space average. The correlations are expressible as $c$-number integrals with the help of the coherent-state propagator, but the associated phase-space density is complex.

• Received 19 March 1984

DOI:https://doi.org/10.1103/PhysRevA.30.890