N-point finite temperature expectation values at real times

In both the real- and imaginary-time formalisms of finite-temperature field theory it is shown exactly what types of real-time expectation value are being calculated. The relationship between these functions is then considered. The conclusion is that neither the well-known results for two-point functions nor the recently obtained results for three-point functions have a simple generalisation to higher-order functions. All the results are valid whatever approximation to the full fields is being used and whatever types of field are involved. (orig.)