The Problem of the Double Time-Ordered Operator Product Calculation in   the Nonequilibrium Quantum Field Theory
  It is shown that calculation of the operator's groups product with the independent time ordering procedure in each one is important for some problems in the nonequilibrium quantum field theory at finite temperature. This problem is solved in terms of path integrals by means of time-integration contour transformation. A role of thermal ghosts for this problem is also discussed. 
Tengiz M. Bibilashvili