Comment on the diagonalization of Green functions

Real-time thermal field theory is known in the two flavors “closed-time path formalism” and “thermo field dynamics”. With a certain choice of parameters the full two-point functions of these models are identical, hence a scheme to diagonalize the Green function in one model can be applied to the other as well. This allows us to compare the diagonalization schemes that have been discussed in the recent literature in order to select the simplest one. Such comparison speaks in favor of a diagonalization method which is motivated from thermo field dynamics.