They are related to the time-ordered Green function by G T ( k , ω ω ) = [ 1 + ζ ζ n ( ω ω ) ] G R ( k , ω ω ) − − ζ ζ n ( ω ω ) G A ( k , ω ω ) , where n ( ω ω ) = 1 e β β ω ω − − ζ ζ is the Bose–Einstein or Fermi–Dirac distribution function.