Positive commutator method in nonequilibrium statistical mechanics

We extend the method of positive commutators, which was very successfully a pplied to zero temperature problems, to positive temperatures, i.e. to none quilibrium quantum statistical mechanics. Using this technique, we give ano ther proof of a fundamental property of large quantum systems, called Retur n to Equilibrium. This property says that if a system is slightly perturbed from its equilibrium state, then it converges back to it as time goes to i nfinity.