On quantum systems in thermal contact

It is shown that under rather general conditions two K.M.S. states ₁ and ₂ of systemsS ₁ andS ₂, respectively, can be simultaneously extended to a K.M.S. state of a system composed ofS ₁ andS ₂, provided both systems have equal temperatures. This result gives further support to the conjecture that K.M.S. states are equilibrium states. In the second part, a model of thermal coupling is constructed which satisfies the assumptions of the first part, thereby showing that the result is also valid in the interesting case of systemsS ₁ andS ₂ in thermal contact.