  The consistent definition of the thermodynamic functions of small open
quantum systems in contact with an environment in equilibrium with a heat bath
has been the subject of many debates in the quantum community. In the present
work we reproduce and comment parts of a recent approach of this subject by
Rivas [15]. This approach overcomes the controversial discussions generated by
the coupling between a system and its environment for any type of coupling
between the two parts and allows for a consistent description of the
thermodynamical properties of the system strongly interacting with the bath
