Imperfect bose system and its mixed state representation. I - Thermal equilibrium state of imperfect bose system

An imperfect spinless bose system is studied with use of the mixed state re presentation, which was originally developed to study highly excited nuclea r states. With this representation, the degrees of freedom are classified i nto two types: One describes the thermal behaviour of the non-condensed bos on and the other mainly describes the dynamical aspects of the condensed bo son. The entropy of the system is specified by only the former type of degr ees of freedom. Comparison is made with the conventional results for a dilu te bose gas with an interaction of the hard sphere type.