Operator formalism in statistical mechanics

We consider the configuration partition function, ZN, of a classical imperfect gas of particles having rigid cores. The rigid cores result in a geometric simplification which makes it possible to find a finite recursion relation between ZN and ZN-1. From this, it is possible to express ZN in terms of the vacuum expectation value of a finite Boson operator raised to the Nth power, N being the number of particles.