Cluster Expansions and the Theory of Many-Boson Systems

The previous configuration-space cluster integral treatmemt of the properties of the ground state of a manyboson system is modified by including those higher order diagrams consistent with the paii excitation approximation, which were previously omitted. The cluster expansions involve a parameter, analogous to the fugacity in classical expansions, whose definition automatically accounts for the depletion of the free particle ground state. The resulting expectation value for the hamiltonian is in agreememt with that obtained by field- theoretic methods for states of pair-excitation type. (auth)