Cluster Expansion in a Bose System

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Abstract
When a trial function for Bose systems contains both one‐particle and two‐particle functions, the cluster expansion is no longer simply related to the classical expansion of Mayer and Mayer. But by observing that the permutations in one‐particle functions can be represented exactly as in the high‐temperature expansion of the spin‐½ XY model, the cluster expansion is developed analogously to the classical case.