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Based on the Cornwall-Jackiw-Tomboulis (CJT) effective action approach and the requirement to preserve the Goldstone theorem within two-loop approximation, we study the universal quantum effect, which is related to the two-body correlation, in a homogeneous weakly interacting Bose gas at zero temperature. We find the state equations that provide the condensate fraction, energy density, chemical potential, and sound speed. They are the series expansions of the gas parameter.

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