### Abstract

A perturbation theory is developed for the logarithm of the normalization integral (or partition function) of an N-body system which is either a Bose liquid in its ground state or a classical fluid in the canonical ensemble. The perturbation in the former is an n-body factor in the ground state wavefunction and in the latter is an n-body potential. The normalization function serves as a generating function for the cumulants (or static correlation functions) of the density fluctuation operator ρ_{k}. The expansions of the perturbed partition function and correlation functions are shown to be linked cluster expansions involving the correlation functions in the unperturbed system; each term in these expansions is manifestly of the porper order in N. Several approximations involving truncations of the cumulants and/or resummation of part of the terms in the linked cluster expansions are discussed.

Original language | English (US) |
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Pages (from-to) | 1076-1086 |

Number of pages | 11 |

Journal | Journal of Mathematical Physics |

Volume | 16 |

Issue number | 5 |

DOIs | |

State | Published - 1974 |

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## Cite this

*Journal of Mathematical Physics*,

*16*(5), 1076-1086. https://doi.org/10.1063/1.522662