The Hubbard Model in the Two-Pole Approximation

@article{Avella1998TheHM,
  title={The Hubbard Model in the Two-Pole Approximation},
  author={Adolfo Avella and Ferdinando Mancini and Dario Villani and Liudmila Siurakshina and V.Yu. Yushankhai Universita' di Salerno and Joint Institute for Nuclear Research},
  journal={International Journal of Modern Physics B},
  year={1998},
  volume={12},
  pages={81-97}
}
The two-dimensional Hubbard model is analyzed in the framework of the two-pole expansion. It is demonstrated that several theoretical approaches, when considered at their lowest level, are all equivalent and share the property of satisfying the conservation of the first four spectral momenta. It emerges that the various methods differ only in the way of fixing the internal parameters and that it exists a unique way to preserve simultaneously the Pauli principle and the particle–hole symmetry. A… 
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