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We show that for low and moderate fillings the ground state of the two-dimensional Hubbard model with positive-U and nearest-neighbor hopping is unstable with respect to d-wave pairing with dxy symmetry of the gap wave function: {Δ}{~}sinkₓsinky. The inclusion of the next-neighbor hopping may either suppress the pairing or change the symmetry of the superconducting state.
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Chubukov et al. (1992) studied this question.
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