Pairing instabilities in the two-dimensional Hubbard model

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.