The discovery of high T_c superconductivity has increased the interest in theoretical models other than BCS. It has been proposed [1,2] that superconductivity can occur in models based on purely electronic interactions; we show an outline of models exhibiting such features in their ground state and discuss the role of the different interaction terms in their Hamiltonians.

1. W.Anderson, Science 235, 1196 (1987)
2. F.C.Zhang, T.M.Rice, Phys. Rev. B 37, 3759 (1988)

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