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Using exact Lanczos diagonalizations we have shown that the pair-hoppingmodel for {\it negative} $W$ exhibits a phase transition into $\eta$-{\itsuperconducting state}. The transition occurs at {\it any band filling} and thecritical value $W_{c}$ varies between $W_{c} \simeq -1.75t$ at half-filling to$W_{c} \simeq -2t$ for two particles on the lattice.Comment: the postscript files of the figures are included (5 figures

η-superconductivity in the one-dimensional Penson-Kolb model