  The one-dimensional t-J model with density-density repulsive interactions is
investigated using exact diagonalization and quantum Monte Carlo methods. A
short-range repulsion pushes phase separation to larger values of J/t, and
leads to a widened precursor region in which a spin gap and strengthened
superconducting correlations appear. The correlation exponent is calculated. On
the contrary, a long-range repulsion of $1/r$-form suppresses superconductivity
in the precursor region.
