  We study by Variational Monte Carlo an extended Hubbard model away from half
filled band density which contains two competing nearest-neighbor interactions:
a superexchange $J$ favoring d-wave superconductivity and a repulsion $V$
opposing against it. We find that the on-site repulsion $U$ effectively
enhances the strength of $J$ meanwhile suppressing that of $V$, thus favoring
superconductivity. This result shows that attractions which do not involve
charge fluctuations are very well equipped against strong electron-electron
repulsion so much to get advantage from it.
