We study the equation of state for symmetric nuclear matter using a ring-diagram approach in which the particle-particle hole-hole ($pphh$) ring diagrams within a momentum model space of decimation scale $Lambda$ are summed to all orders. The calculation is carried out using the renormalized low-momentum nucleon-nucleon (NN) interaction $V_{low-k}$, which is obtained from a bare NN potential by integrating out the high-momentum components beyond $Lambda$. The bare NN potentials of CD-Bonn, Nijmegen and Idaho have been employed. The choice of $Lambda$ and its influence on the single particle spectrum are discussed. Ring-diagram correlations at intermediate momenta ($ksimeq$ 2 fm$^{-1}$) are found to be particularly important for nuclear saturation, suggesting the necessity of using a sufficiently large decimation scale so that the above momentum region is not integrated out. Using $V_{low-k}$ with $Lambda sim 3$ fm$^{-1}$, we perform a ring-diagram computation with the above potentials, which all yield saturation energies $E/A$ and Fermi momenta $k_F^{(0)}$ considerably larger than the empirical values. On the other hand, similar computations with the medium-dependent Brown-Rho scaled NN potentials give satisfactory results of $E/A simeq -15$ MeV and $k_F^{(0)}simeq 1.4$ fm$^{-1}$. The effect of this medium dependence is well reproduced by an empirical 3-body force of the Skyrme type.
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