  We place limits on the mean density of the universe and the slope of the
linear power spectrum around a megaparsec scale by comparing the universal mass
function to the observed luminosity function. Numerical simulations suggest
that the dark matter halo mass function at small scales depends only on
Omega_m(n_eff+3) independent of the overall power spectrum normalization.
Matching the halo abundance to the observed luminosity function requires
knowing the relation between the virial mass and luminosity (separately for
early and late type galaxies) and the fraction of galaxies that reside in
larger halos such as groups and clusters, all of which can be extracted from
the galaxy-galaxy lensing. We apply the recently derived values from SDSS and
find Omega_m(n_eff+3)= (0.15 \pm 0.05)/(1-f_dh), where f_dh accounts for the
possibility that some fraction of halos may be dark or without a bright central
galaxy. A model with Omega_m=0.25 and primordial n=0.8 or with Omega_m=0.2 and
n=1 agrees well with these constraints even in the absence of dark halos,
although with the current data somewhat higher values for Omega_m and n are
also acceptable.

Astrophysics