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The number density of galaxy clusters is extremely sensitive to cosmological
parameters. We have combined X-ray selection of clusters from ROSAT data
with the extensive redshift data in SDSS to obtain a new estimate of the
cluster mass function using virial masses. The cluster abundance is consistent
with independent estimates using X-ray mass estimates. The cosmological
constraints are consistent with Third-Year WMAP results, but interesting
differences remain. I will discuss ways of improving the estimates of the
local mass function and describe how we plan to use the evolution of the
mass function to constrain dark energy.
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