  The bulk of recent cosmological research has focused on the adiabatic cold
dark matter model and its simple extensions. Here we present an accurate
fitting formula that describes the matter transfer functions of all common
variants, including mixed dark matter models. The result is a function of
wavenumber, time, and six cosmological parameters: the massive neutrino
density, number of neutrino species degenerate in mass, baryon density, Hubble
constant, cosmological constant, and spatial curvature. We show how
observational constraints---e.g. the shape of the power spectrum, the abundance
of clusters and damped Lyman-alpha systems, and the properties of the
Lyman-alpha forest--- can be extended to a wide range of cosmologies, including
variations in the neutrino and baryon fractions in both high-density and
low-density universes.
