= Press-Schechter formalism
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{wiki=Press–Schechter_formalism}
The \b[Press-Schechter formalism] estimates <dark matter halo> abundance by comparing a smoothed linear <density contrast> with the <linear spherical-collapse threshold>. A Gaussian upper-tail probability supplies the collapsed fraction, with a factor of two in the original mass accounting. Differentiating that fraction by smoothing mass gives the <Press-Schechter halo mass function>. It connects initial fluctuations with object counts, while idealized spherical collapse and the treatment of nested regions limit its accuracy.<Halo Models of Large Scale Structure by Asantha Cooray and Ravi Sheth>[Halo-abundance discussion: Gaussian threshold model and mass function]{ref}
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