 Press and Schechter observed that the fraction of mass in collapsed objects
more massive than some mass M is related to the fraction of volume samples
in which the smoothed initial density fluctuations are above some
density threshold.This yields a formula for the mass function (distribution
of masses) of objects at any given time.The Press–Schechter formalism predicts that the number of objects with mass between formula_1 and formula_2 is