  The equilibrium magnetization of concentrated ferrofluids described by a
system of polydisperse dipolar hard spheres is calculated as a function of the
internal magnetic field using the Born--Mayer or cluster expansion technique.
This paper extends the results of Phys. Rev. E 62, 6875 (2000) obtained for
monodisperse ferrofluids. The magnetization is given as a power series
expansion in two parameters related to the volume fraction and the coupling
strength of the dipolar interaction, respectively.
