The function fðx1; …; xn;tÞ represents the population density function
defined as the number density of particles per unit volume, i.e., the
net number of particles is then given by R 1
0 … R 1
0 fðx1; …; xn;tÞ
dx1…dxn; the function f 0ðx1; …; xnÞ is the initial density function.
The term Gi refers to the i-th component of the growth rate vector of a
particle in the property space ðx1; …; xnÞ, which is generally size- and
time-dependent; throughout this paper it is assumed in the factorizing
form: