The future size of a population depends on a range of functions. Thus:
where
Pt and
Pt+1 are the sizes of population in an area at two different points in time,
t and
t + 1 are those points,
B is the birth rate,
D is the death rate,
I is the immigration, and
E is the emigration. For an example of its use, see Roos (2005)
J. Econ. Geog. 5, 616.