
212 Population dynamics
it is just the population we considered before. The ratio
P(t,x)
P(t)
describes the age structure of the population. We do not study here
probabilistic properties of P(t,x), but will only consider the behavior
of its expected value EP(t,x) as a function of x and the behavior of the
ratio
EP(t,x)
EP(t)
,
and connect it with the lifetime distributions G
β
(x) and other demo-
graphic characteristics.
The expression (16.25) does not incorporate the migration in and
out of the population, so that we consider a closed population. But if
we do not, the presentation will become rather heavy. It may be not
very useful to strive for complete generality ...