  In the framework of population dynamics, the basic reproduction number R_0
is, by definition, the expected number of offspring that an individual has
during its lifetime. In constant and time periodic environments it is
calculated as the spectral radius of the so-called next-generation operator. In
continuously structured populations defined in a Banach lattice X with
concentrated states at birth one cannot define the next-generation operator in
X. In the present paper we present an approach to compute the basic
reproduction number of such models as the limit of the basic reproduction
number of a sequence of models for which R_0 can be computed as the spectral
radius of the next-generation operator. We apply these results to some
examples: the (classical) size-dependent model, a size structured cell
population model, a size structured model with diffusion in structure space
(under some particular assumptions) and a (physiological) age-structured model
with diffusion in structure space.
