  A population is considered stationary if the growth rate is zero and the age
structure is constant. It thus follows that a population is considered
non-stationary if either its growth rate is non-zero and/or its age structure
is non-constant. We propose three properties that are related to the stationary
population identity (SPI) of population biology by connecting it with
stationary populations and non-stationary populations which are approaching
stationarity. One of these important properties is that SPI can be applied to
partition a population into stationary and non-stationary components. These
properties provide deeper insights into cohort formation in real-world
populations and the length of the duration for which stationary and
non-stationary conditions hold. The new concepts are based on the time gap
between the occurrence of stationary and non-stationary populations within the
SPI framework that we refer to as Oscillatory SPI and the Amplitude of SPI.
This article will appear in Bulletin of Mathematical Biology (Springer)
