 Euler–Lotka equation

For birth rate $b(a)$, death rate $d(a)$ and exponential population growth rate $r$, the Euler-Lotka equation is

The Euler–Lotka equation is a fundamental equation in population dynamics and ecology that describes the relationship between the age-specific birth and death rates of a population and its growth rate. It is particularly useful in modeling the growth of structured populations where individuals can have age-dependent survival and fertility rates.