Frobenius-Peron Equation

$$\rho_{n+1}(x) = \int \rho_n(y)\delta[x-M(y)]\,dy,$$

where $\delta(x)$ is a Delta Function, $M(x)$ is a map, and $\rho$ is the Natural Density.

References

Ott, E. Chaos in Dynamical Systems. New York: Cambridge University Press, p. 51, 1993.