Crater dating; production and chronology functions; equilibrium density কী?

Crater dating is the determination of the age of a surface from the density of its craters. The crater production function is the shape of the size–frequency distribution of the craters a surface acquires, $F(D) = N(D)/N(1\,\mathrm{km})$, assumed the same at every age. The crater chronology function $N_1(T)$ is the number of craters of $1\,\mathrm{km}$ and more per unit area accumulated in a time $T$. A surface is at the crater equilibrium density for craters of a given size when each new crater erases, on average, as many old ones as it adds: their number no longer grows.