Schnirelmann Density

The Schnirelmann density of a sequence of natural numbers is the greatest lower bound of the fractions $A(n)/n$ where $A(n)$ is the number of terms in the sequence $\leq n$.

References

Khinchin, A. Y. ``The Landau-Schnirelmann Hypothesis and Mann's Theorem.'' Ch. 2 in Three Pearls of Number Theory. New York: Dover, pp. 18-36, 1998.