Schnirelmann Density
The Schnirelmann density of a sequence of natural numbers is the greatest lower bound of the fractions 
where
is the number of terms in the sequence 
.
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.
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