LECTURE NOTES IN ANALYSIS (2011)
《LECTURE NOTES IN ANALYSIS (2011)》 · 第 476 页 · Section 11.2「Schnirelmann Density」，Definition 11.2
The Schnirelmann density $d(\mathcal N)$ of a set $\mathcal N$ of non-negative integers is $d(\mathcal N)=\inf_n A(n)/n$.