LECTURE NOTES IN ANALYSIS (2011)
《LECTURE NOTES IN ANALYSIS (2011)》 · 第 473 页 · Section 11.1「The Density of a Sequence」，Definition 11.1
The asymptotic density of a set $\mathcal N$ is $\delta_1(\mathcal N)=\liminf_{n\to\infty}A(n)/n$.