Introduction to Analytic Number Theory
《Introduction to Analytic Number Theory》 · 第 52 页 · Chapter 3「Averages of Arithmetical Functions」
We will prove later that the average $d(n)$ grows like $\log n$ for large $n$; more precisely, $\frac{1}{n}\sum_{k=1}^n d(k)\sim\log n$. This is described by saying that the average order of $d(n)$ is $\log n$.