Question 10
Let $d(n)$ be the number of positive divisors of $n$. Show that
$$
n\left(\frac{1}{2} + \frac{1}{3} + \dots + \frac{1}{n}\right) \le d(1) + d(2) + \dots + d(n) \le n\left(1 + \frac{1}{2} + \frac{1}{3} + \dots + \frac{1}{n}\right)
$$
$$
n\left(\frac{1}{2} + \frac{1}{3} + \dots + \frac{1}{n}\right) \le d(1) + d(2) + \dots + d(n) \le n\left(1 + \frac{1}{2} + \frac{1}{3} + \dots + \frac{1}{n}\right)
$$