Question 139
A function $f: \mathbb{Z} \rightarrow \mathbb{Z}_{+}$, where $\mathbb{Z}_{+}$ is the set of positive integers, is non-decreasing and satisfies $f(m n) = f(m) f(n)$ for all relatively prime positive integers $m$ and $n$. Prove that $f(8) f(13) \geq (f(10))^{2}$.