Let $\frac{ a_1}{ b_1}, \frac{ a_2}{ b_2}, \ldots$ be the Farey fractions of order $n$. We then prove that the inequality $(a_l - a_k)(b_l - b_k) \ge 0$ holds for all $k$ and $l > k$ with $l-k \le \left(\frac{ 1}{ 12} - o(1) \right)n$, sharpening an old result by Erd\H{ o}s. On the other hand, we will show that for all $n \ge 4$ there are $k, l$ with $k < l < k + \frac{ n}{ 4} + 5$ for which the product $(a_l - a_k)(b_l - b_k)$ is negative.
Categories: math.NT