2013 IMO Problems/Problem 6

Problem

Let $n \ge 3$ be an integer, and consider a circle with $n + 1$ equally spaced points marked on it. Consider all labellings of these points with the numbers $0, 1, ... , n$ such that each label is used exactly once; two such labellings are considered to be the same if one can be obtained from the other by a rotation of the circle. A labelling is called beautiful if, for any four labels $a < b < c < d$ with $a + d = b + c$ , the chord joining the points labelled $a$ and $d$ does not intersect the chord joining the points labelled $b$ and $c$ .

Let $M$ be the number of beautiful labelings, and let N be the number of ordered pairs $(x, y)$ of positive integers such that $x + y \le n$ and $\gcd(x, y) = 1$ . Prove that $M = N + 1.$

Solution

This problem needs a solution. If you have a solution for it, please help us out by adding it.

2013 IMO (Problems) • Resources
Preceded by Problem 5	1 • 2 • 3 • 4 • 5 • 6	Followed by Last Problem
All IMO Problems and Solutions

Page

Toolbox

Search

2013 IMO Problems/Problem 6

Problem

Solution

See Also