# 1985 IMO Problems/Problem 2

## Problem

Let $n$ and $k$ be given relatively prime natural numbers, $k < n$. Each number in the set $M = \{ 1,2, \ldots , n-1 \}$ is colored either blue or white. It is given that

(i) for each $i \in M$, both $i$ and $n-i$ have the same color;

(ii) for each $i \in M, i \neq k$, both $i$ and $|i-k|$ have the same color.

Prove that all the numbers in $M$ have the same color.

## Solution

We may consider the elements of $M$ as residues mod $n$. To these we may add the residue 0, since (i) may only imply that 0 has the same color as itself, and (ii) may only imply that 0 has the same color as $k$, which put no restrictions on the colors of the other residues.

We note that (i) is equivalent to saying that $i$ has the same color as $-i$, and given this, (ii) implies that $i$ and $(-i + k)$ have the same color. But this means that $i, -i$, and $i+k$ have the same color, which is to say that all residues of the form $i + mk \; (m \in \mathbb{N}_0)$ have the same color. But these are all the residues mod $n$, since $k$ and $n$ are relatively prime. Q.E.D.