Suppose that a point in the plane is called good if it has rational coordinates. Prove that all good points can be divided into three sets satisfying: 1) If the centre of the circle is good, then there are three points in the circle from each of the three sets. 2) There are no three collinear points that are from each of the three sets.
Problem
Source: 2002 China National Olmpiad
Tags: combinatorics proposed, combinatorics
huricane
08.01.2019 23:02
Fine problem!
Color with color one all rational points $(x,y)$ with $\text{min}(v_2(x),v_2(y))>0$, with color two all those points with $v_2(x)\ge v_2(y)$ and $v_2(y)\le 0$, and with color three all those with $v_2(x)<v_2(y)$ and $v_2(x)\le 0.$
Considering point of the form $(2(1+2k)/q,2(1+2l)/q)$ with $k,l\in\mathbb{Z}$ and $q$ odd we obviously have a point of color one within $2/q$ distance, both $x$-wise and $y$-wise, from any point in the plane. Letting $q$ tend to infinity, and arguing in a similar manner for the other colors, it's clear that $1)$ is satisfied.
Suppose that we have some points $(x_1,y_1)=(2^a\cdot p_1/q_1, 2^b\cdot p_2/q_2)$ (with $a,b>0$ and $p_i,q_i$ odd, hence of color one),$(x_2,y_2)= (2^c\cdot p_3/q_3, 2^d\cdot p_4/q_4)$(with $c\ge d$,$d\le 0$ and $p_i,q_i$ odd, hence of color two) and $(x_3,y_3)=(2^e\cdot p_5/q_5, 2^f\cdot p_6/q_6)$(with $e<f$, $e\le 0$ and $p_i,q_i$ odd, hence of color three) of different colors that are collinear. Then $$\begin{vmatrix} x_1 & y_1 & 1 \\ x_1 & y_1 & 1 \\ x_3 & y_3 & 1 \end{vmatrix}=0.$$Rationalizing and multiplying with $2^{-d}$ the first row and by $2^{-e}$ the second row, we obtain $$\begin{vmatrix} 2^a\cdot\text{odd} & 2^b\cdot\text{odd} & \text{odd} \\ 2^{c-d}\cdot\text{odd} & \text{odd} & 2^{-d}\cdot\text{odd} \\ \text{odd} & 2^{f-e}\cdot\text{odd} & 2^{-e}\cdot\text{odd} \end{vmatrix}=0.$$Mod $2$ this cleans out as $$\begin{vmatrix} 0 & 0 & 1\\ x & 1 & y \\ 1 & 0 & z \end{vmatrix}=0,$$which by expansion along the first row becomes $$\begin{vmatrix} x & 1 \\ 1 & 0 \end{vmatrix}=0.$$But this last determinant is equal to $-1\neq 0$, a contradiction!