In space are marked $ 2005$ points, no four of which are in the same plane. A plane is drawn through any three points. Show that the points can be painted in two colors so that for any two points of the same color the number of the drawn planes separating them is odd. (Two points are separated by a plane if they lie in different open half-spaces determined by the plane).
Problem
Source: Ukraine 2005 grade 11
Tags: combinatorics proposed, combinatorics
26.02.2022 07:32
This is a nice and quite difficult variant of APMO 2004/3. Take a point $O,$ color it red. Let all points an odd number of planes away from $O$ be red, and all others blue. This fails only if three points $A,B,C$ have $d_{AB}+d_{BC}+d_{CA}$ being even, where $d$ is the number of planes separating two points. A plane only matters in the above sum $\pmod{2}$ if it separates $1$ or $3$ of the pairs $\{A, B \}; \{B, C\} ;\{C, A\}.$ The latter case is impossible. In the former case, the plane goes through one of $A,B,$ or $C$ and separates the other two. Claim: Take any two points $X,Y$ out of the other $2002$ points that aren't $A,B,C.$ Then exactly one of the following is true: $AXY$ separates $\{B, C\},$ $BXY$ separates $\{C, A\},$ $CXY$ separates $\{A, B\}.$ Proof: Let $XY$ intersect plane $AXY$ at point $P.$ Then it's easy to see (just draw it out) that no matter where $P$ lies on the plane, exactly one of the following is true: line $AP$ intersects segment $BC$ (equivalent to $AXY$ separates $\{B, C\}$), line $BP$ passes through segment $CA$ (equivalent to $BXY$ separates $\{C, A\}$), line $CP$ passes through segment $AB$ (equivalent to $CXY$ separates $\{A, B\}$). $\square$ There are $\binom{2002}{2}$ such pairs, an odd number. So $d_{AB}+d_{BC}+d_{CA}$ is odd, the end. $\blacksquare$