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Combinatorics Difficulty 7.2 National olympiad, round 2 Prove it Bulgaria

Let MM be convex 2011-gon. Consider 2011 points lying inside MM and such that no three of all 4022 points (the vertexes of MM and 2011 points inside MM) are collinear. A coloring of all points in two colors is called *good* if it is possible to connect some of the points by segments such that the following conditions hold:
(1) Each segment has its endpoints of one and the same color.
(2) No segments intersect in inner points.
(3) Between any two points of the same color there exists a path through the given segments.
Find the number of all good colorings.

Solution

Let the two colors be blue and red. We prove first the following

*Lemma*. Consider ABC\triangle ABC with vertices of both colors. Any coloring of nn points inside ABC\triangle ABC is good.

*Proof*. Without loss of generality assume that AA and BB are blue points and CC is a red point.
We proceed by induction on the number n0n \ge 0 of inner points for the triangle.
When n=0n = 0 we draw a segment with end points AA and BB and we are done.
Assume the assertion holds for any triangle with n=kn = k inner points. Consider triangle with n=k+1n = k + 1 inner points and let these points be colored in arbitrary manner. If all points are blue then we connect AA with BB and with all inner points and the condition holds.
If there exists a red point DD, we apply the induction hypothesis for triangles ABDABD, BCDBCD and ACDACD. It is clear that red points from distinct triangles are connected through DD, and the blue points from distinct triangles are connected through AA or BB. This completes the proof of the lemma. \square

Note that for any good coloring all blue points of MM are consecutive vertices. Indeed, if this is not the case we have two red points AA and BB, dividing the contour of MM into two parts with two blue points CC and DD in each of these parts. It is clear now that the red path between AA and BB intersects the blue path between CC and DD, a contradiction.

Let us first count the number of good colorings of the vertexes of MM. There exist two such colorings with all points having one and the same color. When both colors occur let k>0k > 0 be the number of blue points. For any k=1,2,,2010k = 1, 2, \dots, 2010 there exist 2011 ways of choosing the group of kk consecutive blue points. Therefore we have 20112010+22011 \cdot 2010 + 2 ways of coloring the vertexes of MM.

We show now that any coloring of the inner points is good. If all vertexes of MM have one and the same color (say blue) and all inner point are also blue we connect one vertex with all remaining points. If there exists a red point BB, then we consider all triangles with one vertex BB and sides the sides of MM. The assertion of the Lemma completes the proof in this case.

Consider coloring of the vertexes of MM in which both blue and red points occur. Without loss of generality assume the blue points are A1,A2,,Ak,k<2011A_1, A_2, \dots, A_k, k < 2011. Connect A1A_1 with all red points and Ak+1A_{k+1} with all blue points. Now MM is partitioned into triangles and each triangle has vertexes of both colors. Apply the Lemma for each of these triangles. The points having one and the same color from distinct triangles are connected through A1A_1 or Ak+1A_{k+1} or by the sides of MM.

Therefore the number of good colorings equals 22011(20112010+2)2^{2011}(2011 \cdot 2010 + 2).

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.