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Geometry Difficulty 8.0 National Olympiad, round 2 Prove it Bulgaria

Problem:

Some of the vertices of a convex nn-gon are connected by segments such that any two of them have no a common interior point. Prove that for any nn points in general position (i.e., any three of them are not collinear) there is an one-to-one correspondence between the points and the vertices of the nn-gon such that any two segments corresponding to the respective segments from the nn-gon have no a common interior point.

Solution

Solution:

Let A1A2AnA_{1} A_{2} \ldots A_{n} be a convex nn-gon. Denote by B\mathcal{B} the set of the connected vertices and let B1B2BkB_{1} B_{2} \ldots B_{k} be its convex hull. We shall prove by induction on nn that there is a map with the desired properties that in addition sends two given adjacent vertices of the nn-gon to two given adjacent vertexes of B1B2BkB_{1} B_{2} \ldots B_{k}.

The base of the induction n=3n=3 is obvious. Suppose that our statement is true for any k<nk<n. To prove it for nn, it is enough to find a map for nn that sends A1A_{1} and A2A_{2} to B1B_{1} and B2B_{2}, respectively. Note that there is a unique point AiA_{i} that is connected with A1A_{1} and A2A_{2} (otherwise, some segments will have a common interior point). Consider the points X1,X2,,XsX_{1}, X_{2}, \ldots, X_{s} from B\mathcal{B} such that any of the triangles B1XiB2B_{1} X_{i} B_{2} contains no points of B\mathcal{B}. It is easy to find a point among them, say XlX_{l}, such that the interiors of B 1 B 2 X l\text{B 1 B 2 X l} and B 2 B 1 X l\text{B 2 B 1 X l} contain at most nin-i and i3i-3 points of B\mathcal{B}, respectively. It is clear now that there are a line through XlX_{l} and an interior point of the segment B1B2B_{1} B_{2} that divides the set B\mathcal{B} into two subsets B1\mathcal{B}_{1} and B2\mathcal{B}_{2}, containing ni+1n-i+1 and i2i-2 points, respectively. Let B1XlB_{1} X_{l} and B2XlB_{2} X_{l} be sides of the convex hulls of these two sets. If AiA_{i} is the corresponding point to XlX_{l}, then applying the induction assumption to the sets A2A3AiA_{2} A_{3} \ldots A_{i} and B2{Xl}\mathcal{B}_{2} \cup\{X_{l}\}, and to AiAi+1AnA1A_{i} A_{i+1} \ldots A_{n} A_{1} and B1{Xl}\mathcal{B}_{1} \cup\{X_{l}\}, we see that the statement is true for nn points. This completes the solution of the problem.

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