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Geometry Difficulty 7.1 National olympiad, round 2 Prove it Croatia

Let nn be a positive integer. Points A1,A2,,AnA_1, A_2, \dots, A_n are located on the inside of a circle, and points B1,B2,,BnB_1, B_2, \dots, B_n are on the circle, so that the lines A1B1\overline{A_1B_1}, A2B2\overline{A_2B_2}, \dots, AnBn\overline{A_nB_n} are mutually disjoint. A grasshopper can jump from point AiA_i to point AjA_j (for i,j{1,,n},iji, j \in \{1, \dots, n\}, i \neq j) if and only if the line AiAj\overline{A_iA_j} does not go through any of the inner points of the lines A1B1,A2B2,,AnBn\overline{A_1B_1}, \overline{A_2B_2}, \dots, \overline{A_nB_n}.

Show that the grasshopper can take a series of jumps to get from any point AiA_i to any point AjA_j. (Russia 1994)

Solution

Let a path be any line AiAjA_iA_j and a wall be any line AkBkA_kB_k. We say that a path and a wall intersect if the path goes through an inner point of the wall. A path is good if there is no wall to intersect it, and a wall is irrelevant if it does not intersect any path.

Claim 1. For any i{1,2,,n}i \in \{1,2,\dots,n\} there exists at least one good path from point AiA_i.

Proof. We will prove the claim for point A1A_1, and it holds analogously for all points AiA_i. Define the distance between a point AA and a wall ww as
minTwAT. \min_{T \in w} |AT|.
Since all walls are mutually disjoint, we know that none of the walls A2B2,,AnBn\overline{A_2B_2}, \dots, \overline{A_nB_n} contain point A1A_1. Let AkBk\overline{A_kB_k} be the wall closest to A1A_1. We claim that the path A1AkA_1A_k is good. If we assume the contrary, that means that there exists a wall which intersects A1AkA_1A_k, but then this wall is closer to A1A_1 than wall AkBk\overline{A_kB_k}, which is a contradiction. \square

Claim 2. There exists at least one irrelevant wall.

Proof. Without loss of generality, we can assume that points A1,A2,,AkA_1, A_2, \dots, A_k are the vertices of the convex hull of {A1,A2,,An}\{A_1, A_2, \dots, A_n\}, and that they are labelled clockwise in that order as the vertices of polygon P=A1A2AkP = A_1A_2\dots A_k. If the wall w1=A1B1w_1 = \overline{A_1B_1} is irrelevant, we are done. Therefore, assume that w1w_1 intersects some path. That means that this wall goes through the inner points of polygon PP, and since the walls are mutually disjoint, we conclude that w1w_1 must also intersect a path which is the side of PP, because it cannot pass through any of the vertices of PP except A1A_1. Let CC be the intersection of wall w1w_1 and a side of PP. Consider the arc \overarcA1C\overarc{A_1C} (clockwise from point A1A_1 to point CC), and note that it contains points A2,,AlA_2, \dots, A_l. We can repeat this inference for wall A2B2A_2B_2. Since all walls are mutually disjoint, its corresponding arc is strictly smaller than the arc of w1w_1. Therefore, if all of the walls A2B2,,Al1Bl1\overline{A_2B_2}, \dots, \overline{A_{l-1}B_{l-1}} are relevant, then wall AlBl\overline{A_lB_l} must surely be irrelevant. \square

Finally, we prove the problem statement by mathematical induction on nn. The claim obviously holds for n=1n=1. Assume the claim holds for some positive integer nn. For n+1n+1 it follows from Claim 2 that there exists a point AiA_i such that the wall AiBi\overline{A_iB_i} is irrelevant. By the inductive assumption, we can conclude that the claim holds for all points except possibly point AiA_i. However, by Claim 1, point AiA_i is connected by a good path with at least one of the remaining points, which proves the claim for n+1n+1.

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