olymmath

IMO 2017 Problem 3

  • Day 1

A hunter and an invisible rabbit play a game in the Euclidean plane. The rabbit's starting point, A0,A_0, and the hunter's starting point, B0B_0 are the same. After n−1n-1 rounds of the game, the rabbit is at point An−1A_{n-1} and the hunter is at point Bn−1.B_{n-1}. In the nthn^{\text{th}} round of the game, three things occur in order:

[list=i] []The rabbit moves invisibly to a point AnA_n such that the distance between An−1A_{n-1} and AnA_n is exactly 1.1. []A tracking device reports a point PnP_n to the hunter. The only guarantee provided by the tracking device to the hunter is that the distance between PnP_n and AnA_n is at most 1.1. [*]The hunter moves visibly to a point BnB_n such that the distance between Bn−1B_{n-1} and BnB_n is exactly 1.1. [/list] Is it always possible, no matter how the rabbit moves, and no matter what points are reported by the tracking device, for the hunter to choose her moves so that after 10910^9 rounds, she can ensure that the distance between her and the rabbit is at most 100?100? Proposed by Gerhard Woeginger, Austria

Dataset text AI-MO AoPS. Quoted with source.

Official text: IMO 2017 official English paper, page 1.

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