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KöMaL 2023 A868

    A set of points in the plane is called disharmonic, if the ratio of any two distances between the points is between 100/101100/101 and 101/100101/100, or at least 100 or at most 1/1001/100. Is it true that for any distinct points A1,A2,…,AnA_1, A_2,\ldots, A_n in the plane it is always possible to find distinct points A1′,A2′,…,An′A'_1, A'_2,\ldots,A'_n that form a disharmonic set of points, and moreover AiA_i, AjA_j and AkA_k are collinear in this order if and only if Ai′A'_i, Aj′A'_j and Ak′A'_k are collinear in this order (for all distinct 1≤i,j,k≤n1\le i,j,k\le n)?

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