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MEMO 2017 T4

    Let n≥3n \geq 3 be an integer. A sequence P1,P2,…,PnP_1, P_2, \ldots, P_n of distinct points in the plane is called good if no three of them are collinear, the polyline P1P2…PnP_1P_2 \ldots P_n is non-self-intersecting and the triangle PiPi+1Pi+2P_iP_{i + 1}P_{i + 2} is oriented counterclockwise for every i=1,2,…,n−2i = 1, 2, \ldots, n - 2 . For every integer n≥3n \geq 3 determine the greatest possible integer kk with the following property: there exist nn distinct points A1,A2,…,AnA_1, A_2, \ldots, A_n in the plane for which there are kk distinct permutations σ:{1,2,…,n}→{1,2,…,n}\sigma : \{1, 2, \ldots, n\} \to \{1, 2, \ldots, n\} such that Aσ(1),Aσ(2),…,Aσ(n)A_{\sigma(1)}, A_{\sigma(2)}, \ldots, A_{\sigma(n)} is good. (A polyline P1P2…PnP_1P_2 \ldots P_n consists of the segments P1P2,P2P3,…,Pn−1PnP_1P_2, P_2P_3, \ldots, P_{n - 1}P_n .)

    Dataset text AI-MO AoPS. Quoted with source.

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