olymmath

ISL 1991 Problem 9

    In the plane we are given a set EE of 1991 points, and certain pairs of these points are joined with a path. We suppose that for every point of E,E, there exist at least 1593 other points of EE to which it is joined by a path. Show that there exist six points of EE every pair of which are joined by a path.

    Alternative version: Is it possible to find a set EE of 1991 points in the plane and paths joining certain pairs of the points in EE such that every point of EE is joined with a path to at least 1592 other points of E,E, and in every subset of six points of EE there exist at least two points that are not joined?

    Dataset text AI-MO AoPS. Quoted with source.

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