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game with n circles and k points, one in each circle, circles deleted, min k

Source: Rioplatense Olympiad 2002 level 3 P6

September 6, 2018
combinatoricsgame strategyminimumcombinatorial geometry

Problem Statement

Daniel chooses a positive integer nn and tells Ana. With this information, Ana chooses a positive integer kk and tells Daniel. Daniel draws nn circles on a piece of paper and chooses kk different points on the condition that each of them belongs to one of the circles he drew. Then he deletes the circles, and only the kk points marked are visible. From these points, Ana must reconstruct at least one of the circumferences that Daniel drew. Determine which is the lowest value of kk that allows Ana to achieve her goal regardless of how Daniel chose the nn circumferences and the kk points.