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numbering k from n lines in general position

Source: Mathematics Regional Olympiad of Mexico Center Zone 2018 P3

November 13, 2021
combinatorial geometrycombinatorics

Problem Statement

Consider nn lines in the plane in general position, that is, there are not three of the nn lines that pass through the same point. Determine if it is possible to label the kk points where these lines are inserted with the numbers 11 through kk (using each number exactly once), so that on each line, the labels of the nāˆ’1n-1 points of that line are arranged in increasing order (in one of the two directions in which they can be traversed).