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for each n, exactly 1 of n,2n,3n is element of A, 2 \in A, prove 13824 \notin A

Source: Dutch BxMO/EGMO TST 2012 p5

August 24, 2019
number theorycombinatoricsset

Problem Statement

Let AA be a set of positive integers having the following property: for each positive integer nn exactly one of the three numbers n,2nn, 2n and 3n3n is an element of AA. Furthermore, it is given that 2A2 \in A. Prove that 13824A13824 \notin A.