olymmath

ISL 2020 N7

    Let S\mathcal{S} be a set consisting of n⩾3n \geqslant 3 positive integers, none of which is a sum of two other distinct members of S\mathcal{S}. Prove that the elements of S\mathcal{S} may be ordered as a1,a2,…,ana_{1}, a_{2}, \ldots, a_{n} so that aia_{i} does not divide ai−1+ai+1a_{i-1}+a_{i+1} for all i=2,3,…,n−1i=2,3, \ldots, n-1. (Ukraine)

    Dataset text NuminaMath olympiads. Quoted with source.

    Official text: IMO 2020 Shortlist booklet, page 12.

    Solutions

    Official solution Solution 1 by the IMO Problem Selection Committee

    The terms of the source let olymmath link to this solution but not copy it. Read the solution (IMO official site).

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