olymmath

ISL 2021 C1

    Let SS be an infinite set of positive integers, such that there exist four pairwise distinct a,b,c,d∈Sa, b, c, d \in S with gcd⁡(a,b)≠gcd⁡(c,d)\operatorname{gcd}(a, b) \neq \operatorname{gcd}(c, d). Prove that there exist three pairwise distinct x,y,z∈Sx, y, z \in S such that gcd⁡(x,y)=gcd⁡(y,z)≠gcd⁡(z,x)\operatorname{gcd}(x, y)=\operatorname{gcd}(y, z) \neq \operatorname{gcd}(z, x).

    Dataset text NuminaMath olympiads. Quoted with source.

    Official text: IMO 2021 Shortlist booklet, page 6.

    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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