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ELMO 2021 Problem 4

    The set of positive integers is partitioned into nn disjoint infinite arithmetic progressions S1,S2,…,SnS_1, S_2, \ldots, S_n with common differences d1,d2,…,dnd_1, d_2, \ldots, d_n , respectively. Prove that there exists exactly one index 1≤i≤n1\leq i \leq n such that1di∏j=1ndj∈Si.\frac{1}{d_i}\prod_{j=1}^n d_j \in S_i.

    Dataset text AI-MO AoPS. Quoted with source.

    Official text: ELMO 2021 official paper, page 2.

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