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RU-ALLRUSSIAN 2021 Problem 117

  • Grade 11

Find all permutations (a1,a2,...,a2021)(a_1, a_2,...,a_{2021}) of (1,2,...,2021)(1,2,...,2021) , such that for every two positive integers mm and nn with difference bigger than 202120^{21} , the following inequality holds: GCD(m+1,n+a1)+GCD(m+2,n+a2)+...+GCD(m+2021,n+a2021)<2∣m−n∣GCD(m+1, n+a_1)+GCD(m+2, n+a_2)+...+GCD(m+2021, n+a_{2021})<2|m-n| .

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