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Print2015 Danube Mathematical Competition
Romania 2015 number theory
Problem
Determine all positive integers such that all positive integers less than or equal to and prime to are pairwise coprime.
Solution
Notice that is the first index such that . Now, if for some index , then , by the Bertrand-Tchebysheff theorem, so for all indices .
Consequently, , so . Examination of the possible cases quickly yields the required numbers: .
Consequently, , so . Examination of the possible cases quickly yields the required numbers: .
Final answer
1, 2, 3, 4, 6, 8, 12, 18, 24, 30
Techniques
Prime numbers