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2015 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: .
Final answer
1, 2, 3, 4, 6, 8, 12, 18, 24, 30

Techniques

Prime numbers