Determine all positive integers such that all positive integers less than or equal to and prime to are pairwise coprime.
Danube Mathematical Competition, 2015
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: .
Want a route through all this instead of an archive? The track
puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.