Let , be given primes and the sequence defined recursively as follows: ; and is the largest prime divisor of the number for all . Prove that this sequence is bounded, that is there exists a positive real number such that for all positive integers .
Solution
Put and for each .
We shall show that . In fact, since , it remains to show that .
If then the inequality is trivially true. It suffices to consider the case .
If either or then . Otherwise, both primes and are odd, in particular is even. But , we get
It is clear that there is a positive integer such that . We show, by induction that . Indeed, suppose this is true for , then
If , then with . Hence, must be a divisor of , that is is a proper divisor of . However, this cannot happen since is a prime.
Thus, , and this completes the solution.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.