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Mongolian Mathematical Olympiad

Mongolia number theory

Problem

Prove that if are positive real numbers and there is no integer among them then there exists infinitely many such that .
Solution
Proceeding by contradiction, suppose that there exists such that for all . Consequently, for any prime , (for all ) there exists such that: . It is obvious that when the sequence converges to and because all terms of the sequence are integers, there exists such that . Since the latter is equivalent to , we conclude that are integers. It leads to that are integers and but this is a contradiction.

Techniques

Greatest common divisors (gcd)Prime numbersFloors and ceilings