IMO Shortlist 2011 problem N5

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Dodao/la: arhiva
June 23, 2013
Let f be a function from the set of integers to the set of positive integers. Suppose that, for any two integers m and n, the difference f(m) - f(n) is divisible by f(m- n). Prove that, for all integers m and n with f(m) \leq f(n), the number f(n) is divisible by f(m).

Proposed by Mahyar Sefidgaran, Iran
Source: Međunarodna matematička olimpijada, shortlist 2011