olymmath

ISL 2015 N8

  • Answer

For every positive integer nn with prime factorization n=∏i=1kpiαin=\prod_{i=1}^{k} p_{i}^{\alpha_{i}}, define ℧(n)=∑i:pi>10100αi.\mho(n)=\sum_{i: p_{i}>10^{100}} \alpha_{i} . That is, ℧(n)\mho(n) is the number of prime factors of nn greater than 1010010^{100}, counted with multiplicity. Find all strictly increasing functions f:Z→Zf: \mathbb{Z} \rightarrow \mathbb{Z} such that ℧(f(a)−f(b))⩽℧(a−b) for all integers a and b with a>b.\mho(f(a)-f(b)) \leqslant \mho(a-b) \text { for all integers } a \text { and } b \text { with } a>b .

Dataset text NuminaMath olympiads. Quoted with source.

Official text: IMO 2015 Shortlist booklet, page 8.

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f(x)=a x+b

Solutions

Official solution Solution 1 by the IMO Problem Selection Committee

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