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IMO 2025 — Complete Shortlist Question 30

IMO Shortlist · IMO 2025 — Complete Shortlist

Question 30

Let $\mathbb{Z}_{ \gt 0}$ denote the set of positive integers. A function $f\colon \mathbb{Z}_{ \gt 0} \to \mathbb{Z}_{ \gt 0}$ is said to be bonza if

$$ f(a)\ \mid\ b^{a}-f(b)^{f(a)} $$

for all positive integers $ a $ and $ b $.

Determine the smallest constant c such that $ f(n) \leq cn $ for any bonza function f and any positive integer n.