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MathNet 2016-part-06 — 184 problems (EN–VI) Question 83

Math Olympiad Vault · MathNet 2016-part-06 — 184 problems (EN–VI)

Question 83

Let $a_{n}$ be a sequence of natural numbers defined by $a_{1}=m$ and $a_{n}=a_{n-1}^{2}+1$ for $n \gt 1$. We call a pair $(a_{k}, a_{l})$ interesting if

(i) $0 \lt l-k \lt 2016$,

(ii) $a_{k}$ divides $a_{l}$.

Show that there exists an $m$ such that the sequence $a_{n}$ contains no interesting pair.