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IMO 2018 — Complete Shortlist Question 2

IMO Shortlist · IMO 2018 — Complete Shortlist

Question 2

Find all positive integers $n \geqslant 3$ for which there exist real numbers $a_{1}, a_{2}, \ldots, a_{n}$, $a_{n+1}=a_{1}, a_{n+2}=a_{2}$ such that

$$
a_{i} a_{i+1}+1=a_{i+2}
$$

for all $i=1,2, \ldots, n$.