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IMO 2013 — Complete Shortlist Question 4

IMO Shortlist · IMO 2013 — Complete Shortlist

Question 4

Let $n$ be a positive integer, and consider a sequence $a_{1}, a_{2}, \ldots, a_{n}$ of positive integers. Extend it periodically to an infinite sequence $a_{1}, a_{2}, \ldots$ by defining $a_{n+i}=a_{i}$ for all $i \geqslant 1$. If

$$
a_{1} \leqslant a_{2} \leqslant \cdots \leqslant a_{n} \leqslant a_{1}+n
$$

and

$$
a_{a_{i}} \leqslant n+i-1 \quad \text { for } i=1,2, \ldots, n
$$

prove that

$$
a_{1}+\cdots+a_{n} \leqslant n^{2} .
$$