IMO Shortlist 1991 problem 23


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2. travnja 2012.
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An odd integer n \ge 3 is said to be nice if and only if there is at least one permutation a_{1}, \cdots, a_{n} of 1, \cdots, n such that the n sums a_{1} - a_{2} + a_{3} - \cdots - a_{n - 1} + a_{n}, a_{2} - a_{3} + a_{3} - \cdots - a_{n} + a_{1}, a_{3} - a_{4} + a_{5} - \cdots - a_{1} + a_{2}, \cdots, a_{n} - a_{1} + a_{2} - \cdots - a_{n - 2} + a_{n - 1} are all positive. Determine the set of all `nice' integers.
Izvor: Međunarodna matematička olimpijada, shortlist 1991