IMO Shortlist 2008 problem C2


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Dodao/la: arhiva
April 2, 2012
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Let n \in \mathbb N and A_n set of all permutations (a_1, \ldots, a_n) of the set \{1, 2, \ldots , n\} for which
k|2(a_1 + \cdots+ a_k), \text{ for all } 1 \leq k \leq n.
Find the number of elements of the set A_n.
Source: Međunarodna matematička olimpijada, shortlist 2008