IMO Shortlist 1998 problem C4
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Avg: 7,0 Let , where . A subset of is said to be split by an arrangement of the elements of if an element not in occurs in the arrangement somewhere between two elements of . For example, 13542 splits but not . Prove that for any subsets of , each containing at least 2 and at most elements, there is an arrangement of the elements of which splits all of them.
Izvor: Međunarodna matematička olimpijada, shortlist 1998