IMO Shortlist 1966 problem 24


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2. travnja 2012.
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There are n\geq 2 people at a meeting. Show that there exist two people at the meeting who have the same number of friends among the persons at the meeting. (It is assumed that if A is a friend of B, then B is a friend of A; moreover, nobody is his own friend.)
Izvor: Međunarodna matematička olimpijada, shortlist 1966