2007 IMO Problems/Problem 3

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Problem

In a mathematical competition some competitors are friends. Friendship is always mutual. Call a group of competitors a clique if each two of them are friends. (In particular, any group of fewer than two competitors is a clique.) The number of members of a clique is called its size. Given that, in this competition, the largest size of a clique is even, prove that the competitors can be arranged in two rooms such that the largest size of a clique contained in one room is the same as the largest size of a clique contained in the other room.

Solution

• Nottingham Tuesday Club solution <-- dead link :(

Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.