一次集会有
2005个人参加,其中任意的四个人中至少有一个人认识其余三个人。问在这次集会上,认识全体到会者的人至少有多少位?2002? Suppose A does not Know B. Then the rest 2003 people know each other: If C does not know D, then the group A, B, C, D does not have one person knowing the other three. Now in the 2003 peple, there can only be one person who does not know both A and B. Otherwise they and A, B would form a group that nobody knows the rest three. |
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