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Knowing People at a Party

Date: 08/27/98 at 01:56:53
From: Maylyn Angeles
Subject: Indirect reasoning

Prove that at any party, there are two people who know the same number 
of people. Assume that if A knows B, then B knows A. Assume also that 
everyone knows himself or herself.

It's been suggested that I use indirect reasoning.

Date: 08/27/98 at 05:15:15
From: Doctor Floor
Subject: Re: Indirect reasoning

Hi Maylyn,

Thank you for sending your question to Dr. Math.

Let's say that there are N people at the party. For indirect reasoning, 
assume that all these people know different numbers of people at the 
party. Then there must be one who knows 1, one who knows 2, one who 
knows 3, ..., and one who knows N. This fits exactly for N people. 

Although this is the only possibility, I am going to show that it is 
impossible that one person knows 1 (must be himself) and one person 
knows N. If it is impossible, then it is also impossible to have all 
different numbers of people known. So we have shown by indirect 
reasoning that there must be two people who know the same number of 

We derived that there must be one person who only knows himself (or 
herself), and one person who knows everybody - but then the person who 
knows everybody also knows the person who only knows himself, and we 
assumed that if A knows B, then B knows A. This means that the person 
who was thought only to know himself also knows somebody else. It is 
not possible that one person knows only himself, and one person knows 
everybody, so we're done.

If you have a math question again, please send it to Dr. Math.

Best regards,
- Doctor Floor, The Math Forum
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Associated Topics:
High School Logic

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