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Friends at a party - Revision history
2024-03-29T01:05:27Z
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Zerrakhi: Hint and solution.
2010-10-13T12:46:45Z
<p>Hint and solution.</p>
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<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 12:46, 13 October 2010</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>An extremely popular mathematician threw a party.  Including himself, there were 15 people at the party.  Must there have been at least two people with the same number of friends present?  Explain.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>An extremely popular mathematician threw a party.  Including himself, there were 15 people at the party.  Must there have been at least two people with the same number of friends present?  Explain.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">{{Hint | Assume friendship is mutual, i.e. if A is a friend of B then B is a friend of A.}}</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">{{Solution |</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">The least number of friends a person might have is 0 (gatecrasher) and the greatest is 14 (friends with everyone, excluding themselves). There are as many integers in that range as there are people at the party, so if everyone has a different number of friends, then every number from 0 to 14 must be equal to that number for some person.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">But if some person (very likely the mathematician) has 14 friends, and we assume friendship is always mutual, then there can be no person with 0 friends. Everyone must have at least one friend, namely the person with 14 friends.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">Therefore the assumption that everyone has a different number of friends leads to a contradiction, and by ''reducto ad absurdum'' we conclude that at least two people must have the same number of friends.</ins></div></td></tr>
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Zerrakhi
https://mathpuzzlewiki.com/index.php?title=Friends_at_a_party&diff=523&oldid=prev
Zerrakhi: Clarify that we're only talking about friends who are present at the party!
2010-10-13T12:17:31Z
<p>Clarify that we're only talking about friends who are present at the party!</p>
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<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 12:17, 13 October 2010</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>An extremely popular mathematician threw a party.  Including himself, there were 15 people at the party.  Must there have been two people <del class="diffchange diffchange-inline">there </del>with the same number of friends?  Explain.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>An extremely popular mathematician threw a party.  Including himself, there were 15 people at the party.  Must there have been <ins class="diffchange diffchange-inline">at least </ins>two people with the same number of friends <ins class="diffchange diffchange-inline">present</ins>?  Explain.</div></td></tr>
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Zerrakhi
https://mathpuzzlewiki.com/index.php?title=Friends_at_a_party&diff=31&oldid=prev
Oscarlevin: /* Puzzle */
2010-04-05T13:55:10Z
<p><span dir="auto"><span class="autocomment">Puzzle</span></span></p>
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<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 13:55, 5 April 2010</td>
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Oscarlevin
https://mathpuzzlewiki.com/index.php?title=Friends_at_a_party&diff=25&oldid=prev
Oscarlevin: /* Puzzle */
2010-03-30T20:03:51Z
<p><span dir="auto"><span class="autocomment">Puzzle</span></span></p>
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<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 20:03, 30 March 2010</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>An extremely popular mathematician threw a party.  Including himself, there were 15 people at the party.  Must there have been two people there with the same number of <del class="diffchange diffchange-inline">friend</del>?  Explain.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>An extremely popular mathematician threw a party.  Including himself, there were 15 people at the party.  Must there have been two people there with the same number of <ins class="diffchange diffchange-inline">friends</ins>?  Explain.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Category: Pigeonhole theorem]]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Category: Pigeonhole theorem]]</div></td></tr>
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Oscarlevin
https://mathpuzzlewiki.com/index.php?title=Friends_at_a_party&diff=24&oldid=prev
Oscarlevin: Created page with '==Puzzle== An extremely popular mathematician threw a party. Including himself, there were 15 people at the party. Must there have been two people there with the same number o…'
2010-03-30T20:01:34Z
<p>Created page with '==Puzzle== An extremely popular mathematician threw a party. Including himself, there were 15 people at the party. Must there have been two people there with the same number o…'</p>
<p><b>New page</b></p><div>==Puzzle==<br />
<br />
An extremely popular mathematician threw a party. Including himself, there were 15 people at the party. Must there have been two people there with the same number of friend? Explain.<br />
<br />
[[Category: Graph theory]]<br />
[[Category: Pigeonhole theorem]]</div>
Oscarlevin