If there are 5 points on the surface of a sphere, then there is a closed half sphere, containing at least 4 of them.

It's in a pigeonhole list of problems. But, I think I have to use rotations in more than 1 dimension.


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    $\begingroup$ An answer is given by Calvin Lin here. This is a duplicate in a way, but the topic of that other question is vastly different (a call for trick questions), so I am a bit reluctant to call it a duplicate. $\endgroup$ – Jyrki Lahtonen Feb 13 '14 at 22:50
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    $\begingroup$ So if you ever see a headline like "80% of Olympic Sites In The Last 20 Years Have Been In The ___ Hemisphere," it means bupkis. $\endgroup$ – Bob Stein Feb 14 '14 at 16:36
  • $\begingroup$ @Bob Stein What is the maning of "bupkis" ? I never met this term... $\endgroup$ – Jean Marie Jun 22 at 6:57
  • $\begingroup$ @JeanMarie "bupkis" means "nothing at all". According to yourdictionary it comes from Yiddish. I think it's especially funny when used in a string of synonyms. $\endgroup$ – Bob Stein Jun 22 at 10:15
  • $\begingroup$ @Bob Stein Thanks. Set apart the meaning, I hadn't any idea about the origin of this word. Yiddish is a very interesting language ; I had once the opportunity to listen to old people speaking a language that I thought at first regional german. I asked them the area of Germany they were from, but in fact they were coming from Ukraine... $\endgroup$ – Jean Marie Jun 22 at 18:36

Pick two distinct points out of your 5 (if all 5 are identical then they clearly all lie in a single hemisphere). These two points define at least one great circle (if they're antipodal, they define infinitely many); pick a great circle they define. This circle then cuts the sphere into two hemispheres. Now pigeonhole the other three points between these two hemispheres.

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    $\begingroup$ Wow, this is the most beautiful paragraph I've seen this week. Thanks. $\endgroup$ – Jorge Fernández Hidalgo Feb 13 '14 at 22:55
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    $\begingroup$ But the answer is sort of demoralizing, made me feel bad for not thinking of that. $\endgroup$ – Jorge Fernández Hidalgo Feb 14 '14 at 3:19
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    $\begingroup$ @user4140 Don't be too demoralized! A lot of problems like this come down to whether you can recognize the specific 'trick' - the thing to pigeonhole on; it's easy to see in retrospect but often impossible to spot from the other side. $\endgroup$ – Steven Stadnicki Feb 14 '14 at 5:03
  • $\begingroup$ I was thinking does this still holds if the sphere degenerates into circle, seemingly not. $\endgroup$ – zinking Feb 14 '14 at 6:14
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    $\begingroup$ @zinking: Take 5 points on $\mathbb S^1$ such that the angle of any two neighboring points is $2\pi/5$. Then in every closed hemisphere there lie at most 3 of the points. $\endgroup$ – Wolfgang Spindeler Feb 14 '14 at 14:54

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