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| 112 | + Banach-Tarski - A basic magic of group theory |
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| 115 | + <time datetime="2013-05-02T00:00:00-04:00" itemprop="datePublished"> |
| 116 | + 2 May 2013 |
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| 122 | + - note |
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| 129 | + <p>The Banach-Tarski paradox is a curious theorem from group theory. I was quite taken by it when I first learned of it, and wrote up <a href="/assets/pdfs/2013.05.02-banach-tarski.pdf">these self-contained notes (PDF)</a>.</p> |
| 130 | + |
| 131 | +<p>Banach-Tarski shows <em>how to conjure things out of thin air</em> …but not with magic, just with the <a href="https://encyclopediaofmath.org/wiki/Axiom_of_choice">axiom of choice</a>. You also need an infinitely divisible object, but only on short loan, because after you cut it up and reassemble it, you’ll have two!. Basically, it gives a method for taking a ball, cutting it up into many tiny pieces, and then putting the pieces back together to make two balls just identical the one you started with.</p> |
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| 177 | + <p>Postdoc at MIT, Department of Brain and Cognitive Sciences. |
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