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Q: What's an anagram of Banach-Tarski?

A: Banach-Tarski Banach-Tarski.

I did my undergraduate project on the question of finitely-additive, isometry-invariant measures that extend the Lebesgue Measure and which are defined on all possible bounded subsets of R^n.

And yes, that's relevant. There is (perhaps surprisingly) such a measure on R,and there is such a measure on R^2, but the Banach-Tarski "paradox" shows that there is no such measure on R^3 (and therefore trivially, R^n for n>3).

For contrast, we'd usually like to have a measure that's countably additive, but there is a simple construction (albeit using the axiom of choice) that shows such an object is impossible.

This has, by the way, been submitted and discussed several times in the past. In case you're interested in reading other descriptions, and previous discussions, you can find some of them here:


T-shirt: http://thenerdiestshirts.com/site/math-t-shirts-banach-tarsk...

I suggest buying two and wearing them both. When somebody laughs at the T-Shirt, well, you do the math.

If the shirt's well made, you can buy one and wear them both.

Is Banach-Tarski* a valid answer? (Kleene star.)

No, but (Banach-Tarski)(Banach-Tarski)* would be, or possibly (Banach-Tarski)+ depending on your system. You can't get to zero copies.

Wikipedia says you can collapse two balls into one, so doesn't that mean you can cut your ball in half, mold the halves into balls, and then merge those two half-volume balls into one half-volume ball? And if you can do that, can't you get to an infinitely small ball?

Arbitrarily small, yes, but never zero. You will always have c points, where c is the cardinality of the continuum.

Xeno would agree!

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