Timeline for A set for which it is hard to determine whether or not it is countable.
Current License: CC BY-SA 2.5
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| Mar 22, 2011 at 13:53 | comment | added | Ori Gurel-Gurevich | The thumbtack question can be also done in the plane: how many nonintersecting figure-Ts can one draw in the plane? | |
| Mar 22, 2011 at 11:14 | comment | added | Zsbán Ambrus | This does not quite meet answer the original question, but indeed leads to an interesting class of puzzles. Perhaps it would deserve its own threads. A related question is how many thumbtacks you can place in the 3-space so that no two intersect. A thumbtack is a union of a disk and a segment one of whose endpoints is an interior point of the disk but the other endpoint is not in the plane of the disk. (The answer is countably many.) | |
| Mar 22, 2011 at 3:30 | history | answered | Daniel Litt | CC BY-SA 2.5 |