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 Sep 4 awarded Yearling Aug 19 awarded Popular Question Jun 22 answered Brun's algorithm Feb 17 comment Spectrum of primitive nonnegative integer matrices Right now the best I can do is to enumerate all the matrices (using the trace constraint) and test them one by one... Can anyone think of a less stupid approach? Feb 17 comment Spectrum of primitive nonnegative integer matrices Gerry, I haven't looked at it for hours but it doesn't seem to be obvious even for $2\times2$ matrices. Feb 17 comment Spectrum of primitive nonnegative integer matrices David, unfortunately I would like to avoid the additional zero eigenvalues. Is this question open even without the primitivity assumption? Thanks for the very interesting articles! Feb 16 asked Spectrum of primitive nonnegative integer matrices Feb 17 comment What are the most attractive Turing undecidable problems in mathematics? Note the recent preprint arxiv.org/abs/1312.6700 which improves these results. Undecidability now holds for five 3*3 matrices or two 15*15 matrices. Jun 14 awarded Nice Answer May 27 awarded Notable Question Oct 20 awarded Yearling Jul 9 awarded Good Answer Apr 29 answered Equivalent subshifts Apr 18 revised What's the difference between 2 and 3? added 332 characters in body Apr 18 answered What's the difference between 2 and 3? Mar 22 comment Importance of Poincaré recurrence theorem? Any example? The ergodic theorems can be seen as improvements of the Poincaré recurrence theorem, because they relate the time average with the space average. However, the Poincaré theorem is treated first because it is not hard to prove and it is very nice. Mar 3 answered Computational software in Algebraic Topology? Dec 5 answered More open problems Oct 20 awarded Yearling Sep 21 comment Lecture notes by Thurston on tiling I got the above PDF version from Shigeki Akiyama and I am very glad that putting it on my website is useful to some people. It would be great if more people did this because trying to find a paper that is neither on the internet or in libraries can be extremely frustrating!