DavidLHarden
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 May 18 comment Groups that do not exist It seems I remembered correctly. books.google.com/… Jan 29 awarded Yearling Dec 9 comment Generalizations of the Four-Color theorem $K_{7}$ in the torus has a nice algebraic description: Start with the graph formed by the Eisenstein integers, where $a$ is adjacent to $b$ means $|a-b| = 1$. Every vertex of this graph has degree 6, and it's planar. To make the plane a torus, we quotient by a lattice. To make this well-defined for how we identify vertices and edges, that lattice has to be an ideal. Choose an ideal of norm 7, like $(2+\sqrt{-3})$, and we now have 7 vertices (all have degree 6), yielding $K_{7}$. I don't know if replacing the Eisenstein ring with structures like the Hurwitz ring gives nice generalizations. Nov 18 comment What makes the amenability of Thompsons group $F$ such a tricky problem? The amenability of Thompson's group does not appear to be an isolated instance of this. The complexity of graph isomorphism is somewhat similar, and led the author of this paper to use the term "disease": onlinelibrary.wiley.com/doi/10.1002/jgt.3190010410/pdf So now write 'The Thompson Group Amenability Disease' or something like that? And maybe 'The Jacobian Conjecture Disease' while we're at it? What other examples of 'diseases' are out there? Nov 18 answered Why do Bernoulli numbers arise everywhere? Sep 24 awarded Autobiographer Sep 11 comment In what sense is the classification of all finite groups “impossible”? I had the arxiv link in mind. Sep 10 comment In what sense is the classification of all finite groups “impossible”? 1. Can we make that expository paper about wild linear algebra problems community wiki? 2. Speaking of wildness and computational complexity, how do those problems behave when the field of interest is finite? Simultaneous conjugacy of ordered pairs of matrices is, for example, obviously in NP. Sep 6 comment What are smallest finite images of triangle groups? It is worth noting that there are finite simple groups whose simplicity cannot be proved this way. Since the $(2,3,5)$ triangle group is isomorphic to $A_{5}$, $A_{6}$ and $PSU_{4}(2)$ cannot be proven simple this way. I am not sure if there are any other examples. Jul 7 answered Variations to Cayley's Embedding Theorem for Groups Jul 6 accepted Subgroup property stronger than being characteristic Jul 3 revised Subgroup property stronger than being characteristic "without having" --> "instead of having" because "< Jul 3 answered Fantastic properties of Z/2Z Jul 3 comment Subgroup property stronger than being characteristic Geoff, I am impressed by the nontrivial extent to which you read my mind -- I did not know the Thompson subgroup always did this, but I was trying to get a better handle on some local analysis. Jul 2 asked Subgroup property stronger than being characteristic Jul 2 awarded Curious Jun 20 awarded Popular Question May 27 awarded Enlightened May 27 awarded Nice Answer May 1 comment Non-trivial consequences of Baer's theorem and Lucchini's theorem in subnormality theory A nice consequence I've encountered of the Baer-Suzuki theorem: