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 Feb 23 awarded Nice Question Feb 21 revised Related Forms for the Riemann Hypothesis over Finite Fields added 65 characters in body Feb 21 revised Related Forms for the Riemann Hypothesis over Finite Fields deleted 4 characters in body Feb 21 revised Related Forms for the Riemann Hypothesis over Finite Fields added 85 characters in body Feb 21 revised Related Forms for the Riemann Hypothesis over Finite Fields added 107 characters in body Feb 21 asked Related Forms for the Riemann Hypothesis over Finite Fields Jan 30 awarded Nice Question Oct 24 answered Inference of an identity in Grassmann algebra. Oct 5 revised Radius of convergence of a power series - how can I be sure $\lim \frac{a_{n+1}}{a_n}$ exists? added explanation to inner limit Oct 5 answered Radius of convergence of a power series - how can I be sure $\lim \frac{a_{n+1}}{a_n}$ exists? Sep 27 awarded Popular Question Aug 12 revised Characters appearing naturally in arithmetic functions added 29 characters in body Aug 12 asked Characters appearing naturally in arithmetic functions Aug 12 revised From 4 squares to 2 squares added 28 characters in body Aug 12 asked From 4 squares to 2 squares Aug 9 comment A Combinatorial Proof of Dixon's Identity Thanks, @darijgrinberg! It's a good article to display the diversity of techniques for proving combinatorial identites. Jun 20 revised Number of solutions of $x^2_1+\dots+x^2_n=0,$ $x_i\in \Bbb{F}_q.$ fixed sign mistakes Jun 3 comment From Tilings To Groups (cont.) To get full understanding one should see what Gromov's motivation was for the word-hyperbolic definition, and read his books. I guess the theorem you mentioned will make an appearance there. Jun 3 comment From Tilings To Groups Thanks. I'll go on reading about this theorem (does it have a name, btw?). I've already seen the notion of quasi-isometry and its relation to Cayley graphs (actually, the primer also includes the $\delta$ definition of hyperbolicity). The more I read, the more I understand that the word-hyperbolic def. is useful for proving some theorems (say, "Density 1/2" of Gromov, which I recommend reading), but is not the most intuitive and especially not the most geometric. Jun 3 accepted From Tilings To Groups