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 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 Jun 3 revised From Tilings To Groups added 128 characters in body Jun 3 revised From Tilings To Groups added 36 characters in body Jun 3 asked From Tilings To Groups May 11 awarded Nice Answer May 1 awarded Yearling Apr 11 comment Can someone please explain the Riemann Hypothesis to me… in English? Can anyone provide a free link to the first linked paper? Apr 1 comment Lucas's Cyclotomic Formula Cool, thanks, I've missed that. So, essentially, one uses the identity $\phi_n(x^2) = \phi_n(x) \phi_n(-x)$ (valid for relevant $n$'s) and applies the Gauss identity for $\phi_n$ to get the Lucas identity (with $x^2$ in place of $x$). One needs to prove some properties of $R,S$ in Gauss identity, I might fill in the details in an answer later.