MathOverflow

I have found a new vanishing hypergeometric series. Namely, I conjecture that$$\sum_{k=1}^\infty\frac{\binom{2k}k 16^k(1656k^4-2604k^3+1158k^2-229k+15)}{k^2(2k-1)^2(6k-1)(6k-5)\binom{6k}{3k}\binom{3k}{k}}=0.$$As the series has converging rate $4/27$...

I wonder if there exists a characterisation of Hurwitz integers which are represented as sums of two squares of Hurwitz integers, up to multiplication by a unit. And if so, could you please point to a reference?
By a Hurwitz integer I mean an integer in the ring of quaternions, that is, a quaternion whose components are either all integers or all half-integers....

In the classical setting, the tangent space $T_p X$ at a point $p$ of a smooth scheme $X$ can be defined using equivalence classes of morphisms from the affine line $\mathbb{A}^1$. To avoid the issue where $X$ admits few global maps (e.g., if $X$ is...

This question is probably not research level that's why I asked it previously on MSE a week ago. Unfortunately it doesn't get much attention there and I thought I would try it here.
Choose a arbitrary discrete group $G$. The classifying space $BG$ of $G$ is classically constructed by forming a certain contractable $\Delta$-complex $EG$ (on concrete construction of $EG$: see below) endowed with an action by $G$. $BG$ is obtained...