MathOverflow

The OEIS entry A053121 for a version of the Catalan triangle related to the expansion of the monomials $x^n$ in the Chebyshev polynomials of the second kind $U_k(x/2)$ references p. 60 of On Quaternions and Octonions by J. H. Conway and D. A. Smith ...

Suppose that $(X, H)$ is a polarized abelian variety. Suppose furthermore that $A \subset X$ is an abelian subvariety. Then by Poincaré reducibility there is a complementary abelian subvariety $B$ such that the addition map is an isogeny $a \colon A...

I. About the Quaternion group [the dicyclic group of order 8].
According to this paper Enumeration of dicirculant digraphs, Jing Wang et all, [see table one at page 13] the dicyclic group of order 8 has two non isomorphic connected Cayley digraphs.
Also Scott Effler in his thesis about two-generated Cayley digraphs [see tables in the thesis] says that there are four non isomorphic Cayley Digraphs of order eight with permutation representation of degree eight. There are two groups of order eight...

Let $A$ be an abelian category and $S$ be a right multiplicative system, that is, a collection of morphisms in $S$ satisfying: (MS1) the identity morphisms of all objects in $A$ belong to $S$ and $S$ is closed under composition, (RMS2) if $s:Y\to Z$...