Roots and right factors of polynomials and left eigenvalues of matrices over Cayley-Dickson algebras

Abstract

Over a composition algebra $A$, a polynomial $f(x)\in A[x]$ has a root $\hat{I}\pm$ if and only $f(x)=g(x)\cdot (x-\hat{I}\pm )$ for some $g(x)\in A[x]$. We examine whether this is true for general Cayley-Dickson algebras. The conclusion is that it is when $f(x)$ is linear or monic quadratic, but it is false in general. Similar questions about the connections between $f$ and its companion $C_f(x)=f(x)\cdot \overline{f(x)}$ are studied. Finally, we compute the left eigenvalues of $2\times 2$ octonion matrices.