Points with Commuting Coordinates over Division Rings

Abstract

We investigate the properties of multivariate polynomials evaluated at points with commuting coordinates over division rings and octonion algebras. Given a division ring $D$, this set of points is denoted by $D_c^n$, and in the special case of $D=\mathbb{H}$, Alon and Paran showed that its points correspond to the maximal left ideals of $\mathbb{H}[x_1,\dots ,x_n]$. Here we show that if a polynomial $f$ vanishes at $\vec{a}\in D_c^n$, then any left multiple $gf$ also vanishes at $\vec{a}$. Consequently, over a central division algebra or an octonion algebra, any root of $f$ in $D_c^n$ is also a root of its (reduced) norm. We apply these evaluation properties to discrete algebraic dynamics, proving that if a point in $D_c^n$ is a fixed point of an $n$-tuple $T=(f_1,\dots ,f_n)$ of polynomials in $n$ variables, then it is a fixed point of $T^{\circ m}$ for any positive integer $m$.