Sum-Product Bounds & an Inequality for the Kissing Number in Dimension 16

Abstract

We obtain an inequality for the kissing number in 16 dimensions. We do this by generalising a sum-product bound of Solymosi and Wong for quaternions to a semialgebra in dimension 16. In particular, we obtain the inequality $$k_{16}\geq \frac{\sum_{x \in \mathcal{R}}\left|\mathcal{S}_{x}\right|}{\left|\bigcup_{x \in \mathcal{R}} \mathcal{S}_{x}\right|}-1,$$ where $k_{16}$ is the 16-dimensional kissing number, and $S_x$ and $\mathcal{R}$ are sets defined below. Along the way we also obtain a sum-product bound for subsets of the octonions which are closed under taking inverses, using a similar strategy to that used for the quaternions. We use the fact that the kissing number in eight dimensions is 240 to achieve the result. Namely, we obtain the bound $$\operatorname{max}(|\mathcal{A}+\mathcal{A}|,|\mathcal{A}\mathcal{A}|)\geq \frac{|\mathcal{A}|^{4/3}}{(1928\cdot\lceil \operatorname{log}|\mathcal{A}|\rceil)^{1/3}},$$ where $\mathcal{A}$ is a finite set of octonions such that if $x\in\mathcal{A}$, $x^{-1}\in\mathcal{A}$ also.